Sustainable management of interactions between aquaculture and wild salmonid fish

The predation on seaward migrating salmonid post-smolts can be a major regulating factor for local populations of salmon and sea trout. In freshwater, and especially in the transition zone from river to the sea, avian predators have been reported to be of importance. During the study of the behaviour and migration of salmon smolts, several of the tagged smolts were eaten by gulls or predatory fish within few hours after descent. Through visual observations and stomach analyses, the predation from saithe (Pollachius virens L.) and cod (Gadus morhua L.) on seaward migrating Atlantic salmon (Salmo salar L.) and anadromous trout (Salmo trutta L.) post-smolts, the results indicate that some saithe may have specialized in preying on post-smolts. Wild and hatchery-reared post-smolts were eaten at equal rates. For smolt releases in both enhancement strategies and in telemetry studies, predation as a population regulating factor in various fjord systems should also be considered.

Groups of 2 × 2000 Atlantic salmon smolts were Carlin tagged and released below the fish trap in May at NINAs Research Station for Salmonids at Ims in Rogaland in 2003 and 2004. One group was treated with Substance EX (which protects the fish against salmon lice infection for up to 16 weeks) while the other group was unprotected. The same treatments were performed at the Talvik Research Station in Finnmark and fish were released below the fish trap in the River Halselva by the end of June. Individuals of each species were divided into two groups 1) protected against salmon lice infection with a prophylactic pharmaceutical product and 2) untreated controls. The preliminary results of the present experiment show that whilst Substance EX-treated Atlantic salmon smolts were significantly longer and heavier on return than untreated smolts, there was no significant increase in condition. Increases in size and condition may have an influence on the spawning success of the fish and therefore affect future generations. The final results of these studies are highly relevant for developing management practice both for protecting fish against salmon lice attacks and for monitoring the salmon lice pressure in fjord systems, and will aid authorities in an ongoing process evaluating the farming protection zones in Norway. Experience gained from Norway will assist similar management decisions in other regions of Europe where aquaculture is conducted.

Salmon lice infestation levels in wild sea trout were investigated inside (Eresfjord) and outside (Karlsøyfjord) a salmon farming protection zone in the Romsdalsfjord system, Norway. No new salmonid farms can be established within the protection zone and the existing ones have to operate under a strict management regime. Wild sea trout sampled outside the protection zone (Karlsøyfjord) had higher prevalence and significantly higher abundance of sea lice than sea trout inside the protection zone (Eresfjord). The results indicate that wild sea trout will benefit from a farm protection zone in the fjord. However, the infestation level observed in wild sea trout inside the protection zone was higher than in completely farm free fjords in Norway. This implies that the protection zones are too small to have the necessary effect. The results are therefore highly relevant for establishing and managing new protection zones and for suggesting alternative structures of established zones. The results have been submitted for publication, and will aid authorities in an ongoing process evaluating the farming protection zones in Norway. Experiences gained from Norway will then assist similar managerial decisions in other regions of Europe where aquaculture is conducted.

Differences in infestation risks of salmon lice were studied by sentinel cages in three connected fjord sections of the Romsdalsfjord system, Norway. Eresfjord, in the inner part of the fjord system, is without salmon farming activity. Langfjord, in the middle part, has two farms, while Karlsøyfjord, in the outer part, is exposed to extensively salmon farming. Naive hatchery reared Atlantic salmon smolts were placed in duplicated cages in both pelagic and littoral areas of all fjords. After 14 days, salmon lice infestation on the fish was analysed. Eresfjord (no farms) had lowest infestation risks, intermediate risks were found in Langfjord (two farms), while Karlsøyfjord (extensive farming activity) had highest infestation risks. No clear differences between littoral and pelagic areas of the fjord were found. Results shows that sentinel cages can be used as an alternative method to monitor salmon lice infestation pressure in fjord systems, and confirm the relationship between extensive fish farming and risks of lice infestation. The results are therefore relevant for management of salmon farming protection zones in Europe, and the results have recently been submitted for publication.

