The marine migration of post-spawning anadromous fish remains poorly understood. The present study examined survival and progression rates of anadromous brown trout Salmo trutta L. after spawning ...(kelts) during downriver, fjord, and sea migration. Kelts (n = 49) were captured in the Danish River Gudenaa, tagged with acoustic transmitters and subsequently recorded by automatic receivers. Kelts spent on average 25 d moving down the 45 km river and through the brackish fjord. The fish entered the Kattegat Sea between 14 April and 30 May. Eighteen of the 49 kelts disappeared in the river and fjord during outward migration, likely due to mortality. Survival was not significantly related to gill Na⁺/K⁺-ATPase activity, suggesting that physiological adaptation to saltwater may be less critical for adults compared to juveniles (smolts). Of the 31 fish that entered the Kattegat Sea, 45% survived and returned to the fjord. The duration of the entire marine migration, from leaving to entering the river, was on average 163 d. The fish returned from the Kattegat Sea to the fjord between 22 July and 21 October. Upon return, the fish spent 1–90 d passing through Randers Fjord, with most individuals completing the reach within 4 d, suggesting that the kelts spent limited time foraging after returning to the fjord. The total survival during the entire marine migration, including the fjord, was a minimum of 29%. Our study provides data that are important for management of anadromous brown trout, and the high survival highlights that kelts may represent a valuable resource for both population reproduction and recreational fisheries.
The escape of fish from aquaculture is perceived as a threat to wild fish populations. The escapes problem is largely caused by technical and operational failures of fish farming equipment. In ...Norway, 3.93 million Atlantic salmonSalmo salar, 0.98 million rainbow troutOncorhynchus mykissand 1.05 million Atlantic codGadus morhuaescaped from 2001 to 2009. Salmonids primarily escape after structural failures of containment equipment, while a far greater percentage of cod than salmon escape through holes in the nets. Correlative evidence suggests that after the Norwegian technical standard (NS 9415) for sea-cage farms took effect in 2004, the total number of escaped Atlantic salmon declined from >600 000 (2001 to 2006) to <200 000 fish yr⁻¹ (2007 to 2009), despite the total number of salmon held in sea-cages increasing by 44% during this period. No similar decrease in escaped cod has occurred, suggesting that other measures, such as improved netting materials for sea-cages, are required. In addition to escaping as juveniles or adults, cod may reproduce in seacages, and thus fertilised eggs escape to the environment. The ecological effects of 'escape through spawning' are unclear, and methods to inhibit escape by this mechanism are being explored. To prevent escapes of juvenile and adult fish as sea-cage aquaculture industries develop, we recommend that policy-makers implement a 5 component strategy: (1) establish mandatory reporting of all escape incidents; (2) establish a mechanism to analyse and learn from the mandatory reporting; (3) conduct mandatory, rapid, technical assessments to determine the causes of escape incidents involving more than 10 000 fish; (4) introduce a technical standard for sea-cage aquaculture equipment coupled with an independent mechanism to enforce the standard; and (5) conduct mandatory training of fish farm staff in escape-critical operations and techniques.
Salmon farming increases the abundance of salmon lice, which are ectoparasites of salmonids in the sea. Here we review the current knowledge on the effects of salmon lice on wild sea trout. Salmon ...lice feed on host mucus, skin and muscle, and infestation may induce osmoregulatory dysfunction, physiological stress, anaemia, reduced feeding and growth, increased susceptibility to secondary infections, reduced disease resistance and ultimately mortality of individual sea trout. Wild sea trout in farm-free areas generally show low lice levels. In farm-intensive areas, lice levels on wild sea trout are typically higher, and more variable than in farm-free areas. Lice on wild sea trout are found at elevated levels particularly within 30 km of the nearest farms but can also extend to further ranges. Salmon lice in intensively farmed areas have negatively impacted wild sea trout populations by reducing growth and increasing marine mortality. Quantification of these impacts remains a challenge, although population-level effects have been quantified in Atlantic salmon by comparing the survival of chemically protected fish with control groups, which are relevant also for sea trout. Mortality attributable to salmon lice can lead to an average of 12−29% fewer salmon spawners. Reduced growth and increased mortality will reduce the benefits of marine migration for sea trout, and may also result in selection against anadromy in areas with high lice levels. Salmon lice-induced effects on sea trout populations may also extend to altered genetic composition and reduced diversity, and possibly to the local loss of sea trout, and establishment of exclusively freshwater resident populations.
