A telemetry study for
reintroducing wild Atlantic salmon
(Salmo salar L.) in the Daugava
and Ogre Rivers, Latvia.
En telemetristudie för återinförandet av vild Atlantlax (Salmo salar L.) till
Daugava och biflödet Ogre i Lettland.
Oscar Askling
Faculty of Health, Science and Technology Subject: Master’s thesis in Limnology Points: 30 hp
Supervisor: Olle Calles & John Piccolo Examiner: Larry Greenberg
Date: HT 14
Abstract
Hydropower facilities are obstacles in the rivers, which mostly are impassable for fish. The loss of longitudinally connectivity in many rivers prevents salmon fulfilling their reproductive cycle, leading to worldwide reduction of the Atlantic salmon population. Three hydropower stations today exploit the river Daugava, Latvia. This has resulted in extirpation of the native Atlantic salmon population. To investigate the possibilities of reintroducing salmon, a telemetry study with trap- and
transport-approach was carried out. Eighteen Atlantic salmon were caught near the river mouth of River Daugava and tagged. Two groups were transported past the most downstream obstacle in River Ogre to two different release sites, R1 and R2 (n = 8 and 6), and a third group, R3 (n = 4) group and was released in Riga reservoir, above Riga HES, in River Daugava. All salmon released in River Ogre migrated
downstream, eventually ending up in River Daugava. In total seven salmon passed downstream through the turbines of Riga HES, whereof the recapture success downstream Riga HES was 71%, and the survival rate of turbine passage was at least 60%. This indicates there is a high fishing pressure on salmon in Daugava. The majority in the first group (R1), passed through the turbines of Riga HES, likely due to the high discharge event. However, none in the second group (R2) did. Half of the fish in the third group (R3) released in River Daugava either went downstream passing the turbines of Riga HES or moved upstream standing below the next hydropower station Kegums HES. Six individuals remain missing, presumably because of poaching and/or lack of feedback reporting. After passing downstream from the River Ogre to the Daugava River, about 13% and 17% of groups R1 and R2, respectively, migrated upstream to reach Kegums HES, the furthest possible migration point in River Daugava.
The results from this study give valuable information of the possibilities of reintroducing the wild Atlantic salmon to Daugava. Fishing pressure might be a decisive problem for the reintroduction of Atlantic salmon in the Daugava system. Since all the fish released in Ogre went downstream to Daugava, probable due to flooding, the main objectives of this study were impaired and remain to be investigated to determine if the salmon could find the suitable habitats upstream in River Ogre. Sammanfattning – Abstract in Swedish
Vattenkraft utgör vandringshinder för fisk i vattendragen. Förlusten av konnektiviteten i vattnet har lett till minskning bland laxpopulationer. Vattenkraften i floden Daugava, Lettland, hindrar helt vandrande fiskarter från att nå reproduktionsområden, som har resulterat i att den vilda populationen av Atlantlax (Salmo salar L.) försvunnit. I biflödet Ogre finns lekhabitat för lax. I denna telemetristudie undersöks huruvida en fångst- och transport-lösning skulle kunna återetablera en vild laxpopulation i Daugava. Studien utfördes på lax som fångats i Daugavas mynning och därefter transporterats förbi och delvis släppts ut uppströms Riga kraftstation samt delvis uppströms Ogre kraftstation i biflödet Ogre. Totalt fångades 18 laxar av odlat ursprung i Daugavas mynning. Individerna radiomärktes och sattes sedan ut vid Ogre (n = 8 + 6) och i dammen uppströms Riga kraftstation (n = 4) i huvudfåran Daugava. Samtliga fiskar som släpptes ut i Ogre vandrade nedströms till Daugava. Totalt sju laxar passerade turbinerna vid Riga HES. Återfångsten av de fiskar som passerat Riga HES genom turbinerna var 71 % och
överlevnaden var 60 %. Den höga återfångsten tyder på ett extremt högt fisketryck på lax i Daugava. Majoriteten av de fiskar som sattes ut vid första tillfället (gruppen R1) i Ogre passerade nedströms genom turbinerna i Riga HES, vilket sammanföll i tiden med att det inträffade ett hundraårsflöde i Ogre. Ingen av individerna från den andra utsättningen (R2) passerade turbinerna i Riga HES. Hälften av kontrollgruppen som sattes ut i Daugava passerade via turbinerna vid Riga HES, medan den andra hälften vandrande uppströms mot nästa kraftverk (Kegums HES). Dessutom försvann sex individer spårlöst, vilket sannolikt beror på tjuvfiske och/eller utebliven återrapportering. Av fiskarna som sattes ut i Ogre vandrade 13 % respektive 17 % av fisken från den första respektive andra utsättningen uppströms i Daugava mot Kegums HES.
Resultaten i denna studie ger viktig information om möjligheterna till att återetablera en vild laxpopulation i Daugava. Det höga fisketrycket är sannolikt ett avgörande problem för en fullskalig återintroduktion av lax i Daugava. Eftersom resultaten troligen var påverkade av översvämningen återstår det undersöka om laxen kan hitta habitaten i biflödet Ogre.
3 Introduction
The Atlantic salmon is currently red listed by the IUCN organization and is classified as “least
concern”, which means there is a low risk for extirpation of the species (IUCN 2014). The Baltic stocks of Atlantic salmon are genetically unique (Karlsson & Karlström 1994), having been isolated from North Atlantic stocks for about two thousands years (Lundqvist 1965). Although once considered among the world’s most productive salmon regions with high ocean survival, the vast majority of Baltic stocks have seen drastic declines in production, in part due to hydropower development. Many wild Baltic salmon populations have gone extinct (HELCOM 2007). Since the 1990s, the total salmon harvest in the Baltic Sea has decreased from 5000 to 1000 tonnes per year (HELCOM 2011).