To improve our knowledge on the mortality in the first marine stage of Atlantic salmon post-smolts, we developed and used telemetry techniques to monitor the fish movements in the marine environment with different salmon lice pressure. Wild Atlantic salmon post-smolts were, similar to hatchery-reared post-smolts, not passively drifting with the current, but actively swimming and the fish moved in random directions in relation to the water current. Atlantic salmon post-smolts stayed for a longer time in the inner part than in the outer parts of the fjord system, but in contrast to sea trout, migrated through the whole fjord system into the ocean. Neither salmon lice infection nor pharmaceutical prophylaxis had any effects on survival and rate of progression of migrating Atlantic salmon post-smolts. The proportions and swimming speed of fish recorded did not differ between wild and hatchery-reared post-smolts and indicate that the behaviour of hatchery-reared Atlantic salmon post-smolts may be representative for wild salmon. This information will be used to develop a model for simulating the risk of salmon lice infestation in different areas used by Atlantic salmon post-smolts. By using this model we may better evaluate where and when the observed infestation is acceptable or not.

Sea lice infestations of wild 1SW and 2SW salmon were monitored at Strathy Point, N Scotland. This is an interceptory fishery and the sampled fish are representative of a large proportion of the multiple river populations that comprise the ICES southern European stock component. L. salmonis (mean annual abundance 17-31 lice.fish-1) was numerically more abundant than C. elongatus (3-24 lice fish-1) in all except one of the seven monitored years. Prevalence of infestation was 100% for L. salmonis and 92-100% for C. elongatus. C. elongatus has been markedly under-estimated in its importance on wild salmon in the past and the present data are the most detailed available for wild fish. A positive association in abundance of the two species on individual fish leads to the deduction that fish are equally vulnerable to either species within any one year. Variation in annual abundance patterns was analysed for a range of parameters, with host condition factor being a likely candidate. Host condition was found to vary significantly among years but there were no negative correlations between lice abundances and host condition factor for either species. That is, increased parasite abundances do not determine (reduced) host condition factor, and poor condition fish are no more vulnerable to infestation than are high condition fish.

The Final Report for Workpackage 8 includes the background to SUMBAWS, an overview of the project and our recommendations. A socio-economic perspective underpinned much of the experimental science and we focused on economic impact analyses of the aquaculture and game angling industries in Norway, Scotland and Ireland. Cash values and attributable jobs are notoriously difficult to evaluate but it is clear that angling and aquaculture both are significant sources of employment, often in remote rural areas. The experimental evidence and socio-economic modelling both pointed to interventory treatment of wild smolts (to protect them against sea lice infection) as not offering an appropriate management or conservation strategy. The objective of zero ovigerous lice on farms is attainable, but the costs probably will not be economically and environmentally acceptable as the aquaculture industry is presently structured. Physiological markers of stress of host fish in response to lice infestation allowed us to recommend sublethal threshold lice levels that are indicative of significant stress. These intensities (12 L. salmonis.fish-1 for wild sea trout smolts, and 20 L. salmonis.fish-1 for hatchery salmon smolts) could be utilized as benchmarks by both wild and farmed fish sectors in the interests of fish husbandry and welfare. Intensities <20 L. salmonis.fish-1 probably already is attained for post-smolts placed at sea on most salmon farms. For wild sea trout, we recommend that populations of fish showing <10% of individuals above the 12 L. salmonis.fish-1 threshold should be considered acceptable. For populations exceeding this proportion we would recommend a balanced judgement being taken regarding precautionary intervention if adjacent aquaculture activity is deemed a potential threat to wild salmonids. Monitoring of wild salmonid stocks is increasingly being compromised by the closure of interventory and in-river fisheries. Whilst this is beneficial from a stock conservation standpoint, it has also to be recognised that unique opportunities of obtaining invaluable scientific data on the health and performance of wild stocks is irretrievably lost. The highest priority should be given to monitoring wild stocks wherever possible.

