Resource: Scientific weight of evidence against open net-pen salmon farms

Over the last two decades, a significant body of peer-reviewed evidence has been published in science journals on open net-pen salmon farms and their negative impacts on wild fish in British Columbia. This body of evidence concludes that salmon farms can amplify harmful pathogens, elevate their levels on wild fish, harm wild fish and negatively impact their populations. 

Despite this body of scientific evidence, Watershed Watch staff were recently informed by DFO aquaculture management staff (who are advised by DFO science staff) that the weight of scientific evidence indicates salmon farms do not have a negative population-level impact on wild salmon. 

Further, in January 2023 DFO released a Science Response Report written by aquaculture management and aquaculture science staff that concludes parasitic sea lice on wild juvenile salmon are not significantly associated with sea lice from nearby salmon farms. This report was met with harsh criticism in an open letter from 16 prominent Canadian academics. The open letter details seven criticisms of the report including DFO’s failure to acknowledge and give weight to the large body of peer-reviewed published research that has found significant statistical relationships between parasitic lice on farms and wild salmon. 

This blindness to outside science (and to internal science that reports salmon-farm-linked threats to wild salmon; e.g., DiCiccio et al. 2018; Long et al. 2018) indicates that some in DFO still have a pro-salmon farming industry bias, as Justice Bruce Cohen concluded in the federal Commission of Inquiry into the Decline of Fraser River sockeye. This bias continues to put B.C. wild salmon populations at risk. 

In response to DFO’s bias, we list 39 studies in this body of research below, along with key quotations. Eight studies that report salmon farms are associated with wild salmon population-level impacts are listed first and denoted by “ * ”.

B.C. studies reporting a negative population-level impact on wild fish

* Peacock, S., M. Krkošek, S. Proboszcz, C. Orr, and M. Lewis. 2013. Cessation of a salmon decline with control of parasites. Ecological Applications. 23:606-620. https://doi.org/10.1890/12-0519.1

“Management actions, such as fallowing farms along the migration routes of juvenile salmon and winter treatments with parasiticide, lowered the abundance of sea lice on farm salmon, and therefore reduced infection of wild salmon. Finally, there was a strong negative relationship between pink salmon survival and sea lice infection of juveniles, implicating that efforts by the salmon farming industry to reduce sea lice levels during the wild salmon out-migration have positive implications for wild salmon survival and productivity.”

* Krkošek, M., R. Hilborn. 2011. Sea lice infestations and productivity of pink salmon populations in the Broughton Archipelago. Canadian Journal of Fisheries and Aquatic Sciences. 68:17-29. https://doi.org/10.1139/F10-137 

“Populations exposed to salmon farms (those from the Broughton Archipelago) show a sharp decline in productivity during sea lice infestations relative to pre-infestation years. Unexposed populations (comprising four management areas) did not experience a change in productivity during infestation years and had similar productivity to exposed populations before infestations.”

* Krkošek, M., B.M. Connors, A. Morton, M.A. Lewis, L.M. Dill, R. Hilborn. 2011. Effects of parasites from salmon farms on productivity of wild salmon. Proceedings of the National Academy of Sciences of the United States of America.108:14700–14704. https://doi.org/10.1073/pnas.1101845108 

“Several recent studies have reached contradictory conclusions on whether the spread of lice from salmon farms affects the productivity of sympatric wild salmon populations. We analyzed recently available sea lice data on farms and spawner–recruit data for pink (Oncorhynchus gorbuscha) and coho (Oncorhynchus kisutch) salmon populations in the Broughton Archipelago and nearby regions where farms are not present. Our results show that sea lice abundance on farms is negatively associated with productivity of both pink and coho salmon in the Broughton Archipelago. Other studies have also suggested that lice from farms do not affect the survival of wild salmon in the Broughton Archipelago (30, 31). Beamish et al. (31) interpreted one observation of exceptionally high survival of pink salmon as evidence that lice do not affect survival of wild salmon. However, the cohort studied in ref. 31 was not subjected to major sea lice infestation but rather to a management intervention that fallowed farms along a primary migration route and reduced sea lice abundance on wild juvenile salmon 10-fold relative to other observed infestations (36). Other extrapolations of laboratory studies suggesting louse-induced mortality of wild salmon is negligible (30) ignore the three orders of magnitude difference in infection period between laboratory studies (minutes or hours) and field conditions (months) (37), sublethal effects of infection on predation risk (10), and other indirect effects of lice that could affect survival such as reduced body growth (38).”

