Migratory behaviour of adult wild and escaped farmed Atlantic salmon, Salmo salar L., before, during and after spawning in a Norwegian river

The migratory behaviour of adult wild and escaped farmed Atlantic salmon, Salmo salar L., before, during after spawning in the River Namsen, Norway, was analysed using radio telemetry. The fish were caught, radio tagged and released into the fjord between 7 and 25 km from the river mouth. A significantly higher proportion of wild (74%) than farmed (43%) salmon was subsequently recorded in the river. Wild salmon (33%) were more frequently captured in the sea and in rivers than farmed salmon (14%). The migration speed from release to passing a data logger 11 km upstream from the river mouth was not significantly different between wild (20.6 km day^?1) and farmed (19.8 km day^?1) salmon. Wild salmon tagged when water flow in the river was increasing had a significantly higher migration speed than wild salmon tagged when water flow was decreasing. This was not true for farmed salmon. Farmed salmon were distributed significantly higher up the river than wild salmon during spawning, although both types of fish were found together in spawning areas. Thus, there was no geographical isolation to prevent spawning between wild and escaped farmed salmon. Farmed salmon had significantly more and longer up- and downstream movements than wild salmon during the spawning period. Unlike farmed salmon, the number of riverine movements by wild salmon increased significantly when variation in water flow increased. A smaller proportion of wild (9%) than farmed (77%) salmon survived through the winter after spawning.     

Spawning distribution and abundance of a northern Chinook salmon population

Chinook salmon, Oncorhynchus tshawytscha (Walbaum), is an important biological and cultural resource in Alaska, but knowledge about Chinook salmon ecology is limited in many regions. From 2009 to 2012, spawning distribution and abundance of a northern Chinook salmon population on the Togiak River in south‐west Alaska were assessed. Chinook salmon preferred deeper mainstem channel spawning habitat, with 12% (14 of 118 tags in 2009) to 21% (22 of 106 tags in 2012) of radio‐tagged fish spawning in smaller order tributaries. Tributary spawners tended to have earlier run timing than mainstem spawners. Chinook salmon exhibited extended holding and backout (entering freshwater but returning to saltwater before completing anadromous migration) behaviours near the mouth of Togiak River, potentially prolonging their exposure to fishery harvest. Mark–recapture total annual run estimates (2010–2012) ranged from 11 240 (2011) to 18 299 (2012) fish. Exploitation of Chinook salmon ranged from 36% (2012) to 55% (2011) during the study period, with incidental fishery catches near the mouth of the river comprising the largest source of harvest.     

Prespawning migratory behaviour and spawning success of sea-ranched Atlantic salmon, Salmo salar L., in the River Gudenaa, Denmark

The migratory behaviour of sea-ranched Atlantic salmon, Salmo salar L., was analysed by radio-telemetry in the River Gudenaa, Denmark. The main objectives were to: (1) estimate mortality of returning adults through the fjord; (2) observe rate of progression and migratory pattern in the fjord and river; and (3) record whether spawning occurs in the river. Forty-two returning salmon (19 males and 23 females of total body length from 60–97 cm) reared and released as smolts, were caught and equipped with external radio transmitters in the outer estuary of the River Gudenaa in 1994 and 1995. Of the tagged salmon, 18 (43%) were caught in the estuary, four (10%) were not recorded after release and 20 (47%) entered the river. The mean rate of progression through the fjord was 7.6 km d−1 (range 1.4–18.2) in 1994 and 5.4 km  d−1 (range 1.6–17.1) in 1995. Eleven salmon were alive at the onset of the spawning period. Eight were retrieved dead from the river during or after the spawning period; four with empty gonads assumed to be successful spawners, and four with intact gonads. In 1994, unsuccessful spawners (found dead with intact gonads) entered the river earlier and had a longer total migration distance in the river compared to successful spawners. This suggests that spawning success of sea-ranched salmon is associated with time of river entry and river migration length.     

The prevalence of escaped farmed salmon, Salmo salar L., in the River Ewe, western Scotland, with notes on their ages, weights and spawning distribution

Abstract  The prevalence of escaped farmed Atlantic salmon, Salmo salar L., in the River Ewe, western Scotland, was assessed. After the establishment of smolt cages in the catchment and marine cages near the river mouth during 1986–1987, approximately 425 000 parr and smolts, and 122 000 growers have escaped. Between 1987 and 2001, farmed salmon occurred in the rod fishery in 13 of the 15 years, contributing at least 5.8% of the total catch, with a maximum annual frequency of 27.1%. It was estimated that <1% of fish escaping from the marine cages entered the river, but contributed at least 27% of potential anadromous spawners in 1997. Radiotagged, farmed fish in 2001 probably spawned in three subcatchments also used by tagged wild fish. Despite the likelihood of hybridisation there was no change in the median weight or marine age of wild fish, but smolt age decreased significantly ( P  < 0.02). The Ewe has a depleted wild salmon population (≤900 anadromous adults), and further genetic introgression by escapees should be prevented.     

