FOR THE YEAR 1949

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REPORT No 31

FISHERY BOARD OF SWEDEN

ANNUAL REPORT

FOR THE YEAR 1949

AND

SHORT PAPERS

LUND 1950

CARL BLOMS BOKTRYCKERI A.-B.

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REPORT No 31

FISHERY BOARD OF SWEDEN

ANNUAL REPORT

FOR THE YEAR 1949

AND

SHORT PAPERS

LUND 1950

CARL BLOMS BOKTRYCKERI A.-B.

.

■

:

Director's report for the year 1949; Sven Runnström ... 5

Short papers: Preliminary report of certain experiments with a view to exploiting lakes empty of fish; Gunnar Aim... 19

The sea-trout population in the Åva stream; Gunnar Aim ... 26

Heterogeneous stimulus summation in the release of spawning activities in fish; Eric Fabricius ... 57

The white-fish population of Lake Ocke; Arne Johanson ... 100

Reactions of crayfish on low oxygen pressure; Arne Lindroth ... 110

The fitness of nylon thread for manufacture of fishing tackle; Gösta Molin ... 113

Result of impregnation experiments; Gösta Molin ... 119

A supplement to the fertility of char (Salmo alpinus L.) in the Faxälven watersystem, Sweden; Alexander Määr... 127

The possibility of avoiding winter-kill of fish; Carl Puke ... 137

Lepidurus arcticus Pallas in Indalsälven and Faxälven watersystems, Sweden and Nor­ way; Sven Runnström and Alexander Määr ... 147

The Coregonid problem II. Morphology of two Coregonid species in different environ­ ments; Gunnar Svärdson ... 151

By Sven Runnström

Members of the Staff in Jan. 1950 Director:

Fishery Biologists:

Sven Runnström, fil. dr.

Lars Brundin, fil. dr.

Gunnar Svärdson, fil. dr.

Thorolf Lindström, fil. dr.

Alexander Määr

Eric Fabricius, fil. mag.

K.-J. Gustafson, fil. kand.

Stig Persson, pol. mag.

Gösta Molin

Hendrik Toots

Arne Johanson

Birger Ahlmér

Birgit Sandgren

Anna Hägglund Laboratory Assistants: Ingrid Johannison

Helve Toots Porter: Henning Johanson

Kälarne Research Station (in the province of Jämtland) Fishery Assistant: E. Halvarsson

Secretary :

Fishery Assistants:

Assistent Secretaries :

Fil. dr. Gunnar Alm, Chief of the Bureau of Freshwater Fisheries, who because of scientific work is free from service at the Fishery Board, has retained his office at the Institute during last year. The Migratory Fish Committee (chairman: fil. dr. G. Alm, fishery biologist: fil. dr. B. Carlin, assistant: dr. V. Miezis) also has its office at the Institute.

Scientific and Practical Work by the Staff

In the Director's Report for the year 1948 a general survey was given of the work program of the Institute and during last year the research has continued on the whole along the same lines. This Report confines itself therefore to an account of the progress made during the year 1949 within the limits of the program set up.

Studies on the Bottom- and Planktonfauna

Brundin’s investigations on Chironomidae and other bottom forms in the South-Swedish oligotrophic lakes have been completed and published in a comprehensive paper as Report No 30.

Määr’s investigations on the bottom fauna in the Faxälven water system have also been finished during the year 1949 and the material is being worked up. In all, 457 bottom samples have been taken from 10 lakes. Of these lakes the Blåsjö with a depth of 140 m and the shallow Russfjärden with a maxi­

mum depth of 23 m have been the objects of a closer investigation. The average bentos production in the littoral region in lake Blåsjö is 1680 animals per 1 m2 but in the Russfjärden only 913. In the deeper regions the cor­

responding figures are 638 resp. 245 per 1 m2. The most richly represented animal groups are Chironomidae, Oligochaeta and Mollusca.

The lakes investigated are now regulated by dams and are subject to great water-level variations and Määr's investigations before the regulation now form a sound basis for the future investigations on the production of bottom animals. Investigations have already revealed that the effect is very different in different lakes, depending on depth conditions. Thus in lake Blåsjö only 13.4 per cent of all the bottom animals are exposed to dryness when the water-level sinks during the winter but in the shallow Russfjärden the cor­

responding figure is 76.6. It has however appeared that some of the forms, especially those of Chironomidae and Mollusca, can survive the drainage in winter probably by getting into the frostless depth at the bottom. The damage may therefore be not so great as the above mentioned figures suggest.

In the last year Fabricius with the assistance of Miss Stube has begun an investigation on the fauna on water plants and its importance as food for young fish. In lake Borgasjö at the inflow of the river Sannarå a deltoid territory is formed which is rich in bottom vegetation which probably will disappear because of the planned impoundment of the lake. During the months of June—August a parallel collection of trout stomachs and quan­

titative samples of the water vegetation together with bottom samples have been made. All the animals living on the vegetation have been sifted off and weighed in order to gain understanding of the importance of vegetation as the »producer» of fish food.

Lindström’s investigations concerning zooplankton production and its seasonal variations in lake Ottsjö and in the other lakes in the Storå water- system have been continued in 1949.

Improvement of Lakes by Introduction of New Fish Species

The experiment for an appropriate use of smaller forest lakes which has been going on for several years at the Kälarne Research Station under Alm’s

supervision, has been continued in 1949 according to the former plan. In the preliminary paper in this Report Alm presents some statements on the results gained hitherto.

Testing the Effectivity of Artificial Propagation.

Investigations comprise the control of fish stock in relation to statistics about the catch and plantings of fry and fin-cut fingerlings during certain years.

Pike: The investigations by Svärdsonhave been continued during the year 1949. In the water area of the Institute at Drottningholm 163 pike were caught during the spawning fishing which started on April 8, and after the spawning up to the spawning fishing in 1950, 43 pike. The corresponding figures for earlier years appear in the table given below.

Year

Beginning' of catch

year

Number of spawning

fishing

fish caught other fishing

Total

1945—46 March 27 255 45 300

1946—47 April 4 343 80 423

1947—48 » 18 223 82 305

1948—49 » 3 190 25 215

1949—50 » 8 163 43 206

It would be premature to consider the decline in fishing in the two last catch years as the result of over-fishing. The water-level during these two catch years has been low and thus the pike have been deprived of their spawning grounds to a large extent. As spawning fishing forms the dominant fishing it means that the year’s catch will also be bad. Scale samples have been taken from all the pike counted above but the material has been subjected only to a partial and preliminary work-up.

In the new experimental area, Lake Halmsjö, where the Institute, in con­

trast to conditions at Drottningholm, entirely controls a private body of water, the catch hitherto is as follows:

1949—50 April 6 80 292 372

Pike in Lake Halmsjö show quite a low average weight and are in bad condition. Therefore overpopulation may have occurred here before. The coming catch year as well as scale samples should determine this definitely.

