The abundance and distribution of salmon lice Lepeophtheirus salmons originating from fish farms in a northern Norwegian fjord during the summer of 2010 was investigated by means of a numerical ...model, underpinned by field observations. In order to evaluate the robustness of the simulated distribution of the lice, we re-ran the simulation several times, changing the vertical responses of the lice to environmental cues such as light and turbulence, in addition to altering their vertical swimming velocity. The model was able to realistically reproduce the observed currents and stratification in the region. The simulated distribution of lice was not sensitive to different implementations of surface light nor to the light sensitivity level of the lice. However, the vertical swimming velocity and a mixing parameter influenced both their vertical distribution and horizontal dispersion. The aggregation of lice along land was influenced by their response to turbulent water. The simulated infectious stages of the lice were transported on average 20 to 45 km from their release site. The simulated concentrations of infectious lice varied in synchronisation with lice infestations observed on wild fish in the area. Less than 1% of the simulated lice reached a farm site. The ratio between internal and external exposure ranged from 7 to 57%. Farms in the north of the fjord system were more exposed to lice released in the south than vice versa.
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.
In Norway, 29 fjords and 52 rivers have been designated for protection in order to prevent the infection of important populations of wild salmonids with salmon lice of farm origin. We evaluated the ...effect of this protection on the lice infection pressure for wild salmonids based on lice counts performed on wild-caught sea trout and Arctic charr inside one-third of these protected fjords (known as 'National Salmon Fjords'). Results indicate that these areas may provide a certain extent of protection against lice of farm origin, but their configuration will play a key role in their success. When the size and shape of a protected area are such that fish farms are kept at a minimum distance (calculated here as at least 30 km, but this distance is likely site-dependent), wild fish seem unaffected by the direct lice infection pressure imposed by fish farms. In contrast, the effects of small protected fjords were strongly dependent on the production pattern of the aquaculture industry in the surrounding area, and we found a clear correlation between lice levels on wild salmonids and lice production in nearby salmon farms. To establish more precise management practices, both in National Sal mon Fjords and other fjord systems along the Norwegian coast, the development and validation of accurate distribution and abundance models for the dispersion of planktonic lice larvae is needed; this could also be the basis for an area management system based on 'maximum sustainable lice loads' or 'lice quotas.'
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.
Understanding Atlantic salmon Salmo salar post-smolt coastal migration behaviour is crucial for predicting their exposure to ecological challenges such as the parasite salmon louse Lepeophtheirus ...salmonis. We compared the migration of acoustically tagged, hatchery-reared Atlantic salmon post-smolts of wild and domesticated origins from the inner, middle and outer part of a 172 km long aquaculture-intensive fjord in western Norway. Additionally, we examined if the timing of the release or treatment with an anti-parasitic drug (prophylaxis) altered migratory behaviour. We found no significant differences in mean progression rates among the 3 release locations, among genetic groups or between treatments (range: 11.5–16.9 km d−1). However, individual variation in progression rates and migratory routes resulted in large differences in fjord residence times (range: 2–39 d). Ocean-current directions during and after release affected swimming speed, progression rate and route choice, and for most post-smolts, swimming speeds were much higher than their progression rates out of the fjord. The predicted lice loads based on lice intensity growth rates on smolts held in sentinel cages throughout the fjord indicated that individuals taking >10 d to exit the fjord in periods with high infestation pressure are likely to get lethally high sea-lice infestations. We conclude that, as migratory routes of S. salar post-smolts are hard to predict and migration times can stretch up to over a month, it is important to develop aquaculture management that keeps salmon lice levels down along all potential migration routes and during the full potential migratory period.
Jansen et al. (2016; Aquacult Environ Interact 8:349−350) question the regression analysis presented in Serra-Llinares et al. (2014; Aquacult Environ Interact 5:1−16), which correlates lice ...abundances on farmed and wild fish. Jansen et al. (2016) argue that the correlation might not reflect a cause–effect relationship but be instead a mere artifact of the spatio-temporal covariance in lice abundance on farmed and wild fish driven by temperature. In this Reply Comment we revisit the analysis presented in Serra-Llinares et al. (2014) and further re-analyze our data following the statistical approach used by Helland et al. (2015; Aquacult Environ Interact 7:267−280), to rule out the potential confounding effect of temperature. We conclude that Jansen et al. (2016) were correct in conveying part of the observed correlation to the effect of temperature; however, there is solid evidence of a significant influence of lice originating from nearby farms on the observed lice abundances on wild fish, even after the effect of temperature is accounted for.