* Connors, B.M., D.C. Braun, R.M. Peterman, A.B. Cooper, J.D. Reynolds, L.M. Dill, G.T. Ruggerone, and M. Krkošek. 2012. Migration links ocean-scale competition and local ocean conditions with exposure to farmed salmon to shape wild salmon dynamics. Conservation Letters 5(2012): 304-312. https://doi.org/10.1111/j.1755-263X.2012.00244.x

“Our findings suggest that the long-term decline is primarily explained by competition with pink salmon, which can be amplified by exposure to farmed salmon early in sockeye marine life, and by a compensatory interaction between coastal ocean temperature and farmed-salmon exposure.”

* Connors, B.M., M. Krkošek, J. Ford, L.M. Dill. 2010. Coho salmon productivity in relation to salmon lice from infected prey and salmon farms. Journal of Applied Ecology. 47:1372-1377. https://doi.org/10.1111/j.1365-2664.2010.01889.x 

“During a period of recurring salmon louse infestations in a region of open net-pen salmon farms, coho salmon productivity (recruits per spawner at low spawner abundance) was depressed approximately sevenfold relative to unexposed populations. Alternate hypotheses for the observed difference in productivity, such as declines in coho prey, perturbations to freshwater habitat or stochasticity, are unlikely to explain this pattern.”

* Krkošek, M., A. Morton, J. Volpe, M. Lewis. 2009. Sea lice and salmon population dynamics: Effects of exposure time for migratory fish. Proceedings of the Royal Society of London Series B. 276:2819-2828. https://doi.org/10.1098/rspb.2009.0317

“For Pacific salmon and the parasitic sea louse, Lepeophtheirus salmonis, analysis of the exposure period may resolve conflicting observations of epizootic mortality in field studies and parasite rejection in experiments. This is because exposure periods can differ by 2-3 orders of magnitude, ranging from months in the field to hours in experiments. We developed a mathematical model of salmon-louse population dynamics, parametrized by a study that monitored naturally infected juvenile salmon held in ocean enclosures. Analysis of replicated trials indicates that lice suffer high mortality, particularly during pre-adult stages. The model suggests louse populations rapidly decline following brief exposure of juvenile salmon, similar to laboratory study designs and data. However, when the exposure period lasts for several weeks, as occurs when juvenile salmon migrate past salmon farms, the model predicts that lice accumulate to abundances that can elevate salmon mortality and depress salmon populations.” 

* Ford, J.S., R.A. Myers. 2008. A global assessment of salmon aquaculture impacts on wild salmonids. PLoS Biol 6:e33. https://doi.org/10.1371/journal.pbio.0060033

“Through a meta-analysis of existing data, we show a reduction in survival or abundance of Atlantic salmon; sea trout; and pink, chum, and coho salmon in association with increased production of farmed salmon. In many cases, these reductions in survival or abundance are greater than 50%. Meta-analytic estimates of the mean effect are significant and negative, suggesting that salmon farming has reduced survival of wild salmon and trout in many populations and countries.”

* Krkošek, M., J.S. Ford, A. Morton, S. Lele, R.A. Myers, and M.A. Lewis. 2007. Declining wild salmon populations in relation to parasites from farm salmon. Science. 318:1772-1775. https://doi.org/10.1126/science.1148744 

“We how that recurrent louse infestations of wild juvenile pink salmon (Oncorhynchus gorbuscha), all associated with salmon farms, have depressed wild pink salmon populations and placed them on a trajectory toward rapid local extinction. The louse-induced mortality of pink salmon is commonly over 80% and exceeds previous fishing mortality. If outbreaks continue, then local extinction is certain, and a 99% collapse in pink salmon population abundance is expected in four salmon generations. These results suggest that salmon farms can cause parasite outbreaks that erode the capacity of a coastal ecosystem to support wild salmon populations.”