At Drottningholm a certain number of pike caught has been marked every year. Different marking methods have been tried. In 1945 pike were tagged with a plate and wire of stainless steel round the lower jaw-bone. In 1946 experiments were made with individual fin-cutting according to a system reminiscent of crayfish markings. In 1947—1948 markings with plate and wire of rustless steel placed under the dorsal fin were tried. None of

these methods was good. There were few recaptures and it could be established that the plates fell off or — with jaw marking particularly — seriously influenced the pike’s condition. Fin-cutting in 1946 showed that fins, if not cut at the base, soon grew up again and made it impossible to read off the marking but at the same time so many such pike, which had been cut were recaptured that it appeared clear that the plates must have been lost to a considerable extent.

In 1949 tagging with plates that were fastened under the dorsal fin but with considerably thicker wire, was tried again. Only 29 pike were tagged of which 2 were recaptured during the winter 1949—1950. The new marks seem this time to be a little better and a certain optimism regarding the possibilities of the method can be felt.

The fin-cuttings in 1946 together with numerous good foreign experiences in group-markings by completely cutting off one fin led to the group-marking of young pike being started in 1947. In that year 274 young pike about 10 cm long, which all had had their left ventral fin cut off at the base, were put into the water area of the Institute during July 15—17. In 1948 between June 16—22 699 pike fingerlings of the same size as those in 1947 and whose right ventral fin had been cut off, were introduced into the same waters.

Finally on June 29 1949, about 255 young pike ca 10 cm long whose left ventral fin had been entirely cut off, were put into the Lake Halmsjö.

Of the pike introduced into the Institute’s waters the following interesting recaptures have been made up to the catch year 1949—50 i.e. up to the spawning in the spring 1950:

Introduction of 1947.

1. 16.4.48 cf 22 cm 48 ^a_Ö spawning 2. 28.4.49 ⁵ 53 cm 960 g spawning 3. 29.1.50 5 49 cm 720 g

4. 25.2.50 ^cf 35 cm 245 g 5. 13.3.50 $ 47 cm 730 g 6. 31.3.50 ^cf 49 cm 600 g

Introduction of 1948.

1. 16.4.49 cf 22 cm 52 g spawning 2. 26.2.50 ^cf 40 cm 250 g

These recaptures have shown a well healed scar after the fin-cutting and the fin has not regenerated. Thus it has been easy to identify the recaptures and they show also that the method is promising. They have been in good condition and have been strikingly different from specimens marked with plates. It is interesting to see the opinion confirmed that at least some of the males spawn already as one year old even when they have not reached an

unusually great length of body for this age. The strongly varying individual growth rate, such as is known from scale-analyses, is found even in fin-cut specimens. No. 2 from 1947 year’s introduction shows an unusually good growth and proves that at least some of the females spawn as two years old.

One year old mature pike females are hitherto unknown in Sweden.

Besides the possibility of studying the advantages of the introduction of half-summer-old young pike by the methods now available, the îécaptured fin-cut fingerlings also provide an insight into reliability of the scale-inter­

pretations which is important in regard to the pike. The scales of the fin-cut pike remain »control-scales» since the age is known.

The regional investigations of the advantage of the introduction of newly- hatched young pike has been continued in 1949. All the 15 fishermen together have handed in scales from 4937 pike together with corresponding data on length, weight, sex and catching date. This investigation which was started in 1946 has resulted in scales from 18,267 fish specimens now being put in the archives for future examination.

The loss of eggs in nature was studied in spring 1949 on the spawning grounds at Drottningholm. Small wooden boxes with a handle above the water surface were dug down in the bottom and the upper bottom layer was stirred up so that quite a homogeneous natural bottom was obtained in the boxes. After the water had cleared 100 fertilized eggs were carefully put in each box which was to remain out for 3 or 4 days. Altogether 3 boxes were used so that one box was emptied and one set out practically every day.

The method has certain advantages compared with that with the bottom grab used before.

Naturally one cannot decide with certainty that all the eggs which dis­

appear during the experimental period have been eaten by egg-predators. They may even possibly be carried out over the edges of the box by wave motion and thereby lost.

The contents of every box were sifted carefully and all the eggs were picked out by hand.

The results provided confirmation of last year’s experiences. During the first part of the spawning season the losses are relatively small. The number of eggs left after 3 days is as a rule high comprising 60—90 per cent of the number set out, on the average 85 °/o. After 3 weeks the shore area was invaded as in the past year by the red-eye and above all by the roach but also the perch. As a result of this the accidental losses increased during the 3 days so that in some cases only 5 or 10 percent of the eggs were left.

The results imply that it is hardly possible to give an average number of hatching percentage for pike-eggs in nature. It must vary considerably depending on whether the pike spawn early or late. For the pike spawning late, the losses of eggs should always be very near to 100 per cent while the earlier spawning pike can produce a considerable number of fry.

Since Montén has pointed out the extremely high mortality among the newly hatched pike fry in nature and has even discovered that this is to a high degree related to the extent of suitable habitats in every spawning ground and in every year, the practical interest of numerical value of the hatching percentage has sunk considerably and it does not seem to be worth while to try to produce average figures which after all are burdened with extremely great uncertainty.

1 he colour mutants of pike mentioned in the previous report were put in summer of 1949 into the ponds of the Institute and are for the present hibernated in aquaria. The losses during the second summer were quite con­

siderable but sufficient numerous specimens are left for a new generation which we hope will be born in summer 1950. Only when these offsprings have been examined the genetic nature of pike with diverging colour can be solved.

Ihe salmon, without adipose fin, mentioned in the previous report, unfor­

tunately died in spring 1949 before being transported over to Kälarne for further development.

Char: Alm’s comparative investigations on the results of the natural spawning of char in Lake Vätter and of the earlier planting of char fry which was going on there, have included the collection of statistics and scale samples from certain special char fishing together with the arrangement of data on the intensity of char fishing. Thus preliminary investigations have been made on the number of fishermen and fishing gear in certain years in the twenties, thirties and forties and also on the results of markings made in earlier years in connection with the spawning fishing. In all, 437 males and 523 females were marked with number-plates and in addition about 100 specimens only with silverwire. Of the former not less than 197 and 223 specimens respec­

tively, corresponding to 45.1 and 42.6 per cent have been recaptured. Of those marked with wire several specimens have been recovered. Recaptures have mostly occurred in the year after the marking year but also in the following years even up to 5 years later. Almost half of the recaptures have taken place in connection with spawning and relatively large numbers have been taken on the marking grounds. These char have thus for their spawning returned to the same grounds where they spawned before.