The physiological effects of simultaneous abrupt seawater entry and sea lice infestation were investigated in wild sea trout smolts (Salmo trutta). In a series of laboratory experiments designed to mimic environmentally realistic conditions, we investigated the time course of physiological responses arising from varying intensities of infestation with the sea louse, Lepeophtheirus salmonis. Osmoregulatory, metabolic and stress physiology parameters were assessed alongside observations of general epithelial integrity in skin and gill tissue. Significant lice effects, consistent across all markers, were not apparent until L. salmonis had developed to the mobile preadult and adult stages. Mobile L. salmonis caused significant increases in plasma concentrations of chloride, glucose, lactate, cortisol and plasma osmolality and a significant reduction in haematocrit. In addition, deterioration of skin and gill epithelium and increased numbers of chloride cells, mucus cells and proliferating cells were evident. The novel application of piecewise linear models allowed, for the first time, the derivation of a threshold lice intensity, above which the host fish suffered sublethal physiological stress. This analysis identified a consistent breakpoint of 13 L. salmonis.fish-1 across several physiological markers. Identification of this threshold is an essential prerequisite in formulating effective wild fisheries management and stock conservation policy.

Sea trout smolts were PIT-tagged and a subsection were treated with a prophylactic treatment (Substance EX) designed to protect fish against sea lice infestation. Treatments took place in NW Scotland at the Manse Loch System (2003-2005) and Shieldaig (2004-2005). Despite problems with low water levels and lack of returns of fish at both sites, encouraging results were found. Whilst prophylactic treatment had no significant effect on survivorship, length or weight there were significant effects on condition factor between the treated and control groups. For example at Shieldaig in 2005 significant effects for EX-treated fish were observed using both Fultonęs K and Wr as indices of condition factor. This suggests that EX-treated fish were subject to lesser constraints on their growth at sea and this may have significant impacts on their subsequent growth to adulthood and reproduction. This is of particular importance to stock conservation because of the allometric relationship between female size and fecundity.

Based on the information and data gathered in the project, a computer model was developed of the assumed causal chain between, enhanced lice treatment, better smolt survival and improved returns to the coast, to the economic and social benefits derived from greater income and employment in the commercial and angling sectors. The main result is a series of Cost/Benefit ratios aimed at summarising the economic impacts from treatments with varying levels of effectiveness. A number of scenarios are simulated to illustrate how these ratios vary with changing assumptions. A 10 % improvement in the smolt survival, combined with a moderate response by anglers to the improved runs, would lead to costs of treatment exceeding benefits in Norway while benefits would exceed costs in Scotland and Ireland. A 20 % improvement in smolt runs would lead to benefits exceeding costs in all three countries. In all the cases examined, the benefits would exceed costs in Ireland due to the small size of the farming sector and the good returns in terms of expenditure and employment to improved angling catches. The main value of the modelling is that it clearly highlights what factors are most important for the Cost/Benefit ratio and how they interact.

Results from a study aimed at investigating the temporal and spatial distribution of Atlantic salmon and sea trout post-smolt in a Norwegian fjord system were used for developing a multivariate model for predicting the pattern of the salmon lice infestations on wild salmonids in different parts of the fjord. The model can be used to evaluate if observed infestation levels in fjord systems are acceptable or safe, i.e. if the infestation levels are harmful or not for wild salmonids. Furthermore, the model also enables prediction of the risks and levels of smolts being infested by salmon lice at specific sites in a fjord. Hence, the model can be used to predict effects of establishment of fish farms on new sites in fjords and coastal areas by identifying areas where the negative effects on wild salmonid populations will be minimal. Experiences gained from Norway will then assist similar managerial decisions in other regions of Europe where aquaculture is conducted.