Studies reporting salmon farms can amplify harmful pathogens, elevate their levels on wild fish, harm wild fish and negatively impact their populations

Bass, A.L., A.W. Bateman, B.M. Connors, B.A. Staton, E.B. Rondeau, G.J. Mordecai, A.K. Teffer, K.H. Kaukinen, S. Li, A.M. Tabata, D.A. Patterson, S.G. Hinch, and K.M. Miller. 2022. Identification of infectious agents in early marine Chinook and coho salmon associated with cohort survival. FACETS. 7: 742-773. https://doi.org/10.1139/facets-2021-0102

“While a variety of pathogens had moderate to strong negative correlations with body condition or survival for one host species in one season, we found that Tenacibaculum maritimum and Piscine orthoreovirus had consistently negative associations with body condition in both host species and seasons and were negatively associated with survival for Chinook salmon collected in the fall and winter…Our results, further contribute to this body of evidence and underscore the need for a greater understanding of the pathogenicity of PRV-1a for wild Pacific salmonids. This is of heightened concern considering PRV-1a is ubiquitous on Atlantic salmon farms in British Columbia (DFO 2019), and there is both genomic and epidemiological evidence for continual transmission between farmed and wild salmon, particularly in the fall–winter period (Mordecai et al. 2021b).”

Bateman, A.W., A.K. Teffer, A. Bass, T. Ming, K. Kaukinen, B.P.V. Hunt, M. Krkošek, and K.M. Miller. Atlantic salmon farms are a likely source of Tenacibaculum maritimum infection in migratory Fraser River sockeye salmon. 2022. Canadian Journal of Fisheries and Aquatic Sciences. 79: 1225-1240. https://doi.org/10.1139/cjfas-2021-0164 

“Our data show a clear peak in T. maritimum detections in the Discovery Islands region of British Columbia, where sockeye migrate close to salmon farms. Using well-established differential equation models to describe sockeye migration and bacterial infection, fit to detection data, we assessed support for multiple hypotheses describing farm- and background-origin infection. Our best models (with 99.8% empirical support) describe constant background infection pressure, except around Discovery Islands salmon farms, where farm-origin infection pressure peaked at 12.7 (approximate 95% CI: 4.5 to 31) times background levels. Given the severity of associated disease in related species and the imperilled nature of Fraser River sockeye, our results suggest the need for a more precautionary approach to managing farm–wild interactions in sockeye salmon.”

Mordecai, G.J., K.M. Miller, A.L. Bass, A.W. Bateman, A.K. Teffer, J.M. Caleta et al. 2021. Aquaculture mediates global transmission of a viral pathogen to wild salmon. Science Advances. 7: science.org/doi/10.1126/sciadv.abe2592 

“Our phylogeographic analyses of PRV-1 suggest that development of Atlantic salmon aquaculture facilitated spread from Europe to the North and South East Pacific. Phylogenetic analysis and reverse transcription polymerase chain reaction surveillance further illuminate the circumstances of emergence of PRV-1 in the North East Pacific and provide strong evidence for Atlantic salmon aquaculture as a source of infection in wild Pacific salmon. PRV-1 is now an important infectious agent in critically endangered wild Pacific salmon populations, fueled by aquacultural transmission.”

Long, A., K.M. Garver, and S.R.M. Jones. 2019. Synergistic osmoregulatory dysfunction during salmon lice (Lepeophtheirus salmonis) and infectious hematopoietic necrosis virus co-infection in sockeye salmon (Oncorhynchus nerka) smolts. Journal of Fish Diseases. 42:869-882. https://onlinelibrary.wiley.com/doi/10.1111/jfd.12989 

“Co-infected salmon had elevated osmoregulatory indicators and lowered haematocrit values as compared to the uninfected control. Co-infection did not influence the expression of genes associated with the host response to L. salmonis. Therefore, we conclude that the reduced survival in co-infected sockeye salmon resulted from the osmoregulatory consequences of the sea lice infections which were amplified due to infection with IHNV.” 