The total catch of the char during the last years has been good and has not shown any considerable decline. Since hatching ceased with the last planting of the young in the spring of 1944 and while the usual catch age according to earlier investigations of Alm is 5—7 years, the catches during the past years have been based on age groups which have come into being during those years when at the same time planting has been carried on. Only the chatches of char in the coming years will be entirely based on year groups descended from natural spawning.

Runnström’s control of char population in Lake Torrö (Jämtland) in con­

nection with plantings of fry or fingerlings has been continued in 1949.

Control of Fish Populations

Salmon: On the basis of age-analyses on the one hand and the statistical material obtained on salmon delivered to certain fish-selling associations on the other hand Alm has undertaken preliminary investigations on the com­

position of salmon stock in the Baltic area in recent years. From this it is apparent that the biggest catches have occurred in 1946 in the sea as well as in rivers whereupon a slow decrease has taken place. During the fishing- seasons 1946/47 and 1947/48 the age of salmon taken in the Baltic was unusually high. During the winter 1947/48 there were caught 44.4 per cent six-year-old, 29.9 per cent seven-year-old and even up to 12 per cent eight- year-old and even older salmon. During the autumn 1948 and even more in the spring 1949 again smaller and younger salmon appeared. Age-distribution in the catches in the South Baltic (Gotland, Blekinge and Midsjöbankarna) was now more normal with only 18 per cent seven-year-old and 3 per cent older salmon. The high age during the previous years depended on the pre­

sence of large numbers of salmon from the very rich year classes which arose during the severe winters 1939/40—1941/42 and which as older salmon dominated in the catches 1946—48. A brief account about these investigations was handed over by Alm to the Salmon Committee of the international Council at the meeting in Edinburgh in the autumn 1949.

Grayling-. Gustafson’s investigations regarding the spawning run in the brook Svartbäck (Lake Storsjö, province Jämtland) was carried on during the spring of 1949 in the same way as in the previous year (see Bep. 29: 35—

44). In all, 435 fishes were controlled at the trap (221 cTcf and 214 ??) of which 414 were marked. Of these 28 i.e. ca 6 per cent were recaptured in Lake Storsjö during the summer (mainly June—July), 26 of the fishes being recap­

tured within a radius of 5 km from the marking grounds. Of the 147 graylings marked in the spring 1948, 37 were recaptured during the spawning run in

1949.

Of special interest in last year’s investigations was the discovery of an early migration of the fry from the brook. By means of putting a frame with a fine silk cloth in the dam opening for five minute periods, an approximate idea was obtained of the extent of the migration. This lasted for 10 days (June 7—16) and coincided with a rain period which powerfully increased the water current in the brook. The one-to-two-week-old fry appear to have followed the flow passively. The total number of migrating fry is estimated at ca 80.000 which makes 18 per cent of the total number of eggs deposited in the brook during the spawning in the spring 1949.

In the trap, during the period July—November, 491 down-migrating one- summer-old young graylings and 186 several-summer-old young ones were captured. An inventory of the brook made by means of electrical fish shocking

at the end of August 1949 showed that about 400 young graylings of the year class 1948 had wintered in the brook.

Percentage distribution in different year classes of the spawning population appears of the table given below.

II III IV V VI VI] VIII

1919... °/o — 37 44 12 6 0.8 0.2 1948 ... %> — 22 30 30 16 1 —

Char-. The main spawning place for the char in Lake Jormsjö (Province Jämtland) lies in the running water of the River Blåsjö. Since 1947, under the supervision of Runnström, the spawning run has been controlled here by a weir which is built right across the river. Since however spawning also occurs in the river bed below the weir the whole spawning stock could not be controlled. During spawning run in Sept. 1949 5469 fishes (1884 cfcT and 3585 ?$) were caught in the weir. Of these 474 fishes were tagged with celluloid marks and 4958 were fin cut. Below the weir net fishing was done and of the spawning char taken here, 695 were marked. Of these 135 or about 20 percent were recaptured at the weir. The total spawning stock can thus be estimated at ca 27.000 fishes. During the winter 34 tagged fishes were recaptured in different places in the lake by fishermen. The subsequent control in the autumn 1950 will provide an answer to the question how many of the tagged or fin-cut fishes return to the river for spawning. Of the char tagged (328) and fin cut (465) in the autumn of 1948 only 5 and 15 fish respectively were recaptured in the weir in the autumn 1949. In the meantime only 20 tagged fishes had been fished out so that 1948 year’s spawning stock has given a very poor contribution to the fish yield after spawning.

Trout: Runnström’s control of migrations in the fish ladder at Lake Rensjö was carried on also in 1949 (see Rep. 29:85—88). In all, 612 young trout migrated from the stream to the lake. The migration upstream occurred mainly during June—Sept. During the late summer and autumn only 12 spawning trout (6 $?, 6 cfcf) migrated downstream to spawning grounds in the stream. Of the trout controlled 317 were tagged. Of these 5 were recaptured in the lake in 1949 along with 19 trout which had been tagged in earlier years.

Whitefish: Toots’ investigations regarding the whitefish spawning run at the so called »Vaktfisket» in the River Gimå (see Rep. 29:146—152) have been continued also in the past year. The year class 1945 which dominated in the catches during 1948 asserted itself also in the following year and represents evidently a very rich year class. Catches in 1949 were numerically even higher than in the previous year (9492 fishes) and the catches in both years are considerably above the average. Owing to the numerous individuals in

the 1945 year class the growth has however been bad and the average weight of the whitefish in the catches has gone further down.

Fabricius, with the assistance of Ahlmék, has in 1949 marked 350 spawning whitefish in Lake Vojmsjö (Province Västerbotten) and 77 whitefish in Lake Tåsjö. Recaptures in the coming years should give an insight into the questions of how stationary the stock is at a certain spawning ground and also the percentage of whitefish which return several years in succession to the spawning ground. The marking experiments should also give an idea of the approximate size of the stock.

Investigations on Factors which Release Fish Spawning

In 1949 Fabbicius started investigations which aim at contributing to the elucidation of the combination of factors which cause the selection of spawning ground and the release of spawning activities. His investigations include observations regarding spawning in the pike, whitefish and char and Fabbicius has discussed the problem in a special paper in this Report.

It is to be hoped that these interesting observations on spawning grounds can be completed by experimental studies in aquaria.

Studies in Spéciation of Fish

Whitefish: SvÄKDSON has carried on investigations concerning whitefish problems. The method has been to have a sample made during the spawning of a biologically homogeneous population.

These samples, which comprise 50—100 fishes, sometimes less numerous, are sent to the Institute where the fish are weighed and their length measured.

They are numbered and preserved in formalin. Later a great number of body measurements are taken, scales in lateral line and in height line counted as well as finrays and finally the gillrakers are examined. In the autumn of 1948, 27 such samples arrived in the Institute and 19 more during the 1949 season.

Lakes over all the country are already included in the study.