Brown trout Salmo trutta (L.) is a facultative anadromous species, where a portion of individuals in populations with access to the sea perform migrations to use the richer feeding resources. We ...investigated the effect of salmon lice Lepeophtheirus salmonis (Krøyer 1837) infestation on the survival and behaviour of wild trout post-smolts (average fork length = 180 mm) during their marine migration. Comparisons of the marine migratory behaviour were made between an artificially infested group (n = 74) and a control group (n = 71) in an area with low natural lice infestation pressure. Artificial infestation was estimated to cause 100% prevalence and a mean intensity of 65 lice fish−1 (mean relative intensity of 2.4 lice g−1 fish). Survival analysis showed limited statistical power but revealed lice-induced mortality, with an estimated hazard ratio of 2.73 (95% CI = 1.04−7.13) compared to the control group, when data from a previous pilot study were included. Surviving individuals in the infested group additionally responded by residing closer to fresh water while at sea, and by prematurely returning to fresh water. On average, infested fish returned to fresh water after only 18 d at sea, while control fish spent on average 100 d at sea. The residency in the inner part of the fjord and the premature return to fresh water represent an adaptive behavioural response to survive the infestation, at the probable cost of reduced growth opportunities and compromised future fitness.
Anadromous Arctic charr
Salvelinus alpinus
is a cold-adapted salmonid that is vulnerable to climate warming and anthropogenic activities including salmon farming, hydropower regulation, and ...pollution, which poses a multiple-stressor scenario that influences or threatens populations. We studied the horizontal and vertical behaviour of Arctic charr tagged with acoustic transmitters (n = 45, mean fish length: 22 cm) in a pristine, subarctic marine area to provide insights into the behaviour of first-time migrants. Tagged fish spent up to 78 d at sea, with high marine survival (82% returned to their native watercourse). While at sea, they utilized mostly near-shore areas, up to 45 km away from their native river. Arctic charr showed large variation in migration distance (mean ± SD: 222 ± 174 km), and the migration distance increased with body size. Although the fish displayed a strong fidelity to surface waters (0-3 m), spatiotemporal variation in depth use was evident, with fish utilizing deeper depths during the day and in late July. These results represent baseline data on Arctic charr’s marine behaviour in a pristine fjord system and highlight the importance of near-shore surface water as feeding areas for first-time migrants. Furthermore, the observed dependency on coastal areas implies a vulnerability to increasing human-induced perturbations, on top of impacts by large-scale climate change in marine and freshwater habitats.
We compared the within-river movements and distribution of wild and escaped farmed Atlantic salmon Salmo salar before and during spawning in the Namsen river system of Central Norway. A total of 74 ...wild and 43 escaped farmed salmon were captured at sea, tagged with radio transmitters and released. Based on our examinations, most, if not all salmon (farmed and wild) entering the River Namsen were sexually mature. Farmed salmon entering the river system had a higher probability than wild individuals of reaching the migration barrier in the upper part of the river, 70 km from the sea. During the pre-spawning and spawning periods, farmed salmon were located mainly in the upper parts (50 to 70 km from the sea), whereas wild salmon were evenly distributed along the entire river during both periods. Consequently, the probability of farmed × wild inter-breeding varied among river sections. Our finding that the distribution of escaped farmed salmon may differ from that of wild salmon and among river sections in the prespawning and spawning periods—and that it may also vary over time—must be taken into consideration when (1) designing monitoring programs aimed at estimating the proportion of escaped farmed salmon in rivers and (2) when interpreting monitoring results. Furthermore, targeted fishing in the river aimed at reducing the number of farmed salmon prior to spawning may be more effective in upper rivers sections, and below major migration barriers.
The sea trout (anadromous brown trout Salmo trutta ) displays extensive among-individual variation in marine migration behaviour. We studied the migration behaviour of 286 sea trout (27-89 cm) tagged ...with acoustic transmitters in the spring, in 7 populations located in 2 distinct marine fjord systems in Norway. We examined whether individual nutritional state, sex and body size influenced marine migration behaviour in terms of (1) the decision to migrate to the sea or remain resident in freshwater and/or estuarine habitats, (2) seasonal timing of sea entry, (3) duration of the marine residency and (4) migration distance at sea from the home river. Most sea trout were in a poor nutritional state in the spring prior to migration. Sea trout with low body condition factors and low plasma triglyceride levels were more likely to migrate to sea, and low triglyceride levels were also associated with earlier sea entry. Poor body condition also increased the probability of individuals remaining at sea longer and migrating further offshore compared to fish in better condition. Females were more likely to migrate to the sea than males. Larger fish were also more likely to migrate to the sea instead of remaining in freshwater and estuaries, and dispersed over greater distances from the river than smaller fish. In conclusion, this study documented general trends across multiple populations and showed that nutritional state, sex and body size influence important aspects of the marine migration behaviour of sea trout.
