Mortality risks of Baltic salmon (Salmo salar L.) during downstream migration and the early sea-phase [Elektronisk resurs]: effects of body-size and season

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DURING DOWNSTREAM MIGRATION AND THE EARLY SEA-PHASE EFFECTS OF BODY-SIZE AND SEASON.

by

Torleif Eriksson

**Department of Animal Ecology (*) University of Umeå, S-901 87 UMEÂ, SWEDEN**

**(*) Present address: Department of Aquaculture Swedish University of Agricultural Sciences**

Box 1457, S-901 24 UMEÂ, SWEDEN

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Abstract

Mortality risks in Baltic salmon during early migration was estimated through a sequential release experiment. Effects of time of release and size of fish on survival rate were

studied.

A protected transfer to the sea and an acclimatization prior to release increased the recapture rates by 1.6 to 2.0 times compared to fish released in the river. Furthermore, fish with a delayed release had a 2.8 to 5.0 times higher recapture rate than smolts released in the river.

I found a strong positive correlation between the size of the fish and recapture rates during all three experimental years. Mortality rates peaked during the downstream migration and entry in to the sea. The weekly risk of mortality during the two first weeks was estimated to be 27.8%. Thereafter the mortality risk declined rapidly to 6.1% per week during the following 8-9 weeks. From mid September until the end of November the estimated mortality rate was only 3.5% per week.

Baltic salmon appears to migrate at a sub-optimal size

with respect to survival during migration. A gain in survival

by a larger size during migration,could be obtained by a

prolonged freshwater residency. However this is considered to

be outweighted by the option of an accelerated growth rate in

the sea.

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Introduction

The anadromous Baltic salmon (Salmo salar L.) exhibits two major habitat shifts during its lifecycle. Adult salmon enter the rivers in autumn to spawn. After 1-4 years in freshwater the juveniles leave their riverine environment and migrate to their feeding areas in the central Baltic proper. Compared with the sea, the freshwater environment provides a protected low productive habitat for juvenile salmon (Nikolsky 1963, Thorpe 1982). While in the river, the salmon is mainly a territorial drift feeder. During the early juvenile phase, residency rather than migration may be considered favourable from an energetical point of view. Drifting prey items within an optimal size range are carried to the fish (Northcote

1978). As the young fish grow the amount of suitably sized food items is reduced, leading to a reduced growth rate (Bachman 1982).

Migration to the sea generally leads to a drastically increased growth (Dingle 1980, Elliott 1984). In fish, the advantage of a greater body size includes increased fecundity

(Lagler et al. 1977, Bagenal 1978). Furthermore, egg size increases with increased size of fish (Brännäs et al. 1985, Pope et al. 1961, Sargent et al. 1987), leading to an

increased survival of fry (Bagenal 1969, Elliott 1984). Among male salmonids the situation with respect to size at

reproduction is more complex. Many males reproduce before migration to sea as early maturing males (Lundqvist 1983, Gross 1984, Bohlin et al. 1986). However since dominance is affected by size (Kalleberg 1958, Jenkins 1969), an increased fertilization success is supposed in the large males.

The mortality rate of juvenile salmon during migration is very high (Ricker 1976, Larsson 1984), indicating that seaward migration could be considered as a risky alternative to a prolonged stay in the freshwater. Improved growth from a rich amount of suitably sized food in the sea should be balanced against the elevated mortality risk due to exposure to predators during migration (Thorpe 1984). Mortality of

migrating juvenile salmon seems to be size dependent, and an

inverse relationship with body size is often observed (Carlin

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1969, Mathews & Buckley 1976). Mathews and Buckley (1976) presented a model for natural mortality (inclusive predation) during 18 months of marine life of differently sized coho salmon (Oncorhynchus kisutch). They found an good agreement with the hypothesized inverse weight relationship of

mortality. Ricker (1976), when reviewing growth rate and mortality data in different Pacific salmon species, concluded that the assumption that mortality is inversly proportional to weight, was realistic but unproven.

In this study the mortality risks during early migration were estimated through a sequential release experiment.

Effects of fish size and time of release on non-fishing mortality rate are investigated. The estimates are based on the assumption that estimates of mortality risks can be achieved by protecting smolts and post-smolts from predation for a varying period on the onset of migration.

Material and methods

In each of three years (1980-82) about 9000 two year old

hatchery reared smolts from the Ångerman river stock were used in the experiments. During the winter preceeding the release, the fish used in delayed releases (n^6000) were examined and occurence for early maturing males noted. All the sexually mature males, i.e. precocious male parr, were marked by cutting the adipose fin. In mid-May, all fish were

individually tagged with Carlin tags (Carlin 1955). The fish were measured for fork length to the nearest 0.5 cm and reproductive status was noted (early maturing males vs.

immature fish) on the juveniles used in the delayed releases.

At the time of normal smoltrun in May/June these fish were transferred to netpens in the sea. A control group of smolts

(n=2896-2990) were released in the river at the same time as the other smolts were transferred to the sea. The fish

transferred to the sea were kept in 50m netpens at Ulvön 3

(63°4fN, 18°40fE) in the Bothnian Sea. This site is located

about 30km north of the river mouth. When held in cages, the

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fish were fed at a rate of 2-3% of their body weight per day with a commercial salmon dry food (EWOS).