Salmon lice Lepeophtheirus salmonis Krøyer may affect survival and growth of anadromous salmonids through physiological stress and/or behavioural changes. Using acoustic telemetry tracking, we ...investigated the behaviour of 30 infected sea trout Salmo trutta throughout the summer in a fjord with very high salmon lice infection pressure. Most of the tracked sea trout adopted a movement pattern expected to suppress salmon lice infestation, as they showed a strong preference for fresh or brackish water, spending most of the time close to a river outlet or even migrating into the river. Highly infested sea trout preferred shallower depths, associated with lower salinity. The fish lost to predation stayed further away from the river outlet than non-predated fish, and were likely subjected to a stronger infection pressure. Half of the tracked group were treated with a salmon lice prophylaxis, emamectin benzoate. The effect of treatment on infestation was monitored in a separate group held in a sea cage and found to be moderate; the mortality in this group was associated with infestation by motile lice stages. In contrast, treatment was not found to have an effect on tracked fish behaviour. It is likely that some physiological and behavioural responses to high salmon lice infection pressure may be present even after a prophylaxis treatment, in particular when the treatment is given after exposure to salmon lice infection. We conclude that increased salmon lice infection pressure associated with altered salmon farming practice may have the potential to influence the marine behaviour and growth of sea trout.
Salmon louseLepeophtheirus salmonisKrøyer infection levels in both wild sea troutSalmo truttaL. and sentinel Atlantic salmonSalmo salarL. smolts were investigated inside and outside a temporary ...protected zone with limits on fish farming in the intensively farmed Romsdalsfjord system. Wild sea trout outside the protected zone had higher prevalence and significantly higher abundances than those inside the zone. Furthermore, sentinel caged Atlantic salmon smolts confirmed significant differences in infection pressure: Eresfjord (non-farmed inner part of the protected zone) had the lowest infection risk, a moderate risk was found in Langfjord (intermediately farmed middle part of the protected zone), while Karlsøyfjord (intensively farmed area outside the protected zone) had the highest infection risk. No clear bias between the near-shore and pelagic areas of the fjord were found, although significant differences between pelagic and near-shore cages were occasionally observed. Results show that small sentinel cages can be used as an alternative method to monitor the infection pressure in a fjord system. Overall, our results indicate that wild salmonids may benefit from the protection zone. However, the infection level on wild sea trout inside the temporary protected zone was higher compared to completely farm-free fjords in Norway, and infection levels likely to have a negative physiological impact on wild sea trout were found. This may indicate that the zone is too small to have the necessary protective effect against salmon lice.
The escape of fish from sea-based aquaculture facilities is regarded as a potential environmental threat and is one of the biggest challenges the industry has faced in recent years. In addition to ...preventing escapes by improving farming operations and procedures, effective and operational routines are needed for recapturing fish should an escape occur. This study investigated the post-escape dispersal of juvenile Atlantic cod Gadus morhua L. from commercial sea cages in coastal Norway using acoustic telemetry. It also assessed the efficacy of different techniques (e.g. cod pots, fyke nets, and gill nets) for recapturing escaped fish. Results suggest that escaped juvenile cod can experience a high size-related predation pressure immediately after escape due to high numbers of large fish aggregating around the cages. Consequently, the escapees rapidly disperse from the farms and seek shelter in littoral areas. Once in shallower waters, they stay relatively close (<10 km) to the farm during the first 2 mo following an escape. This suggests a high potential for recapture if the effort is focused on the littoral area and not in the immediate vicinity of the farm. No escapees were caught using live recapture methods, and a low number of recaptures (<5%) were achieved with gill nets deployed in littoral areas. Moreover, high by-catch rates and a subsequent high workload suggest this method may be inappropriate for the recapture of escapees.
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