Furthermore, Palmé et al. (2012) expressed concerns that the genetic diversity of Baltic salmon is being negatively affected by compensatory releases of hatchery fish.
Compensatory releases of hatchery-reared salmon have been used for over 100 years to mitigate for the effects of hydropower development (Thorpe 1998). Dams are barriers that disrupt longitudinal
connectivity and change the assemblages of the fish fauna (Larinier 2001; Rosenberg et al. 1997). Dams and hydropower plants that prevent passage have a fundamental effect on migrating fish populations (Calles & Greenberg 2007, 2009). Upstream migrating fish are prevented from completing their life-cycle since they cannot reach spawning and rearing areas. Downstream migrating fish are often injured or killed when passing through turbines and/or get stuck on trash racks (Calles et al. 2010), and losses can be substantial in rivers where individuals have to pass multiple plants on their way to the sea (Calles & Greenberg 2009; Norrgård et al. 2012). These impacts have contributed to widespread reductions in fish populations (Northcote 1998), and the consequences of hydropower development in lotic ecosystems are related to decreased biodiversity and losses of ecosystem services (HAV 2013a; Rosenberg et al. 1997). Atlantic salmon reproduction depends on intact river connectivity and spawning habitats, they become highly susceptible to these anthropogenic consequences.
Rehabilitation measures such as fish traps, -lifts, -passes or nature-like fishways have been built at many dams, but many still lack such passage solutions (Gough et al. 2012; HAV 2013b). Fish passes make impassable obstacles in a river passable, although in some cases only partly passable though.
Obstacles, for instance a hydroelectric station (hereby called HES), often result in low or no fish
passage efficiency and delayed migration rate for the Atlantic salmon (Chanseau & Larinier 1999; Gowans et al. 1999; Larinier 2008). Delayed migration increases the overwinter mortality, which leads to additional energetic costs (Bardonnet & Baglinière 2000; Lundqvist et al. 2008). Even in rivers with passage, the cumulative effect of multiple obstacles may result in a low proportion of migrating fish successfully reaching spawning areas (Baisez et al. 2011; Gowans et al. 2003). Sometimes a trap- and transport-solution may be the most appropriate approach for introducing or maintaining migrating fish population. It is often used as a temporary solution in restoration programs, when quick actions are necessary, or in rivers with many obstacles, which makes construction of fish passes expensive and sometimes impossible due to high dams (FAO 2001). For example the wild landlocked Atlantic salmon population in Klarälven, Sweden, has been maintained by smolt stocking and a trap- and transport-solution for wild fish for over 50 years (Piccolo et al. 2012). In the Klarälven River adult salmon are trapped at the first HES and transported by truck to be released upstream of eight dams. Here the salmon can continue migrating upstream to reach spawning areas (Hagelin et al. 2015). The wild fish is separated from farmed fish by adipose fin clipping since 1993, and together with fishing restrictions, the population has shown a positive trend, which is believed to be related to rehabilitation measures (Bergman et al. 2014).
Differences between wild and hatchery-reared salmon are both phenotypic and genetic (Jonsson & Jonsson 2006). Hatchery salmon tend to ascend the spawning river later than wild salmon, and they also differ from wild fish in the degree of homing precision and in migration and spawning behavior
(Jonsson & Jonsson 2006). Wild salmon tend to return to their natal sites where they are born and raised
However, one study showed that hatchery salmon spawners migrated to areas more than 25 km from the release site, to reach spawning areas used by wild conspecifics (Dittman et al. 2010). Hatchery-reared salmon homing precision is sometimes less accurate and can differ from that of wild salmon. This could be the result of genetic divergence, a long-term effect of broodstock selection, which impairs natural selection (Jonsson & Jonsson 2006). Sometimes a small percentage of returning adult salmon returns to rivers or areas other than where they hatched. This phenomenon is called “straying” and is considered to be important in the evolution of the salmonids, allowing colonization opportunities to new habitats (Quinn 1993). Studies show that hatchery-reared salmonids stray more frequently than do wild salmon, probably depending on genetic structures (Jonsson et al. 2003; Labelle 1992; Leunda et al. 2013; McIsaac 1990). However straying behaviour, which probably occurs in all salmon populations, can vary among the population and/or even annually (Jonsson & Jonsson 2011).