Return migration of adult Atlantic salmon, Salmo salar, in relation to water diverted through a power station

Abstract  The migration of Atlantic salmon, Salmo salar L., returning to the River Suldalslågen, Norway, was studied in relation to redirection of freshwater flow through a power station. The outlet of the power station is situated in the Hylsfjord, a fjord adjacent to the river mouth. Seventy-two salmon were tagged with acoustic transmitters, released in the outer part of the fjord system and automatically recorded when entering the Hylsfjord or the river. Data were collected during one period when the power station was running and two periods when the power station was closed. The release of water from the power station did not greatly attract the salmon during their return migration. Proportions of salmon entering the river or time from release to entering the river did not differ among salmon tagged in the different periods. The salmon were recorded in the Hylsfjord both when the power station was running and closed and there were no differences in number of times, number of days or hours recorded in the Hylsfjord among salmon tagged in the three periods. The only significant difference found among periods was duration of continuous stays in the inner part of the Hylsfjord. This may indicate a slight attraction to the freshwater release, but the difference seems small (1.8 vs. 0.7 h) compared with the time the fish stayed in the fjord system before entering the river (16–85 days).     

Life‐history differences of juvenile Chinook salmon Oncorhynchus tshawytscha across rearing locations in the Shasta River,California

Salmon from different locations in a watershed can have different life histories. It is often unclear to what extent this variation is a response to the current environmental conditions an individual experiences as opposed to local‐scale genetic adaptation or the environment experienced early in development. We used a mark–recapture transplant experiment in the Shasta River, CA, to test whether life‐history traits of juvenile Chinook salmon Oncorhynchus tshawytscha varied among locations, and whether individuals could adopt a new life history upon encountering new habitat type. The Shasta River, a Klamath River tributary, has two Chinook salmon spawning and juvenile rearing areas, a lower basin canyon (river km 0–12) and upper basin spring complex (river km 40–56), characterised by dramatically different in‐stream habitats. In 2012 and 2013, we created three experimental groups: (i) fish caught, tagged and released in the upper basin; (ii) fish caught at the river mouth (confluence with the Klamath River, river km 0), tagged and released in the upper basin; and (iii) fish caught at the river mouth, tagged and released in the lower basin. Fish released in the upper basin outmigrated later and at a larger size than those released in the lower basin. The traits of fish transplanted to the upper basin were similar to fish originating in the upper basin. Chinook salmon juvenile life‐history traits reflected habitat conditions fish experienced rather than those where they originated, indicating that habitat modification or transportation to new habitats can rapidly alter the life‐history composition of populations.     

An ecological–economic model on the effects of interactions between escaped farmed and wild salmon (Salmo salar)

This study explores the ecological and economic impacts of interactions between escaped farmed and wild Atlantic salmon (Salmo salar, Salmonidae) over generations. An age‐ and stage‐structured bioeconomic model is developed. The biological part of the model includes age‐specific life‐history traits such as survival rates, fecundity and spawning successes for wild and escaped farmed salmon, as well as their hybrids, while the economic part takes account of use and non‐use values of fish stock. The model is simulated under three scenarios using data from the Atlantic salmon fishery and salmon farming in Norway. The social welfare is derived from harvest and wild salmon while the economic benefits of fishing comprise both sea and river fisheries. The results reveal that the wild salmon stock is gradually replaced by salmon with farmed origin, while the total social welfare and economic benefit decline, although not at the same rate as the wild salmon stock.     

Spawning run of Atlantic Salmon (Salmo salar) in the River Tornionjoki monitored by horizontal split-beam echosounding