Di Cicco E., H.W. Ferguson, K.H. Kaukinen, A.D. Schulze, S. Li, A. Tabata, et al. 2018. The same strain of Piscine orthoreovirus (prv-1) is involved in the development of different, but related, diseases in Atlantic and Pacific salmon in British Columbia. Facets. 3:599–641.  https://doi.org/10.1139/facets-2018-0008

“We examined the developmental pathway of HSMI and jaundice/anemia associated with PRV-1 in farmed Atlantic and chinook (Oncorhynchus tshawytscha (Walbaum, 1792)) salmon in BC, respectively. In situ hybridization localized PRV-1 within developing lesions in both diseases. The two diseases showed dissimilar pathological pathways, with inflammatory lesions in heart and skeletal muscle in Atlantic salmon and degenerative-necrotic lesions in kidney and liver in chinook salmon, plausibly explained by differences in PRV load tolerance in red blood cells. Viral genome sequencing revealed no consistent differences in PRV-1 variants intimately involved in the development of both diseases suggesting that migratory chinook salmon may be at more than a minimal risk of disease from exposure to the high levels of PRV occurring in salmon farms.”

Godwin S.C., M. Krkošek, J.D. Reynolds, L.A. Rogers, L.M. Dill. 2018. Heavy sea louse infection is associated with decreased stomach fullness in wild juvenile sockeye salmon. Canadian Journal of Fisheries and Aquatic Sciences. 75:1587–1595. https://cdnsciencepub.com/doi/abs/10.1139/cjfas-2017-0267 

“Caligus clemensi comprised 98.6% of the pre-adult and adult lice and 86.5% of the copepodites (freshly attached juvenile lice); the rest were Lepeophtheirus salmonis. We found that infection status was an important predictor of relative stomach fullness for juvenile sockeye (wet stomach content mass divided by body mass), as indicated by mixed-effects model selection, and that highly infected fish had 17% ± 8% lower relative stomach fullness than did lightly infected fish.” 

Long, A., K.A. Garver, S.R.M. and Jones. 2018. Differential effects of adult salmon lice Lepeophtheirus salmonis on physiological responses of Sockeye salmon and Atlantic salmon. Journal of Aquatic Animal Health. 31:75–87. https://doi.org/10.1002/aah.10053 

“Relative to Atlantic Salmon, infection with L. salmonis caused a profound physiological impact to Sockeye Salmon characterized by loss of osmoregulatory integrity and a stress response. This work provides the first comprehensive report of the physiological consequences of infections with adult L. salmonis in sockeye salmon smolts and helps to further define the mechanisms of susceptibility in this species.”

Morton A., R. Routledge, S. Hrushowy, M. Kibenge, F. Kibenge. 2017. The effect of exposure to farmed salmon on Piscine orthoreovirus infection and fitness in wild Pacific salmon in British Columbia, Canada. PLoS ONE 12: e0188793. https://doi.org/10.1371/journal.pone.0188793

“These results suggest that PRV transfer is occurring from farmed Atlantic salmon to wild Pacific salmon, that infection in farmed salmon may be influencing infection rates in wild salmon, and that this may pose a risk of reduced fitness in wild salmon impacting their survival and reproduction.”

Godwin S.C., L.M. Dill, M. Krkošek, M.H.H. Price, J.D. Reynolds. 2017. Reduced growth in wild juvenile sockeye salmon Oncorhynchus nerka infected with sea lice. Journal of Fish Biology. 91:41–57. https://doi.org/10.1111/jfb.13325 

“Over 98% of the sea lice proved to be C. clemensi and the fish that were highly infected grew more slowly than uninfected individuals. Larger fish also grew faster than smaller fish. Finally, there was evidence of an interaction between body size and infection status, indicating the potential for parasite-mediated growth divergence.”

Peacock, S., M. Krkošek, A. Bateman and M. Lewis. 2015. Parasitism and food web dynamics of juvenile Pacific salmon. Ecosphere 6: art264. https://doi.org/10.1890/ES15-00337.1

“Our experiments show that coho salmon predators (O. kisutch) selectively prey on pink salmon and on parasitized prey. Preference for pink salmon increased slightly when prey were parasitized by sea lice, although there was considerable uncertainty regarding this result. Despite this uncertainty, we show that even the small increase in preference that we observed may be biologically significant. We calculate a critical threshold of pink salmon abundance above which chum salmon may experience a parasite-mediated release from predation as predation shifts towards preferred prey species.”