This result shows the economic significance the Atlantic salmon (Salmo salar L.) in three countries: Ireland, Norway and Scotland. These countries are among the most important countries for conservation and use of Atlantic salmon. The overall gross value of Atlantic salmon was estimated 1,900m., of which approximately 1,500m. relates to salmon aquaculture, 350m.to salmon angling and 10 m. to commercial netting of salmon. When gross values are corrected for import content, displacement effects and multiplier effects, the overall economic impact to the national economies was estimated at a total of 1,320m., of this 1,110m. in aquaculture, 200m. in salmon angling and 8m. in commercial fishing. Overall full-time job equivalents were estimated at 22,000 FTE, of which 15,000 were in the farming industry, 6,400 in angling tourism and 460 in commercial fishing. There are striking differences between the businesses in each country. Norway has a huge farming business and a significant angling tourism sector. Scotland has a significant farming business and a well-developed angling tourism. Ireland has a substantial commercial fishery, which has only limited economic impact, and a small farming business compared to Norway and Scotland. Generally, commercial fishing is insignificant in all countries compared to the other sectors.

The physiological consequences of premature return to freshwater in response to sea lice infestation were examined in laboratory studies by infesting seawater-acclimated wild sea trout smolts with L. salmonis and either, 1) Maintaining fish in seawater or 2) Returning fish to freshwater 19 days post infestation (with respective non-infested controls for both groups). Following freshwater return, the mean infestation intensity and number of mortalities were significantly reduced compared to fish maintained in seawater. Plasma concentrations of chloride and lactate were significantly higher in the seawater infested group than in all other groups after 21 dpi. Liver glycogen content was significantly decreased following sea lice infestation and remained at a reduced concentration in the seawater infested group. There was evidence of recovery of liver glycogen content following return to freshwater. Plasma cortisol concentration increased in both infested groups at 14 dpi compared to non infested controls, but returned to control levels in fish returned to freshwater. These results suggest that premature return to freshwater confers considerable physiological benefit and increased survivorship of sea trout post-smolts previously infested with L. salmonis.

Significant between-year variations in condition factor, CF (= plumpness ) of wild salmon were noted during the analyses of sea lice infestations at Strathy Point (1998-2005). Having confirmed that parasite infestations have no relation to host CF we extended the analyses of CF back to 1993 by incorporating previous Fisheries Research Services (FRS) monitoring data. Detailed analyses with colleagues at FRS Aberdeen have demonstrated clear and geographically defined negative correlations between recent ocean climate warming at the NE Atlantic feeding grounds and reduced CF of salmon. This result runs counter to the emerging paradigm of warmer ocean surface temperatures being beneficial to wild salmon. We have also confirmed that the annual time-series of variation in CF at Strathy Point (multiple stocks) is consistent with a contemporary time-series for a single, definable stock for an east Scotland river. That is, CF varies significantly among years, is set for entire year classes, and applies equally across multiple stocks within the southern European 1SW population component; the effects of global climate warming are, therefore, detectable across all stocks within each year and indicate a major, widespread, chronic and predictable negative impact on fish feeding, growth success and perhaps also their survival to return to spawn. These results have fundamental implications for wild salmon management because reduced condition/size of spawning adults will lead to reduced egg deposition. We estimate a 13-22% difference in fecundity for females in good versus bad years.

Differences in salmon lice infection in Atlantic salmon, sea trout, and Arctic char were studied in the Altafjord system. Fjord migrating Atlantic salmon smolts were only captured during June/July and probably left the fjord after this. Most smolts were uninfected with salmon lice, except a few fish in outer fjord areas, which carried low numbers of lice. Pelagic feeding sea trout and Arctic char had surprisingly high infection levels in June/July, suggesting that their pelagic feeding pattern expose them for high risks of infection. In contrast, littoral feeding brown trout and Arctic char had very low infection during June, slightly increasing in July, and peaking in August. These observations indicate that Atlantic salmon may have a mismatch between time of lice infection and their fjord migration in northern fjords. In contrast, brown trout and Arctic char feed within the fjords throughout the summer and consequentially have higher risk of harmful infections. These results are highly relevant for lice management in Norway, as measures taken have focused on reducing infection pressure on wild Atlantic salmon smolts. The results have recently been submitted for publication, and will aid authorities in an ongoing process evaluating the Norwegian Action Plan Against Lice on Salmonids.