Fixed location split-beam horizontal echosounding was used to assess the size and timing of the Atlantic salmon (Salmo salar) spawning run in the River Tornionjoki. Four transducers, two on each river bank, were mounted across the river at the study site 4 km upstream from the river mouth. Net weirs were used on both shores to direct the passage of fish through the acoustic beams. Hydroacoustic monitoring covered 40–50% of the river cross-sectional area. Also test fishing and yearly catch statistics of salmon were used as an indication of the size of the spawning run in the river. Altogether, 7 700, 5 300 and 4 300 salmon-sized targets (target strength, TS ≥ –29 dB) moving upstream were detected in 1997, 1998, and 1999, respectively. The fish migration began in all the years by early June and peaked during the second half of the month; the migration period of large salmon lasted until mid-July. The observations made by the echosounding and catch statistics were similar in this respect. In 1998 and 1999, however, more targets of TS ≥ –29 dB were detected during late summer than could be expected by the river catches of salmon. It may be that the large targets in late summer were, in fact, whitefish whose run occurred during the same time. Hydroacoustic estimation of the total salmon run at the study site was found difficult. The numbers of salmon-sized targets detected were almost the same as the numbers of salmon caught each year by fishermen. Therefore, only an index of the run timing and the size of the stock can be produced from the data. It was clear that a considerable amount of fish escaped the acoustic monitoring by using areas uncovered by the beam, such as gaps in the bottom and the surface layers of the water column near the shores. Moreover, it was found that species recognition based on TS only is not adequate in multispecies environments. Assessment of spawning runs remains, however, a key issue in the management of Baltic salmon, and with further development, the hydroacoustic monitoring may be the most viable means of doing it.     

Incidence of escaped farmed salmon, Salmo salar L., in commercial salmon catches and fresh water in Northern Ireland

External morphological characteristics were used to identify escaped farmed Atlantic salmon, Salmo salar L., in a coastal salmon fishery in County Antrim, Northern Ireland during four fishing seasons and at an adjacent freshwater location (R. Bush) during a 5-year period. Out of a total of 36 326 adult salmon examined in the fishery, 883 (2.4%) were identified as having escaped from sea cages. Annual average values ranged from 0.26% to 4.04% of the fish caught. Occurrence of escapees entering an adult trap in fresh water averaged 0.88%, with a range of 0.13–2.62%, depending on year. No correlation between presence in the marine fishery and in fresh water was evident, the latter year-round figures probably being more indicative of presence of escapees in spawning stocks. Entry to fresh water was significantly later on average for escaped farmed salmon, compared with wild salmon.     

Escaped farmed salmon, Salmo salar L., in the Glenarm River, Northern Ireland: genetic status of the wild population 7 years on

A previous study described genetic changes in a wild Atlantic salmon , Salmo salar L., population resulting from the spawning of escaped farmed salmon in the Glenarm River, Northern Ireland, in 1990. This study reports an extension of the original investigation with a further follow-up sample that was taken from the river in 1997 to assess the genetic status of the wild population two generations after the original hybridization between the wild population and the farmed strain. Overall genetic variation across eight polymorphic allozyme loci indicated that the wild population remains significantly different from the pre-escape population and from the immediate post-escape population, the presence of an allele not having been previously detected in this population ( GPI-1,2*140 ), suggesting that further incursion(s) of farmed salmon may have taken place.     

Homing patterns of Baltic salmon, Salmo salar L., from smolts released from two hatcheries in the River Dalälven, Sweden

The River Dalälven Baltic salmon, Salmo salar L., population has been maintained by stocking reared fish since the early 1920s. Initially, all rearing was carried out at one hatchery, but since the late 1980s two have been used. Both hatcheries are situated 9–10 km from the river mouth but some 600 m apart. All broodfish were caught in a single fish trap situated some 700 m upstream of the upper hatchery. The salmon smolts were released just below the water outlets of each hatchery, respectively. About 2% of the released smolts from each hatchery were tagged annually with Carlin tags. Total recapture rates were higher for smolts from the lower hatchery. A higher proportion of recaptured fish was reported from the home river for salmon from the upper hatchery. The migration within the river to the fish trap was more precise for fish from the upper station. Strays were very late in the season and of a higher number from the lower hatchery. Observations of jumping salmon by the outlet from the lower station indicated that salmon returned to that point. The lower recaptures in the trap were considered a result of a shorter river migration of salmon from the lower hatchery.     

The relationship between migration speed and release date for chum salmon Oncorhynchus keta fry exiting a 110-km northern Japanese river

The relationship between release date and migration speed was examined for hatchery chum salmon Oncorhynchus keta fry exiting the Nishibetsu River in eastern Hokkaido, northern Japan so that future releases might be scheduled so that fry arrive at the ocean during periods favoring high survival. Separate marked groups of chum salmon released in early April, mid-April, and early May in 2008, late March and mid-April in 2009, and mid-April in 2010 were recaptured with a rotary screw trap 12 km above the river mouth. Chum salmon in later release groups tended to migrate downstream faster than fish in earlier release groups. Those released after mid-April arrived in the lower river on average 9 days after release, while those released before mid-April arrived on average 26–28 days after release. Most marked fish arrived in the lower river during late April to mid-May. These results suggest that chum salmon are adapted to adjust their migratory speed so as to arrive at the ocean during a relatively discrete period, presumably during a time of high productivity favoring good survival.     