Special attention has been given to the transplantation of a whitefish species from one lake into another since in doing so it is possible to compare the same genotype in different environments. Great care must, however, be taken in order to be absolutely sure that no extraneous plantings have occurred which could give possibilities of confusion. Hitherto the material on the following transplantations has arrived in the Institue and has been partly examined.

»Storsik» from Uddjaur transplanted to Ilvåstjärn, Kälarne

»Aspsik» » » » » Vontjärn, »

»Storsik» » Dellen » » Öjungen

River whitefish from Bureå transplanted to Brunträsket

» » » .»..' » » Lajnijaure

» » » Råneå » » Vitträsket

Whitefish » Drogen » » Njarven

» » Gimån » » Sicksjön

» » St. Gla » » Lelången

Transplantings of whitefish from Lake Uddjaur which are more closely dealt with at another place in this Report have shown that the relative size conditions in different parts of the body are more conditioned by growth than characteristic of a species and can therefore probably not be used for identi­

fication. Experiences hitherto reveal, however, that the number of gillrakers are strongly conditioned genetically and therefore probably of systematic value.

Fabricius has investigated whitefish forms from the lakes Vojmsjö, Skik- kisjaure and Dikasjö in the Ångermanälven water system. Three whitefish forms have been determined namely; a large one populary called »Storsik»

and two preferably plankton-consuming smaller forms called »Asp» and

»Sill» (see Rep. 29, fig. lap. 95). The number of gillrakers together with average length in the spawning whitefish included in the samples, appears from the table given below.

Number of Number of Body Number of gillrakers fish length mm fish

M M

»Storsik» (the whole Vojmån watersystem) 21.3 89 346.2 58

»Asp», Vojmsjön ... 45.0 101 194.8 101

»Sill», » 38.0 30 93.5 29

» , Skikkisjaure ... 37.4 101 142.0 101

» , Dikasjön... 38.6 99 185.4 99 Since these whitefish forms within the area investigated occur side by side, it must he assumed that in this area they present themselves as three good species.

»Storsik» can be found in all the three lakes and also in the river streches between them. The presence of the two smaller forms is confined to the lakes and »Asp» occurs only in Lake Vojmsjö while »Sill» is found in all the three lakes.

A remarkable circumstance is that »Sill» in Lake Vojmsjö where »Asp»

occurs, is a dwarf form that lives at a great depth while »Sill» in both the other lakes where »Asp» is absent, is considerably larger and lives at about the same depth as »Asp» in Lake Vojmsjö. A possible explanation is that

»Sill» in Lake Vojmsjö is exposed to hard competition by the »Asp» which has a »biological niche» there, which »Sill» possesses in the other lakes.

* The distribution of »Asp» from Lake Vojmsjö to Lake Dikasjö is apparently prevented by waterfalls in the river stretches between the lakes. In the sum­

mer of 1950 Lake Vojmsjö will be impounded so that the stream between

the two lakes will be changed periodically into a broad sound through which it will be possible for »Asp» to invade Lake Dikasjö from Lake Vojmsjö.

It will be of great interest to examine how this will effect the now compara­

tively large »Sill» in Lake Dikasjö. A more detailed discussion of the material will probably be presented by Fabricius in the Annual Report next year.

Trout-. At Kälarne Alm has carried on investigations on certain race questions in trout. At the Research Station are now to be found offsprings in the third generation from a large native brown trout and a small brook trout, which still show obvious differences in colouration and in sexual maturity.

This further confirms the earlier supposition that in these cases genetical factors and not the environment are decisived (see Alm, Rep. 29:29—34).

In order to investigate these questions still further the Research Station has reared up mature fishes also from Danish and Swiss trout eggs. The aim is to study the newly described characteristics in the offsprings from these

»forms». These are now being reared and are at present one year old.

Perch: For further elucidation of the characteristic relation formerly pointed out between size, growth and sexual maturity in the perch, A^lm has started an experiment on a large scale at Kälarne. During the summer of 1949 a large number of perch fry was reared at the Research Station. In the autumn no less than 3600 one-summer-old young perch with length of 3—7.5 cm were obtained. The intention is during the summer of 1950 by means of dense resp. sparse stock in different ponds to establish a slowly growing and a rapidly growing stock. Later the difference in sexual maturity between different stocks as well as between individuals with different growth rates within each stock will be studied here.

Hybrids: During recent years Alm has carried out certain experiments with hybrids at Kälarne and also at some state hatcheries. The purpose has been to find out whether the growth of bastards is better than that of the parents, whether they become sexually mature and give viable offspring and also whether the results in this case will be different in the different hybri­

dization directions. Earlier experiments in crossing the salmon $ X the trout cf and the trout 9 X the salmon c? have suggested that the latter crossing should give worse fertilization- and hatching results or even prove quite ineffective. A smaller number of bastards of the crossing first mentioned has been reared at Kälarne to an age of 9 years. Here the sexual maturity has been reached at the age of 6—8 years in males as well as females, but the artificially fertilized eggs which have been obtained have died during the hatching time or in some cases given some fry which however have soon died.

In order to obtain a more comprehensive material new crossing experi­

ments on a large scale were started in the autumn of 1948 and were repeated in the autumn of 1949. In the winter 1948/49 the survivors from eggs to fry

were in salmon ? X seatrout cT 29, 37 and 45 per cent against the parallel experiment, salmon ? X salmon cf 75, 87 and 83 per cent respectively and in seatrout ?X salmon c? 0, 0 and 28 per cent against in seatrout ?X seatrout cf 15, 20 and 77 per cent respectively. The rearing of fry during the first sum­

mer showed on the whole the same survival in both hinds of hybrids namely 30—50 per cent and an average length of 7—7,5 cm in both cases.

In the autumn of 1948 21,000 eggs from hybrids which had been obtained by crossing of char ? X brooktrout c? and which were sexually mature at the age of 4 summers were fertilized with milt from the same hybrids.

Another portion of 3,000 eggs were fertilized with milt from the brooktrout.

The losses were high in both cases and only about 1,400 and 700 fry respec­

tively were obtained for further rearing. Of these 320 and 400 one-summer- old fishes respectively, in both cases 7—8 cm long, were fished out in the autumn of 1949. These are still kept in ponds. The fertilization experiments have been repeated in the autumn of 1949.

Researches in Lakes Regulated by Dams.

Many lakes in North Sweden are impounded as water reservoirs in order to give a more even watersupply to the power-generating stations on rivers and several large regulations are being planned. In these lakes great water- level variations occur which influence the fish stock and food production.

The Institute acts as expert to the Water Court regarding these questions and the necessary investigations are payed for by the power-generating industry.