Homing behaviour of spawning salmon is well documented, but still poorly understood. Research suggests that it is related to hormonal activity and cues from the surrounding habitats such as: light, temperature, currents, olfaction, and electrical and/or magnetic stimuli (Putman et al. 2013). These may have been imprinted in the smolt from the natal redd (Jonsson & Jonsson 2011; Ueda 2011). It is widely accepted that returning adult salmon recognize odors (Dittman & Quinn 1996), allowing them to find their natal waters by olfactory discrimination and attraction. The olfaction system depends therefore on imprinting processes from the natal water in the early life-stages. However, thyroxine hormone levels may be influenced by external environmental factors such water flow, temperature, photoperiod and even lunar phase (Dittman & Quinn 1996). When the adult salmon eventually return to the river, the primary factors controlling the migration timing and rates are water flow and temperature (Banks 1969). Hatchery operators have long used the salmon’s homing ability to ensure that spawners return to the release area so that eggs may be collected; although hatchery fish share broadly similar migrations patterns with their wild counterparts (Jonsson & Jonsson 2006), important differences in behaviour and survival between hatchery and wild fish have recently received much attention (Rand et al. 2012). The Daugava River had a historically important fisheries, which was an essential food source for the Latvian people. Catches declined in part due to hydropower development from the 1930s-1970s, when the three largest hydropower plants Kegums HES, Plavinas HES and Rigas HES were built. Historical data show that salmon were caught upstream of the present HES in the River Daugava near Belarus, During the 1950s and 1960s up to 100 tons of salmon were caught annually in the lower Daugava (Malikova 1966). With the construction of the Riga HES close to the river mouth however, natural reproduction of Atlantic salmon among other fish species was impaired, leading to overall decimation of migrating fish populations. By 2012 the total catch of salmon in the Rivers Daugava and Venta was six tonnes of broodstock (ICES 2013).
The HESs in the Daugava River today are impassable for migrating fish. Only the Kegums HES has a fish pass, but this has not been used since the Riga HES was built downstream of it. Atlantic salmon currently do not reproduce naturally upstream of the Riga HES. The existence of a remnant wild population remains unknown, since there is no marking program to differentiate between wild and hatchery salmon. According to HELCOM assessment (2011), the salmon population in river Daugava consists mainly of hatchery-reared fish. Since the construction of the Kegums HES, suitable spawning areas for Atlantic salmon has been surveyed in the tributary Ogre (Evtyukhova 1971). The Ogre River seems to fulfil the morphological parameters and meets the criteria for salmon spawning. The Ogre is also exploited for hydropower, but upstream the Ogre HES multiple suitable habitats for salmon have been identified and therefore possibilities for diadromous reproduction (Birzaks 2013).
The aim of this study is to test if salmon trapped at the mouth of the Daugava will migrate upstream if released in the river Ogre and possibly recognize a trap- and transport-approach as a suitable strategy for reintroducing wild-spawning Atlantic salmon to the Daugava River system, Latvia. The project will serve as a first step towards a restoration and conservation program for wild Atlantic salmon (Comoglio et al. 2013). To evaluate the trap-and-transport approach, adult salmon returning to the mouth of the Daugava were radio-tagged and transported upstream to the Ogre River, and their migration behaviour
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was monitored. The study design comprises of tagging and releasing three groups of ripe Atlantic salmon, which are transported above the dam of Riga HES. Two groups are released in the tributary Ogre River, and the third group is released in the Riga HES reservoir in Daugava River. The purpose of the third group is to see if they move upstream and if they are to find the mouth of Ogre River. The objectives were to determine 1) if the fish would migrate upstream to suitable spawning areas, 2) if there is a difference in migration behaviour (speed, holding times) between groups released at different dates, and between sexes and sizes, and finally 3) if upstream migration does not occur, will
downstream passage through both Ogre HES and Riga HES be successful, and 4) how released salmon behave in the mainstem Daugava River.
Materials and Methods Study areas
Daugava River
Daugava River is the largest river in Latvia and originates from the Valdaja plateau. It flows 1020 km through Russia, Belarus and finally empties into the Gulf of Riga, in the Baltic Sea (57°3′42″N,
24°1′50″E)(Fig. 1). Within Latvia, the Daugava River flows 357 km before reaching the Gulf. The river basin is 24 700 km2, including approximately 40 tributaries in Latvia (Birzaks 2013). The annual mean
discharge in Daugava River is 727 m3s-1 and near Riga the river is approximately 700-m wide and 8-9 m deep (Municipal Portal of Riga 2014; IWAC 2014). The two major tributaries within the Latvian territory are the River Ogre and Aiviekste (Hogan 2012), situated about 50 and 120 km from the river mouth, respectively. Two ecoregions, mixed forests and agricultural land dominate Daugava’s
watershed. A smaller portion of the watershed in Russia consists of Scandinavian and Russian taiga. The low flatland wherein Daugava flows consists mainly of glacio-lacustrine sand (Hogan 2012). There are three major hydropower plants with dams in Daugava: Riga HES, Kegums HES and Plavinas HES, all owned by Latvenergo AS. Riga HES is situated around 30 km from the Gulf of Riga and the head is 18.0-18.8 meters. The HES is equipped with six Kaplan turbines with an intake capacity of 3 345-3 470 m3s-1 depending on the head. The total installed capacity of Riga HES is 402 MW. The Riga reservoir is
15.4 km long (Latvenergo 2014). Kegums HES consists of two powerhouses, one on each side of the river, with spill gates located in between them. Together they comprise seven sets of Kaplan turbines, with the normal intake capacity of 2 150 m3s-1 at the average head of 32 m (Latvenergo 2014). The
discharge intake data for Riga HES and Kegums HES was collected from Latvenergo AS. No spill occurred from either of the two HES’s during the study period.