The seaward migration of wild (n = 61) and hatchery-reared (n = 46) sea trout smolts was investigated in the Danish River Gudenaa and Randers Fjord (17.3 and 28.6 km stretch, respectively) using ...acoustic telemetry. Their riverine and early marine migration was monitored by deploying automatic listening stations (ALS) at 4 locations in the river and fjord. Migration speeds were approximately 3 to 11 times faster in the river than in the early marine environment. Hatchery-reared smolts migrated faster than wild smolts, but the difference was small, especially compared with the large differences in migration speeds among habitats. There was no difference in the diurnal activity pattern between wild and hatchery-reared smolts. Both the riverine and early marine migration activity were primarily nocturnal, although some individuals were also recorded by the ALSs during the daytime. The survival of the wild smolts from release in the river to the outermost marine ALS site, 46 km from the release site, was 1.8 and 2.9 times higher than that of the hatchery-reared smolts in the 2 study years, respectively. Overall, survival from release to the outermost ALS site was 79% for wild and 39% for hatchery-reared smolts. Since the lower survival of the hatchery-reared compared with the wild smolts could not be explained by differences in migration speeds or diurnal migration patterns, behavioural differences on a smaller scale than those recorded in the present study may explain the difference in survival.
Diving behaviour of Atlantic salmon at sea Hedger, Richard D.; Rikardsen, Audun H.; Strøm, John F. ...
Marine ecology. Progress series,
07/2017, Letnik:
574
Journal Article
Recenzirano
Odprti dostop
The diving behaviour of adult Atlantic salmon Salmo salar L. post-spawners in the Norwegian and Barents Seas was monitored with pop-up satellite archival tags (PSATs) and data storage tags (DSTs). ...Salmon from the 3 studied populations showed similar depth use patterns: tagged specimens spent most of their time near the surface (mean of 82% of the time at depths <10 m), with occasional short deep dives (>200 m depth, median time = 2.31 h; range = 0.18 to 22.5 h), the deepest recorded being 707 m. Increased use of greater depths occurred during daytime than night-time in the months between polar day and polar night (August to October). Diel change in depth use around the time of polar night (November to January) was weakest for the population (from the River Alta) that migrated furthest north. Diving was more frequent and shallower when the mixed layer was near the surface during the months of June to October. There was an increase in diving depth (>200 m) when the mixed layer extended to ∼200 or 300 m in winter and spring (December to April). Deep diving consisted of ‘U’ shaped dives, possibly indicative of foraging. We hypothesise that seasonal light conditions, dependent on geographical location, affect Atlantic salmon diving, and that changes in diving depth may be due to seasonal differences in prey aggregation.
Downstream migration of 112 radio-tagged Atlantic salmon smolts was studied in the Diemel, Germany, to examine 1) mortality and migration speeds during riverine migration and at a hydropower station, ...2) choice of migration routes at the power station, and 3) survival and transit speed through Archimedes screw and Francis turbines. Mortality was not elevated in the impounded stretch above the dam compared to a free-flowing control stretch (1.9 and 2.5% loss per km, respectively). Migration speeds did not differ among the control stretch, impounded stretch and in passing the power station, but there was large individual variation. Smolts reaching the power station (n=101) could choose between six possible passage routes. Most smolts passed through the Archimedes screw (43%), or via a route where Francis turbines were installed (33%). Three percent migrated over the dam, 14% used a fishway at the Archimedes screw and 8% used a fishway at the Francis turbines. The smolts used the fishways (instead of the Archimedes and Francis turbines) more often than expected from the proportion of water discharge, especially larger smolts at lower discharge. Smolts passed the power station mainly in the evening and early night. Migration speed past the power station was faster for smolts passing via the Archimedes screw and associated fishway than for those using other routes. Smolts were not markedly delayed in their downstream migration by using the Archimedes screw. Immediate mortality of smolts passing through the Archimedes screw and Francis turbines was probably below 10% and 13%, respectively.
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