The salmon louse Lepeophtheirus salmonis is a common ectoparasite of both farmed and wild salmonids in the marine environment, and can have a significant negative effect on the survival and growth of ...its host. To facilitate development of models of salmon lice population dynamics, we review the available experimental information on its demographic rates and highlight areas where further research is needed. For all stages, the reduced minimum development time of a stage with increasing water temperature (T) was well described by Belehradek's function. However, detailed experimental studies of the development of the parasitic stages at low (T < 7 degree C) and high (T > 15 degree C) water temperatures are needed to cover the whole range of water temperatures experienced in the wild. Little information was available on mortality rates and distributions of developmental times after the initial minimum developmental times. These parameters could only be estimated for a narrow temperature range, but the available estimates suggested that distributions of development times may be assumed to be constant with respect to temperature. Factors affecting female fecundity are presently poorly understood, with a level of unexplained variability in both average egg numbers per string and egg viability, which demands further investigations. In addition, experiments on possible density-dependent effects on salmon lice fecundity and survival are required.
Coastal aquaculture is a globally expanding enterprise. Currently, 1200 salmon farms operate in coastal Norway, yet their capacity to aggregate and subsequently modify wild fish distributions is ...poorly known. Aggregations of wild fish at 9 farms and 9 control locations were counted on 3 separate days in June to August 2007. On each sampling occasion, 6 counts were made at 5 distinct depth-strata at each farm and control location. Wild fish were 1 to 3 orders of magnitude more abundant at farms than at control sites, depending on the location. Gadoid fish (Pollachius virens,Gadus morhuaandMelanogrammus aeglefinus) dominated farm-associated assemblages and were present across a wide range of sizes, from juveniles to large adults. Estimated total farm-aggregated wild fish biomass averaged 10.2 metric tonnes (t) per farm across the 9 farms (range: 600 kg to 41.6 t). Applied across the geographical range of Norway’s 1200 salmon farms, our estimates indicate that salmon farms attract and aggregate over 12 000 t of wild fish into a total of just 750 ha of coastal waters on any given day in summer. Possible consequences of these persistent, substantial aggregations of wild fishes at farms include a heightened potential for the transfer of pathogens from salmon farms to wild fish and among adjacent salmon farms, and altered availability of wild fish to fisheries. Restrictions on fishing in the immediate surrounds of salmon farms may avoid farms acting as ecological traps, particularly for species with depressed populations such asG. morhua, which are highly attracted to farms.
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.
Marine fish farms are widespread in coastal waters throughout the world, yet how they modify the movement patterns of wild fish species is largely unknown. We determined the spatiotemporal ...distribution of saithePollachius virensin a fjord system with intensive salmon cage aquaculture in Norway. Abundances of 8000 to 18 000 saithe were estimated around 2 salmon farms in the fjord using an underwater video system. Residence of saithe around fish farms and movements among farms and throughout the fjord were studied using implanted acoustic transmitters and an extensive array of automatic receivers. Of the saithe equipped with acoustic tags, 63% were observed daily at one or more of the 3 farms in the fjord over a 3 mo period. When resident at a farm, saithe spent 8 to 10 h d–1close to the sea-cages. Periods of residence at specific farms were interspersed with rapid and frequent movements to adjacent farms 1.6 to 4.7 km away. Of 24 tagged saithe, 15 moved among farms 2 to 21 times during the 3 mo period. If the movement patterns of the tagged fish are representative of the movements of untagged saithe, we estimate that fish from 2 different farms resulted in a total (±SE) of 167 112 ± 41 764 and 7768 ± 1831 inter-farm movements during the 3 mo period. Thus, fish farms should be considered as connected, not only through ocean currents, but also through wild fish movements. If saithe share pathogens with farmed salmonids, their behaviours imply that they have the potential to act as vectors for diseases and parasites among salmon farms.
High salmon lice density is a threat to wild and farmed salmonid fish in Norway. To assess and identify areas for high salmon lice infestation pressure, continuous monitoring is necessary. The ...national Norwegian salmon lice monitoring program has until now been based on sampling and counting of salmon lice on wild salmonids and smolts in sentinel cages. The number of lice eggs hatched into the water masses, the relatively long-lasting pelagic life stages and the high spatiotemporal variability of the ocean currents all have a major influence on the local infestation pressure. Thus, a new monitoring system including a numerical ocean model with high temporal and spatial resolution has been established. The plan is that the model will complement, direct or replace parts of the logistically demanding and costly field-based monitoring program. In this study, we evaluate the model's ability to realistically simulate the spread and density of pelagic salmon lice. Results from a 4 yr model run are presented, and the simulated density compared to the mean abundance on smolts in sentinel cages. The comparison demonstrates that the modeled salmon lice density corresponds well with the observational data. Within a slight shift in space, the model matches the observed lice infestation class values in 78% of the cases. Using the modeled lice density, a binary forecast system is proposed to predict areas of elevated lice infestation pressure. For the 2015 test case, the prediction system is correct (elevated/non-elevated) in 32 of 36 cases (89%).
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.'
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.
To be able to design effective management to alleviate wild fish from parasite infestation pressure from fish farms, it is pivotal to understand when post-smolts migrate past areas of potential ...exposure to salmon lice Lepeophtheirus salmonis. Here, data from release groups of coded-wire-tagged Atlantic salmon Salmo salar smolts and their subsequent recaptures in a trap net in the outer fjord 12 to 97 km from the various release sites were used to estimate the smolts' progression rate and their arrival time in an outer fjord in Norway. The arrival time estimates to the outer fjord are compared with modelled infestation pressure from local fish farms. The overall progression rate varied from 0.8 to 31.2 km d⁻¹ (0.05 to 2.20 body lengths s⁻¹), with mean and median values of 8.8 and 7.8 km d⁻¹, respectively (0.60 and 0.54 body lengths s⁻¹). The progression rate varied with water discharge from the rivers into the fjords, fish length, condition factor and smolt origin. Simulated arrival time and capture of wild smolts suggest that smolts from the different rivers arrive in the outer fjord system with a difference of up to 4 wk. The arrival time for the rivers with the longest migration was estimated to be from mid-May throughout June. Infestation pressure from fish farms increased from the beginning of June in 2 of 3 study years, suggesting that an increase in lice exposure from fish farms will overlap with smolts from late-migrating populations in some but not all years.
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.
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.
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