Ogre River
The Ogre River (56°48′39″N, 24°36′6″E) is the largest tributary within the reach between the first (Riga HES) and the last (Plavinas HES) dams in Daugava River (Fig. 1). The annual mean discharge at the river mouth is approximately 18 m3s-1 (range: 2 – 177 m3s-1). The Ogre River basin is 1 730 km2 and the river meanders 188 km from the Latvian boarder northeast of the Daugava River. It has mainly sandy riverbeds and the local environment is partly mixed forests and agricultural land. A total of 223 sites of potential migratory fish habitats with hard riverbed substrate were reported in BIORs surveys (Birzaks 2014). The hard riverbed substrate consists of dolomite, boulders, pebbles and gravel and these habitats comprise a total area of 29.6 ha in fast-flowing water sections. The survey indicates that the Ogre River has potential
spawning areas for Atlantic salmon. The oxygen content in water generally exceeds 8 mg/l, but temperatures may reach 24 °C in summer periods. There are three small HES on the Ogre with the first, Ogre HES, located 6 km upstream the river mouth. The next HES is located 85 km upstream. Ogre HES is a small power station with an installed capacity of 630 kW.
F igu re 1. Ma p of R ive r D auga va a nd tr ibut ar y O gr e, inc ludi ng st udy ar ea a nd se tu p in L at vi a.
7 Abiotics
Abiotic variables were measured by Latvenergo AS personnel 24 October, upstream Ogre HES in the Ogre River. The recorded values (± standard deviation) were: pH = 7.78 ± 0.09, phosphorus (Pk) = 0.070 ± 0.006
(mg l-1), and nitrogen (Nk) = 2.21 ± 0.30 (mg l-1), ammonia (N-NH4) = <0.09 (mg l-1), chemical oxygen
demand (COD) = 68.5 ± 5 (mg l-1), dissolved oxygen (DO) = 11.9 (mg l-1), and turbidity = 29.8 ± 1.8 (NTU). During the study, heavy rain led to a flood event with high discharge and turbid water. Due to the high water flow, the spill gates of the Ogre dam were open for a long period, making the initially impassable obstacle passable at least for downstream-migrating fish. Water level data for both rivers were supplied by Latvenergo AS or obtained from a gauge station in Ogre River. Temperature was measured with temperature loggers (model: HOBOwater temp pro, V2; Onset, Bourne, MA, U.S.A.), which were placed in each river attached to iron bars, near R1 in the Ogre River and hanging from the weir at Riga HES in the Daugava River.
A visual survey of the suggested potential spawning habitats along the Ogre River was carried out, starting around 70 km upstream the confluence with River Daugava. A total of 27 locations and observations along the Ogre River were made to validate the spawning habitats previously investigated by BIOR
(Fig. 2)(Birzaks 2013).
Figure 2. The left picture shows a fast-flowing section in the Ogre River and the picture to the
right shows a site in which the riverbed is mixed with gravel, pebble and cobble. This is one site that was included in the BIOR survey for salmonid spawning areas. Water levels were high when the
photos were taken. PHOTO: Oscar Askling, 15-11-2014. Tagging programme
The study period occurred from 10 October to 19 November 2014. Atlantic salmon were caught with fyke-nets by local fishermen in the Gulf of Riga in the Baltic Sea, near the Daugava River mouth in early October. The captured fish were most likely of hatchery origin and were released as smolts in the lowermost parts of the River Daugava after which they spent some years in the Baltic Sea before returning to the river to spawn. This method is commonly referred to as “ocean ranching”. The salmon were tagged and released in three groups: R1) 1 km upstream Ogre HES in the Ogre River, R2) 20 km upstream of the Ogre HES in the Ogre River near the Lobe tributary, and R3) in the Riga HES reservoir in Daugava River (Table. 1, Fig. 1). The purpose of the third group was to see if there is any propensity for upstream migration in the main stem and also if the salmon could find and be attracted to enter the mouth of the Ogre River. The groups of R1 and R3 were tagged and released on 14 October and the R2 group was tagged and released on 17 October. The different tagging and release dates were due to limited availability of study fish.
Fish were held in a cage inside the River Daugava (maximum five days), before they were brought to the River Ogre for tagging. The fish were transported to the tagging station by car in a 0.8 m3 tank filled with well-oxygenated river water that was continuously supplied with oxygen. The tagging was
performed in a tent outdoors, near R1 in Ogre. The salmon were initially anesthetized in an aereated 50 L tub mixed with water and benzocaine in alcohol (75%) solution (0.5 g / 10 ml, per 10 L water). When anesthetized (average anaesthesia time: 3 min and 10 s ± 3 s), the salmon were surgically tagged with body implant transmitters (model F1835 w/ antenna, 14 g; Advanced Telemetry Systems (ATS), Isanti, MN, U.S.A.). The incision was made ventrally and longitudinally (Jepsen et al. 2002) between the pectoral fins and the pelvic fins, and approximately 3 cm long. The transmitter was inserted inside the abdominal cavity of the salmon together with a PIT-tag (PIT system Oregon RFID, Portland, USA; 23 mm HDX tags, 0.6 g) to allow identification of the fish in case the radio-tag would cease to function. The transmitter’s antenna was led through the body wall on the side of the fish, beneath the lateral line, a couple of centimetres from the incision with a needle. The incision was closed using a dissolvable suture with three separate stitches (Vicryl V391H, FS-2 45 cm, Ethicon Inc., Cornelia, GA, U.S.A.). The tagged salmon was then held under water in the river until it had recovered and could manage to swim against the current. In the second tagging session, where the release site was further away from the tagging station, the tagged fish where allowed to recover in the tank on the car before released in the river. A total of seventeen salmon, with an average weight of around 4 kg, were tagged (Table 1). One individual from group R3 was recaptured soon after release and was reused in group R2 to provide more data.
Table 1. Groups of tagged fish (n) with release info, mean length ± SD, mean weight ±SD, and sex ratio. The grey shaded row shows the total.