River/ Bothnian sea

1980 -y*! ^

n: 2990 1992 1989 1750

1981

~ n --- î --- *

n: 2978 1993 1991 1724

1982

n: 2896 1987 1844 1774

“ 1 I I I I I

June July Aug Sept Oct Nov

Time of year

Fig. 1. Release schedule for the sequential release

experiments in 1980-82. Number of fish (n) in each release group indicated.

Fish were released from the netpens on three occasions (n=1724-2990 per occasion) each year (Fig. 1). Prior to the first release in the sea, smolts were acclimatized to the brackish water (4—5*/.• ) for 10-14 days. Two delayed releases were performed after 8 and 15 weeks and after 9 and 17 weeks upon transfer to the sea in 1980 and 1981, respectively (Fig.

1). In 1982 the fish were kept for 15 and 25 weeks

respectively, prior to release. Before release a sample of fish were measured for length and weight from which the size of the fish at time of release was calculated.

Test of proportions was used in testing differences in

per cent recoveries of adult salmon between years. Data were

considered as valid observations if more than ten recoveries

per size class (1 cm) were made.

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Survival was estimated as:

where is survival at time t and survival at time t-x weeks.

Survival rate per week was then given as follows:

where x is number of weeks between estimated survival rates.

Thus the weekly mortality rate will be:

m = 1 - S w w

Recaptures reported from the open sea fishery, the coastal and the riverine fishery were used when estimating survival. The Ångerman river salmon stock is totally dependent on hatchery propagation, and adult spawners are collected in a brood stock fishery. This fishery is supposed to catch the majority of escapement returns. The recapture data from tagged fish were collected in computer files at the Salmon Research Institute, previously described by Carlin (1971). The routine analytical procedures has been given by Larsson (1984). In the Baltic the fishermen’s proneness to report tagged salmon has decreased during late 70fs and early 80*s. During the actual years the frequency of recaptures reported has been estimated to about 40% (Eriksson 1982). Therefore, the survival rates was adjusted (to a higher value) with a factor 2.5 when calculating natural mortality.

In the same research program analyses on migratory distance (Eriksson, in prep.) and on flexibility in life- history tactics (Eriksson et. al., 1987) were made.

Results

During the three years of experiments, a transfer of fish to

the sea and an acclimatization approximately two weeks prior

to release increased the recapture rate from 1.6 to 2.0 times

compared to the control group released in the river (Fig 2).

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1982

ö

may jun aug sept may jun aug sept may jun sept dec

Release in: River Bothnian Sea River Botnian S ea River Bothnian Sea

Fig. 2. Recapture rates reported (left scale) and recapture rates correlated for reporting rate 40% (right scale) in release experiments in 1980-82. In each year fish were

released (n=1700-3000 per release) in the river (end of May) and in the sea (0.5-6 month’s delay) after transfer to cages at normal time of smoltrun.

Furthermore, fish released at a delay of 2 to 6 months, showed a 2.8 to 5.0 times higher rate of recapture than did smolts released in the river (Fig 2). The recapture rates reported among the fish released in the river varied between 4.2% and 7.9%, and in all three years there was a significant

difference (p<0.05) between smolts released in the river and fish transferred to cages and kept in the sea for 10-14 days before release. Similarly there were significantly higher recapture rates (p<0.05) obtained for each release group after a longer delay period prior to release compared with the

previously released group (Fig. 2).

Within each group released into the sea and in all three

years a positive correlation between body length at release

and recapture rate was found (Tab. 1). The correlation

coefficient ranged between 0.58 and 0.99.

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Table 1. Correlation coefficient between length at release and recapture rate within each group released during the

experiments 1980-82.

Release year 1980 1981 1982

Release site River Sea River Sea River Sea

Release month 05 06 08 09 05 06 08 09 05 06 09 12

Correi, coeff 0.98 0.63 0.99 0.97 0.86 0.93 0.91 0.64 0.58 0.95 0.92 0.92

A:

50-1

4 0 -

3 0 -

a >

CO

3

a

CO

o

CC

-1 8 .4 6 + .19* X 1 0-

0.8 8 p <0.001

140 190 240 290 340

Size at release (mm)

Fig. 3. Per cent recapture of tagged Baltic salmon in rela

tion to size at release when released from sea cages at Ulvön in A: 1980, B: 1981 and C: 1982. Regression lines indi

cated. Each observation con

sist of 10 recaptured fish.

30-1

2 5 -

2 0-

< i )

CO

<D

I. 10 -

co

o

-1 3 .8 2 + .1 3 « x

r = 0.82 p <0.001 CC

310

160

210 260

C:

3 0 -

5

25 -

<d

3 20 -

_a

co

o

CD

CC 15 - y = -2 .6 7 + .0 9 * X

r = 0.78 p<0.001 10

390 340 240 290

140 190

The correlation between recapture rate and size at

release into the sea of all recaptures was estimated each year

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(Fig. 3a-c). The correlation coefficient was 0.88, 0.82 and 0.78 for 1980, 1981 and 1982, respectively. For all three years the correlation coefficients were highly significant

(pcO.OOl). The effect on recapture rate by size at release expressed by the regression slopes ranged between 0.9 to 1.8 % per cm.