Through this it has been possible to attach a relatively large staff to the Institute and many of the general fishery-biological investigations referred to in this Report have been carried out in connection with investigations in the regulated lakes. It is necessary first to make clear the biological condi­

tions under natural circumstances before one can determine the effects which rise after regulation. At present 150 lakes are included in the investigation program, where a routine work is carried out by the fishery assistants, which comprises the collection of fishery statistics, practical fishery experiments, mapping of spawning grounds etc. The Institute has to make proposal to the water court concerning compensation for damage which is done to the fish stock.

Practical Research on Fishing Gear.

Molin has finished certain experiments concerning the impregnation of fishing-nets and also investigations regarding the suitability of nylon yarn for making fishing gear and he has published the results in two papers in this Report.

Library

As the result of publishing the Annual Report the Institute has been grati­

fied by the offers of exchanges from a large number of institutes in Europe and the USA and we hope that these exchanges will be extended still further during the present year.

Publications in the Year 1949

The following papers by the staff of the Institute and other members of the Fishery Board have been published during the year:

Rep —Report from this Institute.

SFT=Svensk Fiskeri Tidskrift (Swedish Fishery Journal). Only Swedish language.

Alm, G. Ökar eller minskar gäddan och abborren utefter våra kuster? Sportfiskaren 15: 3—5.

— Vandringsfiskutredningen och dess hittillsvarande verksamhet. SFT 58: 115—

118.

— Influence of heredity and environment on various forms of trout. Rep. 29:

29—34.

— De fisktomma sjöarnas produktionsmöjligheter. Fisken vid disken. Svängsta, Årg. 1, 2: 4—7.

— Protection of fish and wildlife. Fish United Nations Scient. Confer, on the Con­

servation and Utilization of Resources. Water 8 (cll):3—16.

Brundin, L. Chironomiden und andere Bodentiere der südschwedischen Urgebirgs- seen. Rep. 30: 1—914.

Dahr, E. Kräftpestens utbredning år 1949. SFT 58: 122—124.

— Biologiska betraktelser över nyttan av fiskinplanteringar i naturliga sjöar och vattendrag. SFT 58: 171—175.'

Fabricius, E. Fiskodling i naturliga klippdammar. Akvariet 23: 58—61, 73—77.

Furuskog, V. Fiskvägar. Byggnadshandboken IV: 493—495.

Gustafson, K.-J., Movements and growth of grayling. Rep. 29: 35—44.

Hult, J. Regleringsproblem i skogssjöar. SFT 58: 76—73.

— and Johnels, A. Predators on salmonfry in the river Mörrumså in 1948. Rep.

29: 45—48.

Lindroth, A. Nya försök över giftverkan hos sulfitfabriksavloppsvatten. Sv. Pap­

perstidning 52: 237—238.

— Vitality of salmon parr at low oxygen pressure. Rep. 29: 49—50.

— Synpunkter på vattenhygienens organisatoriska ställning i Sverige. Vattenhygien 5: 64—69.

— The » Waste-pool system» as purification method, XII Int. Dairy Congress, Stockholm 1949: 652—658.

Lindström, J. Etude quantitative des planctons crustacés dans quelques lacs du Jämtland. Rep. 29: 51—57.

— Om samspelet mellan avfiskning och ett fiskbestånds tillväxt. SFT 58: 130—131.

MääR, A. Fertility of char (Salmo alpinus L.) in the Faxälven water system, Sweden.

Rep 29: 57—70.

Olofsson, O. En dålig och en bra ålyngelsamlare. SFT 58: 6—7.

Puke, C. Experiments in Lake Väner on the influence on fish of bomb-dropping.

Rep 29: 71—74.

2

— A new water sampler. Rep 29: 75—76.

— Bottom fauna and environmental conditions in the littoral regions of lakes.

Rep 29: 77—80.

— Environment and productivity of the lakes near Stockholm. Rep 29:81—84.

Rosén, C. Laxfisket i Östersjön. SFT 58: 140—141.

Runnström, S. Director’s report for the year 1948. Rep 29: 5—28.

-— Control of trout migration by a fish ladder. Rep 29: 85—88.

Svärdson, G. The Coregonid problem. I. Some general aspects of the problem. Rep 29: 89—101.

— Note on spawning habits of Leuciscus erythrophthalmus (L.), Abramis brama (L.), and Esox lucius L. Rep. 29: 102—107.

— Competition between trout and char (Salmo trutta and S.alpinus). Rep 29:

108—111.

— Salmon (Salmo salar L.) with no adipose fin. Rep 29: 112—114.

— Natural selection and egg number in fish. Rep 29: 115—122.

— Sex differentiation in eel (Anguilla anguilla) and the occurrence of male eels in the Baltic. Rep 29: 123—128.

— Eels (Anguilla anguilla) found in Sweden in partial nuptial dress. Rep 29: 129— 134.

— Stunted crayfish populations in Sweden. Rep 29: 135—143.

— Den hemliga döden. Svensk Jakt 87: 171—176.

— Den hemliga döden och rovdjuren. Svensk Jakt 87: 223—229.

— Competition and habitat selection in birds. Oikos I: 157—174.

— Gamla siklöjor. SFT 58: 29—30.

— Gäddans lekakt. SFI' 58: 60—61.

— Varför lägga fiskarna många ägg? SFT 58: 155—159.

Sorensen, I. Om kulturbetingad förstörelse av sydsvenska vattendrag. Skånes Natur:

56—80.

Toots, H. Maränenfischerei im Fluss Gimån (Jämtland). Rep. 29: 146—152.

— En sikrompump. SFT 58: 97—98.

Vallin, S. Disposal of dairy waste water. XII Int. Dairy Congress. Stockholm 1949, General Report: 204—210.

— Sockerbrukens avloppsvattenfrågor. Socker meddel. 5: 197—206.

to Exploiting Lakes Empty of Fish

By Gunnar Alm

In 1931, when the program of work was initiated at Kälarne’s Research Station, it comprised, originally, various experiments concerning growth, sex maturity, race questions etc. These experiments were designed to take place not only in the large and small ponds that are to be found at the Research station but also in small lakes and tarns in the neighbourhood. Thus many of these were utilized for the experiments. It soon became evident to the author, who was directing the work at Kälarne, that another important question should be included in the plan of investigation, viz. the best way to exploit these lakes and tarns, many of which were empty of fish. Others contained only stunted populations of perch while, in a third group, perch, as well as pike and roach, were to be found. In addition to these fish the somewhat larger lakes also included whitefish and burbot and, occasionally, trout; the latter being also found in some small brooks. Lakes of the above type are very common in large parts of the forest areas in northern Sweden.

Though they may often be rather insignificant in size they can offer consider­

able advantages to people living in the vicinity provided the supply of fish is good. Sport fishing also helps to turn these lakes into valuable assets.