Godwin, S.C., L.M. Dill, J.D. Reynolds, Krkošek, M. 2015. Sea lice, sockeye salmon, and foraging competition: Lousy fish are lousy competitors. Canadian Journal of Fisheries and Aquatic Sciences. 72:1113-1120. https://doi.org/10.1139/cjfas-2014-0284 

“Highly infected sockeye were 20% less successful at consuming food, on average, than lightly infected fish. Competitive ability also increased with fish body size. Our results provide the first evidence that parasite exposure may have negative indirect effects on the fitness of juvenile sockeye salmon and suggest that indirect effects of pathogens may be of key importance for the conservation of marine fish.”

Rees, E., S. St-Hilaire, S. Jones, M. Krkošek, S. DeDominicis, M. Foreman, T. Patanasatienkul and C. Revie. 2015. Spatial patterns of sea lice infection among wild and captive salmon in western Canada. Landscape Ecology. 30, 989-1004. https://doi.org/10.1007/s10980-015-0188-2 

“Fish length, sampling year and method were strong explanatory factors. Infection was greatest in higher salinity water. Farmed and wild juvenile salmon infection levels were correlated, on average, within 30 km.”

Peacock, S., B. Connors, M. Krkošek, J. Irvine, and M. Lewis. 2014. Can reduced predation offset negative effects of sea louse parasites on chum salmon? Proceedings of the Royal Society B. 281: 20132913. https://doi.org/10.1098/rspb.2013.2913

“In Pacific Canada, sea lice can spread from farmed salmon to migrating juvenile wild salmon. Low numbers of sea lice can cause mortality of juvenile pink and chum salmon. For pink salmon, this has resulted in reduced productivity of river populations exposed to salmon farming. However, for chum salmon, we did not find an effect of sea louse infestations on productivity, despite high statistical power. Motivated by this unexpected result, we used a mathematical model to show how a parasite-induced shift in predation pressure from chum salmon to pink salmon could offset negative direct impacts of sea lice on chum salmon. This shift in predation is proposed to occur because predators show an innate preference for pink salmon prey.”

Jakob, E., T. Sweeten, W. Bennett, and S. Jones. 2013. Development of the salmon louse, Lepeophtheirus salmonis and its effects on juvenile sockeye salmon Oncorhynchus nerka. Diseases of Aquatic Organisms. 106:217-227. https://doi.org/10.3354/dao02642 

“In conclusion, the mild inflammatory response of juvenile sockeye salmon to infection with Lepeophtheirus salmonis was most similar to that of Chinook salmon, whereas the loss of lice during mobile stages of development and the evidence for somatic perturbations were similar to those reposted for Atlantic salmon, chum salmon and sea trout.”

Krkošek, M., B. Connors, P. Mages, S. Peacock, H. Ford, J. Ford, A. Morton, J. Volpe, R. Hilborn, L. Dill, and M. Lewis. 2011. Fish farms, parasites, and predators: Implications for salmon population dynamics. Ecological Applications. 21:897-914. https://doi.org/10.1890/09-1861.1

“The experimental results indicate that lice may increase the rate of prey capture but not the handling time of a predator. Based on this result, we developed a mathematical model of sea lice and salmon population dynamics in which parasitism affects the attack rate in a type II functional response. Analysis of the model indicates that: (1) the estimated mortality of wild juvenile salmon due to sea lice infestation is probably higher than previously thought; (2) predation can cause a simultaneous decline in sea louse abundance on wild fish and salmon productivity that could mislead managers and regulators; and (3) compensatory mortality occurs in the saturation region of the type II functional response where prey are abundant because predators increase mortality of parasites but not overall predation rates. These findings indicate that predation is an important component of salmon–louse dynamics and has implications for estimating mortality, reducing infection, and developing conservation policy.”

Morton, A., R. Routledge, A. McConnell, and M. Krkošek. 2011. Sea lice dispersion and salmon survival in relation to salmon farm activity in the Broughton Archipelago. ICES Journal of Marine Science. 68:144–156. https://doi.org/10.1093/icesjms/fsq146  

“Results indicate that fallowing reduces the abundance and flattens the spatial distribution of lice relative to that expected in areas without farms. Active farms remained the primary source of lice, but transmission was reduced 100-fold relative to previous epizootics in the study area. On the migration route containing active farms, 50% of the juvenile salmon showed evidence of louse damage to surface tissues and the estimated direct louse-induced mortality was approx 10%, not including indirect effects of infection on predation risk or competition.”