0.30-1

0.25-

£

^

0

.

20

-

<D

<D 5

^

0

.

15

-

w

‘i .

'a0.10- +-*

2 o 0

.

05

-

0

J i--- 1--- 1--- 1--- 1--- 1--- 1--- 1

May June July Aug Sept Oct Nov Time of year

Fig. 4. Mean mortality rates per week during summer/autumn for Ångerman river smolts released 1980-82.

The calculated mortality rate (Fig. 4) peaked during the

downstream migration and entry in to the sea. The weekly risk

of mortality during the two first weeks after release was

estimated to 27.8%. After these first critical weeks the

mortality risk declined rapidly, and during the following 8-9

weeks it was 6.1% per week. The mortality rate decreased

further during the summer-autumn and from mid September until

the end of November it was only 3.5% per week (Fig. 4).

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Discussion

The mortality curve for Baltic salmon smolts and postsmolts obtained in this study emphasizes the high mortality risk during downstream migration and during the first weeks in the sea. Mortality seems to decrease rapidly during the first summer in the sea. This indicates that the natural (non

fishing) mortality of Baltic salmon mainly takes place up to the end of the first autumn of seaward migration. A high mortality rate during the first year in the sea has earlier been assumed (Carlin 1969, Larsson 1984). However, no

estimates of the dynamics of the natural mortality during the first year in sea have been possible to make.

In Baltic salmon a positive relationship between smolt size and survival has been reported by several authors (e.g.

Carlin 1969, Österdahl 1964, Lundqvist et al. 1988). However this study clearly points out that this relationship between size and survival is acting on the fish from the time of release in the river, through summer and autumn. According to this study postsmolts that have reached a size of about 40 cm seem to face a reduced natural mortality risk.

Maximizing body size should be important in reducing mortality when entering the sea. Wild Baltic salmon smolts also show a size related survival (Österdahl 1964). Still the size of Baltic salmon smolts at time of migration range between 12-18 cm (Österdahl 1964,1969). This implies that strong selective forces must act against a prolonged freshwater residency counteracting the advantage of an increased size before migration. Considering the size distribution and the seasonal abundance of food organisms available in Scandinavian rivers, the growth rate of juvenile salmonids can be expected to decrease markedly at a fish length of about 15cm. In Baltic salmon the gain in migration survival due to a larger size because of a prolonged

freshwater residency should be outweighed by an accelerated growth in the sea. Since food productivity varies both

seasonally and geographically, the best option between these

two forces leading to maximimal reproductive sucess should

vary.

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The consistent positive correlation between survival rate and size at release for all release groups strongly supports the importance of a seasonal timing of entrance in to the sea.

Migrating juveniles have to cope with a seasonal variation in encountered conditions. A seasonal migration synchronized to optimal food availability in the sea should be of selective importance (Eriksson, in prep). Salmon smolt migration during night (Thorpe and Morgan 1978, Österdahl 1969) and at high flows (Solomon 1982) are also assumed to enhance survival.

Contrary to the results presented here, delayed release experiments in rivers have resulted in high mortality rates despite large size prior to release (Peterson 1973, Larsson 1979). This discrepancy could be explained by the release of behaviourally non-migrants in rivers. If so, this difference may emphasize the effects of a circannually governed migratory period, expressed as a smoltification-desmoltification cycle

(Lundqvist and Eriksson 1985, Eriksson et al. 1987).

As previously demonstrated (Eriksson et al. 1987), the survival of early maturing males differs between males

released into the river and those transferred to the brackish water in the Bothnian Sea before release. Early matured males released into the river exhibit a low survival rate to large size compared with immature fish (Lundqvist et. al 1988). On the other hand early matured males released into the sea show survival rates similar to immatures of the same size (Eriksson et al. 1987). The low survival rate of early matured males released into the river is assumed to result from impaired migration due to incomplete smoltification (Lundqvist et al.

1988). Accordingly, the high mortality observed in fish

released in to the river may partly depend on a difference in

performance between non-river and river released males. If all

previously matured males die before leaving the river, the

mortality rate per week during the first two weeks (when

leaving the river and entering the coastal area) will be

reduced for the immature fish from 0.28 to about 0.23. Thus

even if this reduced mortality is taken into consideration,

the general pattern with extremely high risk of mortality

during the first two weeks of the migratory period remains

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unchanged. This indicates that the figures obtained in this study can be regarded as valid estimates.

Acknowledgements

I thank Prof. L-0 Eriksson, Dr. H Lundqvist and Prof C Otto for constructive criticism on earlier drafts of this

manuscript and to B Bjurström for revision of the English.

This study was funded by the County Council of Väster

norrland, the Swedish Council for Forestry and Agricultural

Research and the Swedish Natural Science Research Council. The

Salmon Research Institute covered tagging costs.

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