Group (n) Release site and date
Mean Length (± 1 SD)
Mean Weight
(± 1 SD) Male/Female
R1, Ogre 1 (n = 8) 1 km upstream Ogre Dam, 14th Oct. 67 (± 1) 4 (± 1) 5/3 R2, Ogre 2 (n = 6) 20 km upstream Ogre Dam, 17th Oct. 68 (± 6) 5 (± 2) 3/3 R3, Daugava (n = 4) Riga HES Reservoir, 14th Oct. 59 (± 2) 3 (± 0) 4/0
Total (n = 18) - 66 (± 3) 4 (± 1) 12/6
Tracking
The radio-tagged fish were monitored by two automatic logger stations (ATS; R4500S automatic receiver, Isanti, MN, U.S.A.). The first automatic logger station (hereafter called Automatic Logger Station 1, ALS1), was placed 1 km upstream the Ogre HES dam, 7 km from the river mouth, using two series of 6-el Yagi-antennas facing upstream respectively downstream. The station was situated in a hill area, mounted in trees, giving a detection range of up to 1 km. The antenna facing downstream could detect fish at the Ogre weir. When the coverage of each station was tested using transmitters mounted below a float, some blind spots were recorded close to the weir. The detection range of the antenna facing downstream had complete coverage to approximately 150-200 m upstream of the weir. The second automatic logger station was placed on the Riga HES dam (Automatic Logger Station 2, ALS2), connected to an omnidirectional antenna mounted on the roof of a shed. The signal map that was produced showed that the ALS2 had a detection radius of approximately 1 km. The only exception was when a transmitter was present close to the spill gates, at which the signal strength was greatly reduced or even disappeared. Both stations were connected to mainline electricity, with 12 V batteries connected to maintenance chargers acting as backup power supplies. Only frequencies that were used on the tagged fish were inserted and the ALS scan rate was set to seven seconds. Also the transmitters had a function that transmitted a half pulse rate if movement had stopped for a longer period.
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In addition to the stationary loggers, manual tracking along the Ogre and Daugava Rivers was performed. Manual tracking was done using ATS manual receivers models: R410 and R2000 (Isanti, MN, U.S.A.) with 4-el Yagi antennas and an omnidirectional antenna (car antennae). The fish were positioned as precisely as possible, with the case-specific precision noted as within 10 m, between 10 – 100 m, or more than 100 m. Along the Ogre River, tracking was mostly carried out by foot along the sides of the river between the river mouth and the logger station ALS1. Tracking along the river stretch between ALS1 and the second release site (R2), 20 km upstream, was done by car. Tracking on the Daugava River was mainly performed from a car, with either a 4-el YAGI antenna or a car antenna. When tracking by car the detection range was restricted by the availability of roads, and so the tracking efficiency was reduced in some places. Boat tracking was only conducted on the Daugava River, moving from Riga reservoir upstream to the Kegums HES. While boat tracking, one or both of the car antenna and the 4-el Yagi antennas were used. The boat tracking in the Riga reservoir most likely had a high uncertainty in tracking efficiency due to the large area and deep water. Thus, the reservoir is not considered as a monitored part of the Daugava River, except the area within the detection range of the ALS2, in front of Riga HES. The tracking sessions were mainly executed during daytime and
occasionally after dusk.
A total of 20 tracking sessions was conducted, whereof five were executed by boat. Analysis
The data set consists of time-stamped positions for the fish in the three groups. The parameters analysed were: direction (i.e. analysing detection time to determine direction of movement); time to passage (i.e. time elapsed from release to make complete passage at logger stations) and, thereby speed; differences in time between sexes and sizes; holding time between groups and sexes (i.e., time individuals were tracked upon multiple times until they moved); passage success and mortality through turbines at Riga HES. Statistical programs used were Microsoft Excel and StatPlus from Analyst Soft (statistical plug-in program to Microsoft Excel for Apple Macintosh computer).
Results
Visual observation of potential spawning habitats in River Ogre
The observations made in the River Ogre confirmed the potential spawning places for salmonids, accordingly to the literature (Armstrong et al. 2003; Louhi et al. 2008)(Fig. 2).
Tracking
The manual tracking frequency, i.e. how many times a tagged fish was detected by manual tracking during the whole period, was highly variable: R1, Ogre site 1 (median 3, range 1-8 times), R2, Ogre site 2 (median 4, range 0-16 times) and R3, (median 2, range 0-9 times). One individual from group R2 was never tracked manually after release, but was recorded when passing ALS1.
Ogre River – Passage of ALS1
All fourteen fish released in the Ogre River were logged at the ALS1, and logger data complimented by manual tracking records, show that all individuals passed the Ogre HES, heading downstream towards the Daugava River (Fig. 3). None of them migrated upstream towards potential spawning areas in the Ogre. The R1 group passed ALS1 soon after release, and since they were released near the station the distance they had to travel to reach the Ogre HES was short (median 0.63 h, range 0.22-1.45 h; speed median 0.63 km h-1, range 0.22-1.45 km h-1). The group R2 varied more in time from release to
reaching the ALS1 20 km downstream (median 72.35 h, range 11.72 – 157.93 h; speed median 0.28 km h-1, range 0.13 – 1.71 km h-1). There was no difference between sexes in time after release to reach the ALS1 in the R2 group (Mann-Whitney, p > 0.05). Nor was there significant correlation between fish length and time from release to passage at ALS1 in R1 or R2 (Spearman’s ranks correlation: R1, r = -0.26, p = 0.54; R2, r = -0.09, p = 0.87). One fish in the group R2 was reported dead after passing the Ogre HES, but the exact passage route was not known. All passages occurred during the period with high water levels in the middle of October (Fig. 4).