Price, M.H.H., S.L. Proboszcz, R.D. Routledge, A.S. Gottesfeld, C. Orr, J.D. Reynolds. Sea louse infection of juvenile sockeye salmon in relation to marine salmon farms on Canada’s west coast. PLoS ONE 6: e16851. https://doi.org/10.1371/journal.pone.0016851

“Fraser River sockeye migrating through a region with salmon farms hosted an order of magnitude more sea lice than Skeena River populations, where there are no farms. Lice abundances on juvenile sockeye in the salmon farm region were substantially higher downstream of farms than upstream of farms for the two common species of lice: Caligus clemensi and Lepeophtheirus salmonis, and changes in their proportions between two years matched changes on the fish farms.”

Connors, B.M., N.B. Hargreaves, S.R.M. Jones, and L.M. Dill. 2010. Predation intensifies parasite exposure in a salmonid food chain. Journal of Applied Ecology. 47: 1365-1371. https://doi.org/10.1111/j.1365-2664.2010.01887.x

“We used a hierarchical modelling approach to examine the abundance and sex ratio of salmon lice on juvenile pink and coho salmon, collected from a region of salmon aquaculture during sea louse infestations, to test the hypothesis that trophic transmission of salmon lice increases infection on coho that feed upon infected pink salmon prey. As predicted, coho had higher adult and pre-adult louse abundance than their pink salmon prey, and louse abundance was more adult male biased on predators than sympatric prey. We estimate that trophic transmission accounts for 53–67% of pre-adult and adult louse infection on coho.”

Price, M.H.H., A. Morton, J.D. Reynolds. 2010. Evidence of farm-induced parasite infestations on wild juvenile salmon in multiple regions of coastal British Columbia, Canada. Canadian Journal of Fisheries and Aquatic Sciences. 67:1925–1932 https://doi.org/10.1139/F10-105 

“We compared sites of low and high exposure to farms and included an area without farms (Bella Bella) to assess baseline infection levels. Louse prevalence and abundance were lowest and most similar to natural baseline levels at low-exposure sites and highest at high-exposure sites in all farm regions. A significantly greater proportion of the lice were Lepeophtheirus salmonis at high-exposure sites. Exposure to salmon farms was the only consistently significant factor to explain the variation in prevalence data, with a secondary role played by salinity. Our results support the hypothesis that salmon farms are a major source of sea lice on juvenile wild salmon in salmon farming regions and underscore the importance of using management techniques that mitigate threats to wild stocks.” 

Jones, S.R.M., B. Hargreaves. 2009. Infection threshold to estimate Lepeophtheirus salmonis associated mortality among juvenile pink salmon. Diseases of Aquatic Organisms. 84:131-137. https://www.int-res.com/abstracts/dao/v84/n2/p131-137/ 

“A threshold of lethal infection was estimated from previous controlled laboratory exposures to be 7.5 Lepeophtheirus salmonis g–1 for pink salmon Oncorhynchus gorbuscha averaging <0.7 g. This threshold was used to assess the risk of mortality caused by L. salmonis among pink salmon of the same size class in the Broughton Archipelago, Canada from 2005 to 2008. Virtually all (≥98.9%) pink salmon collected in late March belonged to this size class, and this proportion declined to ≤1% by early July. The proportion of these small pink salmon with infections equal to or exceeding the threshold declined from 4.5 in 2005 to 0% in 2008, coincident with an overall decline in parasite prevalence and intensity during this period.”

Jones S., E. Kim, W. Bennett. 2008. Early development of resistance to the salmon louse, Lepeophtheirus salmonis (Krøyer), in juvenile pink salmon, Oncorhynchus gorbuscha (Walbaum). Journal of Fish Diseases. 31: 591–600. https://doi.org/10.1111/j.1365-2761.2008.00933.x 

“This study examined the effect of fish weight on the susceptibility of post-emergent pink salmon to Lepeophtheirus salmonis (Krøyer). Three trials were conducted, each with two stocks of pink salmon, Oncorhynchus gorbuscha (Walbaum), at starting weights of c. 0.3, 0.7 and 2.4 g, respectively. In each trial, duplicate tanks of fish were exposed to 0, 25 (only in Trial 1), 50 or 100 copepodids per fish. Mortality in Trial 1 was c. 37%, regardless of stock following exposures to 50 or 100 copepodids. Mortalities occurred up to 26 days after exposure, and more than 80% of the lice on the dead fish were chalimus stages. Infections with adult or preadult lice were observed on c. 35% of fish surviving to 37 days after exposure. Mortality was 5% in Trial 2 and there was no mortality in Trial 3.”