Similarly to previous studies in result 11 with wild seatrout smolts, we conducted a further series of laboratory experiments infesting farmed Atlantic salmon with varying intensities of L. salmonis. This allowed us to determine, for the first time, a threshold level of sea lice infestation which induces physiological stress for farmed Atlantic salmon. Results from the high infestation group(25 lice.fish-1) indicated significant decreases in haematocrit and increases in plasma concentrations of chloride, glucose lactate and cortisol and plasma osmolarity, once sea lice had reached the mobile stages. Piecewise linear statistical modelling again allowed the objective determination of abrupt changes in these physiological markers, attributable to the intensity of L. salmonis infestation on individual fish.20 L. salmonis.fish-1 was a consistent breakpoint across several physiological markers indicating the onset of physiological stress above this lice intensity. This allowed identification of 20 as an overall threshold lice burden. This important information will be made available to the aquaculture industry and contribute to management and policy development.

In the present study SLICE®, the commercial in-feed treatment against sea lice, was used to treat hatchery-reared salmon smolts to determine its efficacy in protecting salmon smolts from sea lice infestation and determine its effects on their marine survival. Results from the present study reveal a significant difference in the return rate of SLICE®-treated and control salmon smolts in three of four release groupings; these data suggest reduced mortality of smolts in the treated groups and hence perhaps protection from sea lice infestation in aquaculture bays. Given the presence of a significant source of sea lice infestation from marine salmon farms in both bays in the present study, increased mortality of salmon smolts can be expected. The greater weight of returning adult salmon treated with SLICE® could be expected from enhanced growth of fish protected from high sea lice infestation. The findings of the present study demonstrate the potential impact of marine salmon farms on salmon post-smolt marine mortality in very short coastal bays in the West of Ireland.

Field studies were undertaken over two years to examine sea trout population dynamics in two rivers in the west of Ireland entering bays with salmon aquaculture. SLICE®, the commercial in-feed treatment against sea lice, was used to treat wild sea trout to determine its efficacy in protecting smolts from sea lice infestation and determine its effects on their marine survival. Sea trout smolts were held in tanks and divided into experimental (SLICE® treated) and control groups. The SLICE® group were administered small quantities of SLICE® treated food at regular intervals over a ten day period while the control group were fed untreated pellets. Although a large number of sea trout smolts were included in these experimental and control groups, few were recorded returning in upstream traps. Equal numbers of fish from both groups were recorded from the Gowla, whereas a greater number of SLICE®-treated fish returned to the Invermore in 2004. In 2005 sufficient fish returned from control and experimental groups on the Gowla group to allow statistical analysis. There were no statistical differences between SLICE®-treated and control groups in terms of growth, weight, chalimus abundance, total lice level or days spent at sea.

To improve our knowledge on the mortality in the first marine stage of sea trout post-smolts, we developed and used telemetry techniques to monitor the fish movements in the marine environment with different salmon lice pressure. Manual tracking of wild sea trout post-smolts showed that they were actively swimming in the fjord. The actual movements of sea trout were in all compass directions, with no systematic patterns. By automatic tracking it was shown that the rate of progression of wild sea trout smolts was different during their outward migration from the outlet of the home river. Both wild- and hatchery-reared sea trout post-smolts had a slower rate of progression between different receiver sites than wild- and hatchery-reared Atlantic salmon post-smolts and remained to a larger extent compared to Atlantic salmon post-smolts in the inner part of the fjord system. It was also shown that the wild sea trout post-smolts stayed rather closer to the shore than in the middle of the fjord. This information will be used to develop a model for simulating the risk of salmon lice infestation in different areas used by sea trout post-smolts. By using this model we may better evaluate where and when the observed infestation is acceptable or not.

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