The first questions that had to be answered were as follows: Why do so many of these lakes completely lack fish? Can planting of fish in those lakes be regarded as a success economically and, if so, what species should be tried and what principles applied? Would the stunted perch populations be improved by decimation or could they be brought to a better standard of growth and a higher economic yield by some other means? Would it be advisable to introduce new species of fish in small lakes of this type where several other species are already to be found?

Preliminary investigations were initiated in a great many of these lakes with regard to vegetation, fauna, the condition of the shores and the depth of the water, temperature, oxygen content, pH value and the transparency and colour of the water. The proprietors of the fishing rights, i.e. Domän­

styrelsen (the Forestry Board), forest companies and private owners, were approached and they authorised the Research Station to organise planting

of fish and subsequent test fishing in order to check the results. Unfortu­

nately, it often proved impossible to secure the desired data. Owing to a great deal of poaching in many of the lakes whei'e the experimental planting had produced a fine supply of fish, the tests resulted in a misrepresentation.

In spite of this, several small lakes, which were previously looked on as fairly worthless, were found to bring considerable catches while, in other instances, for various reasons, this failed to occur. Also some of the lakes which contained fish before the planting experiments, gave a higher yield than ever expected. When further experience has been gained a comprehen­

sive report of all these activities will be submitted. So far as the stunted perch populations are concerned, an extensive account was published in 1948 (Alm, 1948).

In the present preliminary paper I propose to deal only with lakes empty of fish. These lakes, as well as other minor waters in this vicinity, are often of the type of so-called tarns, with areas of a few hectares and depths varying from two to ten or twelve metres, without any noteworthy water circulation, i.e. with only very small affluents and outlets. The shores usually consist of quagmires and loose bottoms of mud.

The first question which must be examined is why certain of these small lakes are empty of fish. A certain minimum of oxygen, a relatively high temperature in the water, absence of harmful or poisonous matter and sufficient food are, of course, indispensable to the fish. Properly established fish populations require, in addition, suitable spawning grounds. In the lakes which lack fish the first mentioned prerequisites are not alwTays to be found.

Hydrographic examinations of nearly a hundred small lakes, with or without fish, within the experimental area, have shown that, in summer as well as in winter, exceedingly pronounced stratification is formed. It has a high- situated metalimnion and a relatively large hypolimnion of an extremely low temperature and with either a markedly reduced oxygen content or a total absence of oxygen. (Brundin, 1942, Alm, 1943.) This applies to shallow, as well as to relatively deep lakes. During the winter a complete absence of oxygen will sometimes, and in the shallow lakes fairly regularly, be notice­

able in the entire water mass. Naturally no fish can survive from one year to another in lakes of this latter type. However, in most of the lakes a total lack of oxygen need not, ordinarily, be anticipated. Nevertheless, during a severe winter, when the waters have long been frozen, such an oxygen defi­

ciency may occur and the fish population inevitably die of suffocation.

Finally, many of these small lakes have, either long ago or more recently, contained permanent fish populations which have now disappeared. I have suggested previously that such a disappearance may have been due to changes in the climate affecting the hydrography of the lakes (Alm, 1937). Later, more favourable climatic conditions may, very conceivably, have become re-established, while no opportunities have occurred to introduce any fish.

The investigations have also shown that many of these lakes empty of fish contain adequate fauna (Brundin, 1942, 1949) thus offering the fish com­

paratively fine opportunities of growth.

Proceeding from the aforementioned observations, concerning lakes empty of fish a great many planting experiments have been carried out. The material used for this purpose has sometimes consisted of fingerlings and yearlings from the ponds at Kälarne or sometimes of fry hatched at the Research Station. The planting should be made as economically profit­

able as possible. Therefore, in spite of the fact that the lakes have not seemed particularly ideal for such a purpose, the material used has been chiefly salmon, trout, char and brook trout, with the addition of whitefish and eel and, finally, in a few isolated instances, pike and perch. Sometimes only one, on other occasions several, species have been introduced. In order to check the results, test fishing has been regularly arranged every autumn with various types of fishing tackle, though nets, with meshes of varying width, have mostly been used The fish thus caught have, broadly speaking, concluded their proper period of growth. In connection with the test fishing length measuring and, occasionally, weighing, have taken place. Fish capable of further life have sometimes been replanted. Samples of scales and stomach contents have been taken from some fish and sex and sex maturity have been determined. Some fish have been preserved.

Several planting experiments have proved futile and, in other cases, itera­

tive test fishing has produced only occasional specimens. The probable explanation is that poachers have caught the planted fish. Nevertheless, in some lakes good results have been recorded, as shown by the following few examples.

East Vontjärn. 3 hectares, depth 4 m, pH 6.5, solid, sparsely wooded, as well as marshy, shores. The water vegetation comprises, partly, Equisetum and Nitella and a red alga, Pseudochantransia serpens Isr. Planting in the Spring of 1937, of 180 young trout (small river trout and trout from Lake Vättern), of a length of 15—16 cm, produced, during the first year, 44 21—

31 cm long specimens weighing about 10 kg. During the second year, the corresponding number was 16 specimens of a length of 31—44 cm, weighing about 7 kg and, during the third year, 8 specimens, 32—56 cm long, of close on 12 kg, some weighing 2 kg a piece. In the autumn of 1940 only 1 trout of 1.8 kg was captured. Since that time no trout have been caught.

That population has, evidently, ceased to exist, owing to suffocation during the severe winters of 1940/41 and 1941/42.

In the autumn of 1944, planting took place of 250 river trout and 300 brook trout, both one-summer old, and in the spring of 1945 of 25,000 fry of whitefish (»asp» from Arjeplog). In the same autumn 26 brook trout, 22—29 cm long, weighing about 4.9 kg were obtained, as well as 10 white- fish, 18—20 cm long, of 0.6 kg. In the autumn of 1946 catches were recorded

of 17 brook trout, 30—38 cm in length, of about 10.5 kg, and 75 whitefish, 30—33 cm, weighing 25.5 kg. After that no brook trout were caught, and the planting of trout proved completely negative, probably owing to the superior competitive power of the brook trout who, though of the same age at the planting, were a little larger. On the other hand, the planting of the whitefish continued to produce fine catches. In 1947, 20 specimens of 15.5 kg were captured, in 1948 22 specimens, 45—51 cm long, of about 26 kg, and, finally, in 1949 7 specimens, 48—54 cm long of 11.4 kg. Further, during the autumn of 1949, 24 graylings, 22—30 cm long and weighing about 1.6 kg, were caught, from a planting performed in the spring of 1949 of 180 two-summer grayling. Altogether, approximately 125 kg of fish, i.e. about 10 kg per annum, and 3.3 kg per hectare, have been captured during the past 13 experi­

mental years.