Figure 3. The movements of tagged salmon in the Rivers Ogre and Daugava. The black columns represent complete passages of ALS1 in the Ogre River, the grey columns represent complete passages at ALS2 in the Daugava River, and the white columns represent individuals that moved upstream in the River Daugava. Eight fish were tracked on the same site multiple times, displaying holding behaviour (median 83.24 h, range 19.23 – 168.10). Individuals in all groups displayed this behaviour, although only one in R3 did so in the Daugava River (Tab. 2). There was no significant difference in holding time between groups (Kruskal-Wallis test, H = 0.86 (2, N = 18), p > 0.05). However, there was a significant difference in holding time between sexes, with females holding for a longer period than males (Mann-Whitney, U8 = 15, p < 0.05).
Figure 4. Passages of Ogre dam (ALS1) related to water level and temperature in the Ogre River. The cross-symbols indicate how many fish that passed the dam.
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Table 2. Holding time (h) and location for each individual salmon (S) that was tracked on the same site for a period of time, but that eventually left the holding location.
Individuals Group Sex
Length (cm) Weight (kg) Holding time (h) Location
S1 R1 Male 61 3.2 54.9 Ogre HES
intake channel
S4 R1 Female 77 6.3 168.1 Near Ogre
river mouth
S5 R1 Male 58 2.5 72.4 Ogre HES
intake channel
S7 R1 Female 74 5.5 123.2 Bend below
Ogre HES
S12 R3 Male 55 2.3 70.4 Kegums HES Below
S13 R2 Male 53 2.1 94.1 Ogre HES
intake channel
S16 R2 Female 79 8.1 96.9 Ogre HES
intake channel
S18 R2 Male 61 2.9 19.2 Ogre HES
intake channel
Daugava River – Passage at ALS2 and upstream migration
Out of the four fish tagged and released in Riga reservoir in the Daugava River, none of them was ever sighted in the Ogre River. Two out of four (50%) went downstream through the turbines, and were logged by ALS2 at the Riga HES, about 24 h after release (Fig. 3). Local fishermen reported one recaptured the morning after and it was collected and released as a part of group R2. No obvious injury was observed. The remaining two (50%) fish in the group R3 were later tracked near the Kegums HES about 10 km upstream.
Downstream migrating fish from groups R1 and R2 eventually ended up in the Daugava River, which means that all fish released in the Ogre River swam downstream. However, only fish from R1 (the first release near the Ogre HES) were recorded passing the Riga HES, whereas no fish from R2 (the second release 20 km upstream the Ogre HES) passed downstream through the turbines at the Riga HES (Fig. 3). The downstream passage events at the Ogre HES for fish from group R2 occurred during the peak discharge at Riga HES (Fig. 5). In total, seven (R1 + R3) out of eighteen fish (39%) eventually went through the turbines at Riga HES (Fig. 6), although only five (36%; from R1) out of fourteen fish released in the Ogre River. The distribution between the sexes of the fish that passed Riga HES, they were six males (86%) and one (14%) female.
Five (≈ 70%) out of seven fish that went through the turbines at Riga HES were reported recaptured by fishermen, whereof three (60%) were still alive (Fig. 6). The cause of death of the remaining two recaptured fish could not be determined.
Four (22%; one from R1, one from R2, and two from group R3) out of eighteen fish migrated upstream to the Kegums HES. These were all in the swift water, released from the Kegums turbine outlets (Tab. 3).
Figure 5. Passages of ALS2, i.e. tagged fish passing through the turbines at Riga HES (cross-symbols represents how many fish passed), in relation to turbine discharge and temperature at Riga HES. The grey dashed line represents Kegums HES intake discharge.
Figure 6. Recaptured fish that went through the turbines at Riga HES. Shaded area represents how many fish that were still alive when the nets were emptied and the black represents dead fish.
13
A total of ten tagged fish remained in River Daugava at the end of the study, of which six were lost (i.e. not positioned for an extended period of time). One of the lost fish passed ALS1 on 17 October, two were never found again after being positioned at “church island” in the Daugava River (Fig. 1), and the remaining three were never found again after 22 October. The signal for one of these lost fish had a decreased pulse rate, indicating a lack of movement
Table 3. The distribution of the final fate of radio-tagged fish in Rivers Daugava and Ogre. The grey shaded row represents all groups together. N/A = not applicable.