Morton A., R. Routledge, and M. Krkošek. 2008. Sea lice infestation of wild juvenile salmon and herring associated with fish farms off the east-central coast of Vancouver Island, British Columbia. North American Journal of Fisheries Management. 28:523-532. https://doi.org/10.1577/M07-042.1 

“Here, we report on 2 years of sea louse field surveys of wild juvenile pink and chum salmon, as well as wild sockeye salmon O. nerka and larval Pacific herring Clupea pallasii, in another salmon farming region, the Discovery Islands region of British Columbia. For pink and chum salmon we tested for the dependency of sea louse abundance on temperature, salinity, sampling period, host species, and farm exposure category. For both louse species, farm exposure was the only consistently significant predictor of sea lice abundance. Fish exposed to salmon farms were infected with more sea lice than those in the peripheral category.”

Krkošek M., A. Gottesfeld, B. Proctor, D. Rolston, C. Carr-Harris, M.A. Lewis. 2007. Effects of host migration, diversity and aquaculture on sea lice threats to Pacific salmon populations. Proceedings of the Royal Society B. 274:3141–3149. https://doi.org/10.1098/rspb.2007.1122

“We studied this characteristic for sea lice (Lepeophtheirus salmonis and Caligus clemensi) and pink salmon (Oncorhynchus gorbuscha) from one of Canada’s largest salmon stocks. Migratory allopatry protects juvenile salmon from L. salmonis for two to three months of early marine life (2–3% prevalence). In contrast, host diversity facilitates access for C. clemensi to juvenile salmon (8–20% prevalence) but infections appear ephemeral. Aquaculture can augment host abundance and diversity and increase parasite exposure of wild juvenile fish.  An empirically parametrized model shows high sensitivity of salmon populations to increased L. salmonis exposure, predicting population collapse at one to five motile L. salmonis per juvenile pink salmon. These results characterize parasite threats of salmon aquaculture to wild salmon populations and show how host migration and diversity are important factors affecting parasite transmission in the oceans.”

Jones S.R., M.D. Fast, S.C. Johnson, D.B. Groman. 2007. Differential rejection of salmon lice by pink and chum salmon: disease consequences and expression of proinflammatory genes. Diseases in Aquatic Organisms. 75:229–238. https://www.int-res.com/abstracts/dao/v75/n3/p229-238/ 

“The consequences of high (735 copepodids fish-1) and low (243 copepodids fish-1) level exposures of size-matched juvenile pink and chum salmon to Lepeophtheirus salmonis copepodids were examined. At both levels of exposure the prevalence and abundance of L. salmonis was significantly higher on chum salmon. In addition, the weight of exposed chum salmon following the high exposure was significantly less than that of unexposed chum salmon. At both exposures, the haematocrit of exposed chum salmon was significantly less than that of unexposed chum…To initiate the exposure, water flow was stopped and the fish were sedated by dissolving 0.07 mg L1 metomidate*HCl in the tank water. The water volume in each tank was reduced to 3 or 4 L, supplemental aeration provided and the required number of copepodids was added. Water flow was resumed after 2 h.”

Beamish, R.J., S. Jones, C. Neville, R. Sweeting, G. Karreman, S. Saksida, E. Gordon. 2006. Exceptional production of pink salmon in 2003/2004 indicates that farmed salmon and wild Pacific salmon can coexist successfully in a marine ecosystem on the Pacific coast of Canada. ICES Journal of Marine Science. 63:1326-1337 http://icesjms.oxfordjournals.org/content/63/7/1326.full 

“The processes responsible for the high marine survival cannot be identified with certainty, but they could include increased freshwater discharge in 2003, which may have resulted in lower salinity less favourable to sea louse production, increased inflow of nutrient-rich water to the study area, and the introduction of a Provincial Action Plan that required mandatory louse monitoring and established a fallowed migration corridor for pink salmon.”