Stocking location and predation by marine fishes affect survival of hatchery‐reared Atlantic salmon smolts

Abstract Release strategies of hatchery‐reared Atlantic salmon, Salmo salar L., smolts were compared by studying survival and migration of smolts (n = 99) and their predators (Atlantic cod, Gadus morhua L., n = 8; and saithe, Pollachius virens (L.), n = 2) during the first 37 km of the marine migration using acoustic telemetry. Survivorship was higher in smolts released at the river mouth (30%) compared with smolts released in the river (12%). This was likely due to mortality or reduced migratory behaviour in fresh water. The marine mortality was 37% during the first 2 km after leaving the river (at least 25% mortality because of predation from marine fishes), and total marine mortality over 37 km was 68%. Detection‐depth data were useful for evaluating whether the tagged smolts were alive or predated; mortality during the first 2 km of outward migration would have been underestimated at 26% instead of 37% without the analysis of depth detection. Transmitters from consumed post‐smolts remained in predators for up to 47 days (average 29 days).     

Movements and spatiotemporal distribution of escaped farmed and local wild Atlantic cod (Gadus morhua L.)

Commercial farming of Atlantic cod (Gadus morhua L.) is now being developed in several countries. The ecological consequences of cod culture are poorly understood, but recent research suggests that Atlantic cod are more prone to escape from net pens than Atlantic salmon. Here, we describe the movements and the spatiotemporal distribution of farmed cod after escape relative to wild cod, both during and outside the natural spawning season. The experimental design included simulating escape incidents of farmed cod tagged with acoustic transmitters and using an array of automatic listening stations to monitor their dispersal and distribution. For comparison, local wild cod were monitored using the same array of receivers. The farmed cod dispersed rapidly after a simulated escape, they randomly distributed over large areas and their distribution overlapped with local wild cod. Moreover, escaped farmed fish were found at local cod spawning areas during the spawning season. The study also indicated that the recapture rate of escaped farmed cod was high compared with that of escaped farmed salmon. Thus, while our results showed that there is a considerable potential for ecosystem effects caused by escaped farmed cod, mitigating actions such as an efficient recapture fishery for escapees may be possible.     

Aggressive interactions in pure and mixed groups of juvenile farmed and hatchery-reared wild Atlantic salmon Salmo salar L. in relation to tank substrate

Total aggression and individual behavioural traits of equal-sized juveniles of farmed Atlantic salmon Salmo salar L. selected for five generations in the Norwegian Breeding Programme for Salmonids and a wild strain originating from the River Rauma, Norway, were compared in pure (10 fish) and mixed (5 + 5 fish) groups. Total aggression was defined as the sum of the following five offensive behaviour patterns: intentional movements, lateral display, frontal display, charge and bite. Experiments were carried out in 2-m² tanks with either a grey fibreglass substrate (pure and mixed groups) or diagonally divided into a fibreglass and river cobble substrate (mixed groups only). In mixed groups kept on a fibreglass substrate, the aggressive behaviour directed by wild salmon towards farmed salmon was more frequent than the reverse (84.2 and 27.0 aggressive acts h^–1, respectively, P < 0.025). Total aggression was not significantly different between wild and farmed juveniles in tanks with divided substrate. The aggressive behaviour of wild and farmed salmon was significantly increased when kept in tanks with fibreglass substrates (437%, P < 0.005, and 296%, P < 0.05 respectively). Pure groups of either wild or farmed salmon kept on a fibreglass substrate exhibited similar frequencies of total aggression (85.7 and 101.6 0 aggressive acts h^–1, respectively, P > 0.1) and single behaviour patterns. The aggressive behaviour of farmed offspring was not influenced by vertical distribution in the tanks with a fibreglass substrate. Farmed salmon tended to position pelagically (78%) and used the water column more frequently than wild salmon (19%), whereas wild salmon hid more. It is concluded that social interactions among groups of juvenile Atlantic salmon are influenced by substrate.     