Group (n) Downstream passage Recap. below Riga HES Positioned below
Kegums HES
ALS1 Ogre ALS2 Riga HES Total Dead
R1, Ogre 1 (n = 8) 8 of 8 (100 %) 5 of 8 (63 %) 3 of 5 (60 %) 1 of 3 (33 %) 1 of 8 (13 %) R2, Ogre 2 (n = 6) 6 of 6 (100 %) 0 0 0 1 of 6 (17 %) R3, Daugava (n = 4) N/A 2 of 4 (50 %) 2 of 2 (100 %) 1 of 2 (50 %) 2 of 4 (50 %) Total (n = 18) 14 of 14 (100 %) 7 of 18 (39 %) 5 of 7 (71 %) 2 of 5 (40 %) 4 of 18 (22 %) Discussion
This study demonstrated that adult salmon that had not been imprinted on spawning grounds in the Ogre River did not migrate upstream when released in the lower parts of River Ogre, past the Ogre HES. All of the fish released in River Ogre went downstream and eventually ended up in the main River Daugava. All fish were released, however, during a period of exceptionally high discharge when the spill gates at the Ogre HES were fully opened. This is not the normal situation, so it remains unknown if these fish would have held at the Ogre HES and perhaps even migrated upstream towards the potential spawning grounds during less extreme discharge conditions. Approximately 23% of all tagged fish eventually migrated upstream to Kegums HES in the Daugava River, so upstream migration behaviour in general could be observed for these fish. Since it is widely reported from other systems that non-imprinted salmon migrate upstream and eventually spawn with some success (Fleming et al. 2000; Jonsson et al. 1991), it is possible that under normal flow conditions the trap-and-transport method might work to some extent in Ogre. During no spill conditions fish should have been forced to either hold near the Ogre HES, or move upstream to explore the river for spawning areas and
potentially spawn. This might indicate that a trap-and transport-approach would be appropriate to start with in the rehabilitation of the wild Atlantic salmon in Daugava. This was not the case, making it hard to determine the possibilities if in fact River Ogre is suitable for salmon reproduction. Other factors like stress from handling the salmon and them being anesthetized can affect the behaviour. Additionally, the degree of maturity was not closely investigated, but during tagging one could see they were ripe. Also the small sample size of tagged salmon may influence the strength of the tests, and therefore also the results. The study is planned to be repeated in 2015, hopefully under less extreme conditions.
Since the spill gates were fully opened during an extended period, the fish went through the spill gates and ended up in River Daugava. Whether this behaviour is because of the absence of imprinting processes or a response to extreme flooding conditions is hard to distinguish. Since the smolts are released in the mouth of the River Daugava, near the Gulf of Riga, the fish are not imprinted on the River Ogre. Other factors controlling upstream migration for anadromous salmonids are mainly water flow and temperature (Banks 1969). Extreme discharge conditions can result in delayed migration or out-migration (Trepanier et al. 1996). Still, other studies show that discharge does not always affect upstream migration. This could be because of the river sizes relative to the amount of water required for upstream migration for the fish (Jonsson & Jonsson 2011). In this case, River Ogre is rather small,
which suggests that high flow might influence migration negatively (Enders et al. 2005; Wootton 1998), which can explain why the salmon moved downstream. The high passage success at Ogre HES
indicates that most adult salmon could pass Ogre dam without major problems, at least during high water levels when all spill gates were open.
Impacts on the water quality are often a consequence of high discharge. It is known that water quality can sometimes affect the migration of Atlantic salmon (Thorstad et al. 2005). The abiotic parameters collected in this study show high values in chemical oxygen demand (COD) and turbidity. According to WHO water quality assessment guide (1996), the parameters indicate moderate polluted water, typically an effect of run-off on terrestrial surfaces, which is common during flooding. Since the parameters were collected on the 24 October, there is a chance that the values would have been even more extreme during the peak of the flooding on 16 October. Thorstad et al. (2005) showed that Atlantic salmon responded by avoiding wastewater from a wood pulp mill. Salmons that were holding position before spawning rapidly moved either upstream or downstream, with the majority moving downstream (Thorstad et al. 2005).
Another variable tested in this study was holding/delaying time. A total of eight individuals were observed to hold, the majority (five) in the intake channel of the Ogre HES. There was no significant difference in holding time between groups, but there was a significant effect of sex. Females tended to hold longer than males, which indicate sexual differences in migration behaviour. Females have in general more willingness to migrate than males (Jonsson & Jonsson 2011; Klemetsen et al. 2003), hence the result could possible be indicating that the females were more motivated to locate spawning grounds. One question is why did they hold? It could be debated if this is a matter of holding phase in prespawning behaviour or a delaying response. The maximum holding time was seven days, which is probably too short to represent a holding phase prior spawning (Finstad et al. 2005; Heggberget 1988). The majority of the fish were positioned in the intake channel of Ogre HES, which probably indicates that the salmon ended up here due to the presence of an obstacle, rather them selecting the site based on habitat characteristics. Delaying might also be a response to the flooding event, which caused the salmon to maintain position to save energy or take shelter (Enders et al. 2005). Additionally, there were two salmon holding in the River Ogre and one in the River Daugava. The two sites in the River Ogre may be refuges where the fish rest or avoid environmental stressor (Thorstad et al. 2005). The one fish holding in the River Daugava was positioned below Kegums HES and probably tried to continue upriver (Chanseau & Larinier 1999; Gowans et al. 1999). All in all it seems that the salmon were delayed rather than holding position prior to spawning.
Half (50%) of the control group in River Daugava moved upstream towards Kegums HEP. This indicates the propensity of these salmon to migrate upstream despite lacking imprinting from any specific spawning grounds. Since the areas of the upper parts of the river near Kegums HES were not fully monitored, it is possible that more fish showed a tendency to move upstream than we observed. The other two individuals released at Riga HES (R3) went through the turbines of the dam. The
passages occurred during high discharge, which may have influenced them to pass through the turbines. The result from the R3 group indicates that there were no attempts to enter the River Ogre, but
nevertheless some fish showed a motivation to move upriver to Kegums HES. The mouth of the Ogre River is probably not easy to find. It is situated just upstream of a promontory, quite far from the main stem of the River Daugava. In addition, there is a relatively small attraction flow from the River Ogre compared to the flow in the main river (Gowans et al. 1999; Lundqvist et al. 2008).