Grästjärn. 10 hectares, 5 m deep, pH 6, surrounded by quagmires and, thereabout, pine wood. No solid shore, but on one side of the tarn a steep rock. The vegetation very sparse. In the autumn of 1935, a planting of 80 young whitefish (ova from Ansjön), of a length of 15—20 cm, produced, in 1936, 27 specimens 26—33 cm of length weighing 6.4 kg. During the following year, 6 specimens of 39—42 cm and 5.5 kg were caught and in the subsequent years up to, and including, 1948 altogether 36 specimens weighing nearly 43 kg.

The planting, in the spring of 1936, of 150 two years old specimens of brown trout, 9—16 cm long, produced during the first following year, 1937, 26 specimens 17—27 cm of length weighing almost 3 kg, during the second year 40 specimens, 23—36 cm long, weighing 8 kg, during the third year 13 specimens, 31—38 cm long, of 6 kg, and during the fourth year 6 trout of 33—

40 cm and 3.6 kg. Planting in the autumn of 1940 of 740 one-summer char (ova from Dikanäs) has, during six subsequent years, produced altogether 138 specimens of, in all, about 35 kg, and in later years several specimens of a weight of 1 kg each. A few of the captured char have been replanted.

A new planting experiment, in the autumn of 1943, including 400 one- summer trout has produced a disappointing catch of only 8 specimens. In all probability, they have been eaten by ihe older and larger char. In this tarn, poachers have also been extremely busy in recent years. This explains the unsatisfactory catches of 1947 and 1948, as well as the complete failure in the autumn of 1949. In this year 500 one-summer salmon had been planted.

The favourable result of the planting of char in this lake is noteworthy, in view of the fact that the entire habitat completely differs from that usually connected with the occurrence of char. In spite of this, the char has, evidently, flourished. The aforementioned figures, as well as the appended fig. 1, dis­

close a vigorous growth. Mention will be made below of a further instance of successful planting of char. This latter case concerns a lake of another type.

Total catches in Grästjärn amount to about 111 kg of fish, or 8.6 kg per

Number

H 1945

Fig. 1. Growth of the char in the lake Grästjärn. Number of fish at different size in different years.

year. Counted per hectare this capture is not big, only 0.9 kg, but considering that it has consisted chiefly of trout and char, it must, nevertheless, be regarded as fairly satisfactory.

West Rörtjärn. 6 hectares, depth 5 m, pH 6.4. Shores partly stony, wooded.

Vegetation sparse, with Equisetum and Nupliar. In the autumn of 1940, 560 one-summer char (ova from Rensjön) were planted here. In the course of the years 1941—1949, captures reached a total of 172 char. 59 of these were returned to the lake. The length varied, in the autumn of 1949, from 42 to 44 cm. The weight of the 113 collected specimens was about 48 kg, i.e.

5.3 kg per annum and almost 1 kg per hectare.

Nissetjärn. 1.5 hectare, depth 8 m, pH 6.8. Quagmire, as well as stony shores. Vegetation sparse. In the spring of 1939 and the autumn of 1944 one-summer old brook trout was planted, viz. 150 and 350 specimens, respec­

tively. Test fishing during the years 1940—1949 produced annual catches of between 6 and 40, part of which were replanted. During the summer of 1947, a great deal of poaching went on. It was rumoured to have been very remunerative. During the years 1947 and 1948, the test fishing proved nega­

tive. The annual total, earlier accounted for, amounted to 87 specimens of 30 kg’s weight, i.e. 3 kg per annum and 1.5 kg per hectare. The largest specimens had a length of 44 and 47 cm with a weight of 1.1 and 1.2 kg, respectively.

Ilvåstjärn. 1.5 hectares, depth 9 m, pH 6.8. Surrounded by quagmires.

Vegetation sparse. Planting of 32 four-summer whitefish (ova from Ansjön) took place in the autumn of 1935, and of 500 offspring of brook trout in the spring of 1937. In the autumn of 1936 19 specimens of whitefish were caught, of a length of 31—37 cm and a weight of about 5 kg, in the autumn of 1937 3 specimens of 38—42 cm and 2.7 kg. In the years of 1938 and 1939 no whitefish were caught, nor any brook trout. The last-mentioned planting must, therefore, have been a complete failure. Also a planting experiment of 400 one-summer char in the autumn of 1940 (ova from Kallsjön) proved completely negative. No doubt, a total lack of oxygen occurred in this tarn during the winters of 1940—42, thus causing the death of the planted char, as well as of such whitefish and brook trout as may have remained. The lake was, therefore, again regarded as empty of fish and, in the spring of 1945, a new planting of whitefish took place with 15,000 fry of large white- fish from Arjeplog. In the autumn of 1947 4 specimens were caught, in the summer and autumn of 1948 53 specimens of 19—25 cm and about 4.5 kg and, finally, in the autumn of 1949 34 specimens, 19—26 cm long of about 2 kg’s weight. Provided that the population is not annihilated by suffocation during the coming winters this later planting experiment seems to promise a favourable result.

These instances of successful planting in lakes empty of fish are no new occurrence. Similar observations have been made in many places in Sweden.

The chief purposes of the present experiments at Kälarne have, however, been to ascertain why the results are sometimes positive, sometimes negative and, when positive, the possibilities of growth and pi'opagation in the new environments. There is practically no doubt that the previously mentioned hydrographic factors play the greatest part as far as the negative results, which are not included here, are concerned.

As to the growth, it has, as already mentioned, in several instances been very good. This might, of course, have been expected, considering that no other fish have competed for the previously untouched food. However, in this respect the conditions differ from one lake to another, in view of the comparatively marked variations in the supply of plankton, as well as in the fauna of the shores and bottoms. These questions will, however, not be dis­

cussed in the present connection.

On the other hand, it should be noted that successful propagation in these populations has occurred only exceptionally. The planted fish have become mature, ova and milt have formed quite normally and spawning has often occurred. This is shown by the specimens with different numbers of remaining ova which have been collected. Thus, the spawning act has been induced, notwithstanding the absence of suitable spawning grounds. However, no results of the spawning of, e.g. the trout or the char, over the loose mud

bottoms have been discernable. On the other hand, in Grästjärn, several specimens of a new generation of whitefish have been caught. As far as can be judged they originated from spawn of the implanted specimens of white- fish. Yet, usually, renewed implantations are required with intervals of a few years.

Furthermore, the experiments have shown that in these types of lakes there is always the possibility that severe winters, by their unfavourable hydrographic and chemical effects, may prove disastrous to the fish. Never­

theless, in spite of this risk, strong arguments can be advanced in favour of these implantations. Had the catches been marketed, the income from the sales would have much more than covered the costs of the implantations.

A very good income would have been obtained had these waters been let for sport fishing.

References

Alm, G. 1937. Sötvattensfiskarnas utbredning och den postarktiska värmeperioden.

Y mer 57: 299—SU.

— 1943. Beiträge zur Kenntnis der Limnologie kleiner Schwinguferseen. Arch. f.

hydrobiol. AO: 555—575.