Incidence and timing of wild and escaped farmed Atlantic salmon (Salmo salar) in Norwegian rivers inferred from video surveillance monitoring

Run timing of escaped farmed Atlantic salmon Salmo salar vs. wild fish was compared by the use of video camera surveillance in 15 rivers over several years, covering 1600 km of the Norwegian coastline (from 58°N to 69°N). Annual runs of wild salmon varied among rivers from <200 fish to more than 10 000. During the surveillance period that for most rivers extended from late May to early October, larger‐sized salmon (fish ≥ 65 cm) generally entered the rivers earlier than small fish. The percentage of salmon identified as escaped farmed fish ranged from 0.1% to 17% across rivers with an average of 4.3%. Estimates of escapees are, however, assumed to represent minimum values because an unknown number of farmed fish passing the video cameras may have been misclassified as wild fish. By the use of a linear mixed model and generalised additive mixed models, it was found that the relationship between run timing and fish length differed significantly between farmed and wild salmon. While small‐sized farmed and wild fish (<65 cm) entered the river at about the same time, wild large salmon returned on average 1–2 weeks earlier than similarly sized escapees. The proportion of large‐sized farmed escapees also increased until late August and decreased thereafter. In contrast, there was a relatively constant and lower proportion of small‐sized escapees throughout the season. Within the surveillance period, there was no evidence of any exceptionally late runs of fish classified as escaped farmed salmon.     

Content of synthetic astaxanthin in escaped farmed Atlantic salmon, Salmo salar L., ascending Norwegian rivers

Abstract  Isomeric ratios of astaxanthin in eggs and alevins of Atlantic salmon, Salmo salar L., have proven useful in identifying female spawners of farmed origin, but the method underestimates the proportion of fish of farmed origin. The rate of underestimation was studied by analysing astaxanthin content in tissue of 55 farmed Atlantic salmon ascending two Norwegian rivers in the autumn of 1991. The astaxanthin content fell into two distinct classes. Fifty-one per cent of the adult escaped salmon had isomeric ratios similar to salmon fed synthetic astaxanthin, whereas all the remaining fish had ratios similar to wild fish. Discriminant analysis classified 96.4% of the fish with known astaxanthin content into the correct astaxanthin class on basis of tail-fin erosion, length, weight and gill-cover damage. This discriminant function was used to estimate the astaxanthin classification of 1017 farmed salmon caught in nine rivers during 1989–1991. The classification success varied among years from 52 to 64%. Corresponding numbers for females and males were 45–48% and 54–70%, respectively. Thus, estimates of spawning rates of farmed female salmon via astaxanthin content in eggs or alevins from redds should be adjusted accordingly. The observed isomeric ratios of astaxanthin in the escaped farmed salmon and the relationship with morphology indicates that a significant proportion of the escapees ascending rivers have spent more than 1 year in the wild after escape.     

Linking habitat characteristics with juvenile density to quantify Salmo salar and Salmo trutta smolt production in the river Sävarån,Sweden

Abstract Habitat mapping along 85 km of river was related to juvenile (15 years of electric fishing) and smolt (3 years of screw‐trapping) abundance data to estimate salmon, Salmo salar L., and sea trout, Salmo trutta L., smolt production in the River Sävarån, northern Sweden. Spawning site selection by radio‐tagged salmon (n = 12) and sea trout (n = 4) was also assessed. Fifty‐one hectares of potential spawning and nursery habitat was found in the main stem river, representing 25% of the total river area. These areas were estimated to yield 1300–7580 salmon and 630–3540 sea trout smolts based on juvenile densities, equating with 3 years of screw‐trap data (2990–5080 salmon and 680–2520 trout smolts, respectively). A hypothetical maximum production of about 19 900 salmon smolts was predicted for the river at a density of 40, 0+ salmon 100 m^?2. Tracking adults during the spawning period identified optimal and potential reproductive areas.     

Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 1970s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon × trout hybrids have been recorded in rivers situated near intensive salmon farming in Norway and Scotland, which may be indicative of a breakdown in reproductive isolation between salmon, Salmo salar L., and brown trout, Salmo trutta L. In the present study, salmonid fry and 0+ parr were collected from rivers in western Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2%). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic salmon female × brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 catchments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the first extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.     

Upstream migration of Atlantic salmon at a power station on the River Nidelva, Southern Norway

The upstream migration of 17 radio-tagged adult Atlantic salmon, Salmo salar L., past hydroelectric developments on the River Nidelva, Southern Norway, was examined. Salmon migrated quickly from the site of release in the lower part of the river up to the tunnel outlet of Rygene power station, but were substantially delayed at the outlet. The salmon stayed in the outlet area for 0–71 days (median = 20), and mainly took up a position inside the dark power station tunnel. Water discharge in the tunnel was 57–176 m³ s⁻¹, while residual flow in the river between the outlet and the dam 2.5 km further upstream was 3 m³ s⁻¹. Ten salmon passed the outlet and entered the residual flow stretch, but none passed the dam. Six of the 10 salmon returned to the tunnel outlet. No major migration barriers were identified in the residual flow stretch, suggesting lack of motivation among the salmon to migrate due to either low water discharge compared with the main river, or several minor migration barriers along the river stretch.