The final fates of the fish that ended up in the River Daugava showed that 63% from the first group (R1) but none from the second group (R2) passed through the turbines at Riga HES, which makes up a total of (39%) for all groups. However, in R3 only one fish was tracked upriver near Kegums HES. This may correspond to the effect of more “normal” environmental conditions, in terms of water quality. The first group of fish (R1), which entered River Daugava during flooding conditions, may have passed ALS2 at Riga HES as a response to water quality (Thorstad et al. 2005). The passages evidently occurred during the peak of water discharge, and the associated high water levels. Why the fish in the
15
second group (R2) did not follow this pattern, may be explained due to sedimentation, sediment
transport and reduction of water levels, which culminates the effects of flooding inflicting on the water quality. Thus, since the time to reach River Daugava was much longer for the second group (R2), the water quality was closer to normal than during the peak flood, which could have reduced the impacts of the water quality.
Commercial fishermen caught fish that passed through the turbines in their nets downstream of Riga HES, near the river mouth. The recapture rate was 71% (five out of seven). The majority (60%) were still alive, indicating moderate survival success of turbine passage at Riga HES. However, this
proportion may be higher since the cause of death of the dead fish is unknown. During the study period no water was spilled at Riga HES, indicating the fish that passed did so through the turbines. Technical factors influencing turbine mortality are typically turbine type, size, and numbers of revolutions. Even though the passage survival was moderate, the cumulative survival (including winter survival and post-spawning survival) for kelt and possibly for smolts may be very low (Östergren & Rivinoja 2008). An additional result is that the fishing pressure seems quite high at the river mouth of the River Daugava. Therefore, implementations of bypass solutions at the Riga dam may be needed.
A total of six individuals could not be found since early in the study period (last seen 22 October). Two of them disappeared from the area around Church Island; the others disappeared after last being
positioned at different sites. The Riga reservoir was poorly covered when boat tracking which could explain why the fish were not detected. However, one of them was found multiple times late in the study period emitting a half pulse rate, suggesting it had lost the tag or that it was dead. A plausible explanation could be that the fish died naturally, but if this was the case one might think that the dead fish would have drifted downstream and eventually reach the Riga reservoir. Therefore, another
possible explanation may be that someone had taken the transmitter from the fish and thrown it into the water. In fact, reports from anonymous people indicate that on two occasions recreational fishermen caught tagged salmon. The fact that recreational fishermen were often seen close to where we
positioned the salmon, combined with the anonymous reports, suggest that poaching may be a problem. Whether or not a trap-and-transport approach is an appropriate method of reintroducing wild Atlantic salmon to Daugava remains unanswered. Even though imprinting may be important for Atlantic salmon, prior studies show that successful spawning of hatchery-reared salmonids is possible. A radio telemetry study of tagged hatchery-reared Atlantic salmon and brown trout transported above an obstacle in the River Rhine in France reported successful migration and spawning (Gerlier & Roche 1998). However, similar research setup in River Klarälven in Sweden showed erratic movements and a high frequency of downstream migrating hatchery-reared Atlantic salmon after release, which resulted in low reproductive success (Hagelin et al. 2015).
Conclusion
This study showed that adult salmon transported above an obstacle in the tributary Ogre River did not migrate upstream, but instead they moved downstream and left the Ogre River. This behaviour can have been a result of 100 year flood. The study also showed that downstream passage of both Ogre HES and Riga HES was possible. The high recapture rate of fish downstream of the Riga HES indicated a high fishing pressure. Even though the study’s main objective was impaired by the flood, the information obtained may serve as valuable resource for future activities associated with the reintroduction of Atlantic salmon in Daugava.
Additional measures that might facilitate re-establishment of wild spawning populations would be to release fry or pre-smolt near potential spawning habitats in the Ogre or other suitable tributaries. These fish would then imprint on the local conditions and would be more likely to return there if transported upstream past Riga HES, or passing through a fishway, when returning as adult spawners. This study showed that downstream passage of adults was possible at the two key-HESs, and taking the large size of the study fish into account, the turbine-induced smolt mortality might also be limited. Further
investigations of downstream fish migration success are without a doubt needed. If the turbine-induced mortality for smolts would turn out to be substantial after all, some structure may have to be added to divert them away from the intake channel and the turbine intake. Since the head at the Ogre weir is limited, installing a fishway would be feasible. The creation of the Riga reservoir altered this part of the Daugava, which now resembles a lake, and so the predator pressure may be high and could be an additional future problem for downstream migrating smolts (Jepsen et al. 2000; Olsson et al. 2001). Besides smolt survival, kelt survival is an important issue due to the loss guiding devices and bypasses for downstream migrating fish, and the presumably high fishing pressure in the main stem and the mouth of the Daugava River.
Acknowledgments
I wish to thank all involved that took part in planning, supporting and cooperating in this project. Special thanks to Mrs. Alona Bolonina and Latvenergo AS, in Latvia for cooperation, taking initiative and partly financing this project. Also thanks to Dr. Matiss Zagars from the Institute for Environmental Solutions in Latvia with colleagues, friends and students, also initiative taking, providing equipment, and valuable insight and help with the field work. Thanks to BIOR Latvian Fish Resource Agency for supplying valuable information. I am very grateful to have been working together with such high competence and experienced mentors in Olle Calles, Associate Professor at Karlstad University in Sweden, and Claudio Comoglio, Assistant Professor at Politenico di Torino in Italy. Also thanks to John Piccolo, Associate Professor at Karlstad University in Sweden, for helping forming this report. References
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