— 1946. Reasons for the occurrence of stunted fish-populations (with special regard to the perch.) Inst. Freshwater Research, Drottningholm, Rep 25:

1—U6.

Brtjndin, L. 1942. Zur Limnologie jämtländischer Seen. Inst. Freshwater Research, Drottningholm, Rep. 20: I—10A.

— 1949. Chironomiden und andere Bodentiere der südschwedischen Urgebirgsseen.

Inst. Freshwater Research, Drottningholm, Rep. 30: 1—91A.

By Gunnar Alm

Contents

Page

Introduction ... 26 The water system of the Ava Stream and the present experiments ... 27 The biology of the trout of the Ava Stream... 28 Seasons of ascent and descent ... 28 The size of the spawning population... 30 Sex ratio ... 31 Food of the young... 31 Age distribution at the first descent and ascent... 31 Growth ... 33 Results of the tagging experiments ... 39 The method of tagging and the possible effect on the condition of the fish... 39 The results of the tagging ... 42 Migrations in the sea ... 45 Spawning migrations in the stream... 47 Year-classes and survival in different ages... 49 Annual variations in the year-classes... 49 Survival from eggs to adults . ... 51 Causes of differences in survival of different year-classes ... 52 Summary... 53 References ... 55

Introduction

A great many authors have studied the sea-trout (Alm, 1919, 1936, Dahl, 1910, Dixon, 1931, Hessle, 1935, Huitfeldt-Kaas, 1927, Nall, 1930, Poul-

sen, 1934, Rosén, 1918, Segerstråle, 1937, Wendt, 1949, and others).

Data are available regarding various matters, such as the seasonal migrations of the sea-trout in different waters, the average size in different localities, age, growth, etc., as well as of the food of the young fish. On the other hand, there are no particularly comprehensive investigations to be found of some given trout population over a prolonged period, or of the variations in dif­

ferent year-classes and their causative factors. For a number of years the author has been engaged in such investigations of the sea-trout in the Åva Stream, south of Stockholm. Though it has not proved possible to carry out these investigations to the full extent, as originally planned, the results have been thought to justify the present account.

The water system of the A va Stream and the present experiments The Åva Stream has its outlet in Morviken in the parish of Tyresö, about 30 km south of Stockholm, and has a precipitation area of approximately 15 square kilometres. About 2.5 km from the outlet, there was at one time a dam, earlier connected with a mill, and a steep fall of about 4 metres. The breadth of the stream varies between 1 and 2 metres, the depth of the water from a few decimetres up to one metre. In its upper course, below the aforementioned dam (Lake Kvarnsjö), the water is fast moving and the bottom mostly consists of sand, gravel and stones. In its lower part the water flows more slowly and there the bottom is loose. The surrounding shores are partly meadow land and covered with vegetation. The water volume has been estimated at an average of 0.1 m3/sec. In the upper part of the water system, there are several small lakes such as Kvarnsjön and Stensjön, to which reference will be made below. Some of these lakes are regulated. The water as a rule contains some clayey mud from the many ditches of the adjacent fields that discharge immediately below Lake Kvarnsjö. Above this lake, there are forest and moraine grounds, and the water is fairly clear.

Apart from a sparse stock of pike, perch and, in the spring, ide, a popula­

tion of sea-trout is to be found in the Åva Stream. The aforementioned fall has served as an upward boundary to the sea-trout in the stream. Trout have always been captured in the innermost region of Morviken, as well as in the stream itself. The total annual catch has, as a rule, amounted to but a few hundred kg. Some years the catches have approximated to a total of 1.000 kg, i.e. about, 400—500 fish, while, in other years, the total catch has not exceeded some tens of kg.

In 1926, the fishing in the whole water system, including Morviken, was leased to Stockholm’s Sport Fishing Club. From the first, it seemed evident to the Board of the club that some interesting investigations could be carried out here. In cooperation with the author, certain experiments were performed during the year 1926. The Government made some grants from the Fund of Fishing Dues and in the following year, the County Government licensed the building of a permanent fish-trap for the control of ascending and descending fish about 100 metres from the outlet of the stream. By tagging the trapped fish, it should prove possible to estimate the size of the popula­

tion, decimation at different years of age, growth, etc. Finally, it was hoped to obtain material for comparing of the results of normal spawning and artificial cultivation, respectively. A plan was made up by which the ascending trout would some years be left to spawn themselves, while, in other years, the ova would be collected and hatched out in a fish hatchery built by the Club. After that the fry would be planted out. According to the aforemen­

tioned, and a subsequent, special licence from the County Authorities, the Club was further entitled to fish, even during close seasons, and to under-

take experimental transfers of certain numbers of trout to lakes situated higher up. Finally, pike and perch were captured and their ascent to the stream prevented by the trap.

Unfortunately, this interesting scheme could not be entirely carried out.

Occasionally, some fish, while ascending or descending, would escape observa­

tion at the station owing to accidental heavy flows of the stream that would carry the fish past the traps. The seaward migration of young fish also practically eluded notice since, at the time of their descent, high water, mixed with leaves, twigs and other rubbish that could not be cleared away, obstructed the gratings, causing the water to flow over. For several years, especially during the forties, labour was short, and it became impossible to maintain sufficient control, or even to keep the fishing trap in proper repair.

The present observations regarding the ascent of the fish include, for the most part, data of the length and weight of the specimens. Sometimes only the weight has been noted down and, unfortunately, in some instances only the number of fish without any particulars of size. The length has been measured from the tip of the nose to the hindmost edge of the spread-out tail-fin. For the purpose of determining age and growdh, a large quantity of scale material was collected during the years 1927—1935. This has chiefly consisted of samples from specimens in their first spawning migration, numbering altogether 771, viz. 201 males and 570 females. Age and growth analyses from this material are included in tables 4—10. Additional scale samples have been taken from a number of tagged, and recaptured, trout, in order to check the age determinations.

The biology of the trout of the A va Stream Seasons of ascent ancl descent

The trout spawn chiefly in the upper accessible part of the stream. In some years, spawning may take place also in its lower part. Ascending trout find their way into Morviken at the beginning of the autumn, assembling at the mouth of the stream, to await suitable conditions for their ascent, i.e. an abundant flow of water, rain, storm and darkness. As will be seen from table 1, the greatest number, viz. 67 per cent, ascend during the month of November. In some years, the principal ascent will, however, occur in October. In September, as well as in December, the ascent is negligible. The males may ascend somewhat earlier than the females, though no definite difference has been ascertainable, in this respect. Contrary to what happens in big rivers, no ascent will take place in the Åva Stream during other seasons. Nor can any ascent of trout that is not going to spawn be traced in the Åva Stream, apart from rare occasions in connection with the spawning ascents of their fellows.