Krkošek, M., M.A. Lewis, A. Morton, L.N. Frazer and J.P. Volpe. 2006. Epizootics of wild fish induced by farm fish. Proceedings of the National Academy of Sciences USA. 103:15506-15510. https://doi.org/10.1073/pnas.0603525103 

“Farm-origin lice induced 9–95% mortality in several sympatric wild juvenile pink and chum salmon populations. The epizootics arise through a mechanism that is new to our understanding of emerging infectious diseases: fish farms undermine a functional role of host migration in protecting juvenile hosts from parasites associated with adult hosts.” 

Krkošek, M., M.A. Lewis, J.P. Volpe. 2005. Transmission dynamics of parasitic sea lice from farm to wild salmon. Proceedings of the Royal Society of London Series B. 272:689-696. https://doi.org/10.1098/rspb.2004.3027

“Our calculations suggest the infection pressure imposed by the farm was four orders of magnitude greater than ambient levels, resulting in a maximum infection pressure near the farm that was 73 times greater than ambient levels and exceeded ambient levels for 30 km along the two wild salmon migration corridors.”

Morton, A., R. Routledge. 2005. Mortality rates for juvenile pink Oncorhynchus gorbuscha and chum O. keta salmon infested with sea lice Lepeophtheirus salmonis in the Broughton Archipelago. Alaska Fishery Research Bulletin. 11(2):146-152. www.adfg.alaska.gov/static/home/library/pdfs/afrb/mortv11n2.pdf 

“In each trial or series, significantly more fish died in the categories with sea lice than in the lice-free category. The majority of fish infected with motile-stage sea lice died. These observations indicate that short-term mortality of wild juvenile pink and chum salmon is increased by infestations of 1-3 sea lice.”

Morton A., R. Routledge and R. Williams. 2005. Temporal patterns of sea louse infestation on wild Pacific salmon in relation to the fallowing of Atlantic salmon farms. North American Journal of Fisheries Management. 25:811-821. https://doi.org/10.1577/M04-149.1 

“Overall, L. salmonis levels were significantly reduced (P < 0.0001) at the study sites during fallowing but returned to the original level after fallowing. The decline was age specific. While the abundance of the earliest attached sea louse phase (the copepodid stage) declined by a factor of 42, the mean abundance of adult L. salmonis did not decline significantly. Changes in salinity and temperature could not account for the decline. This study provides evidence that the fallowing of Atlantic salmon farms during spring juvenile salmon migrations can be an effective conservation and management tool for protecting wild salmon.”

Morton, A., R. Routledge C. Peet and A. Ladwig. 2004. Sea lice (Lepeophtheirus salmonis) infection rates on juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon in the nearshore marine environment of British Columbia, Canada. Canadian Journal of Fisheries and Aquatic Sciences. 61:147-157. https://doi.org/10.1139/f04-016 

“A 10-week study in the Broughton Archipelago found sea lice were 8.8 times more abundant on wild fish near farms holding adult salmon and 5.0 times more abundant on wild fish near farms holding smolts than in areas distant from salmon farms. We found that 90% of juvenile pink and chum salmon sampled near salmon farms in the Broughton Archipelago were infected with more than 1.6 lice·(g host mass)–1, a proposed lethal limit when the lice reach mobile stages. Sea lice abundance was near zero in all areas without salmon farms.”

Morton, A., and R. Williams 2003. First report of a sea louse, Lepeophtheirus salmonis, infestation on juvenile pink salmon, Oncorhynchus gorbuscha, in nearshore habitat. Canadian Field-Naturalist. 117:634-641. https://doi.org/10.22621/cfn.v117i4.834

“High infestation rates of the Sea Louse (Lepeophtheirus salmonis) have been reported on juvenile salmonids in Europe since 1989; however, this species has not been reported on juvenile Pacific salmonids until now.  Abundance (Kruskal-Wallis statistic = 100.95, p<0.0001) and intensity (KW= 70.05, p<0.0001) of lice, and mean number of lice/g host weight (K-W= 112.23, p<0.0001) were significantly higher in juvenile Pink Salmon in close proximity to salmon farms, than in Pink Salmon distant from salmon farms.”