Broughton Archipelago Research Articles

Effect of government removal of salmon farms on sea lice infection of juvenile wild salmon in the Discovery Islands

In response to a federal government order, the number of salmon farms operating in the Discovery Islands region declined from eight in 2020, to one in 2022. Over this period, 1627 juvenile pink ( Oncorhynchus gorbuscha) and chum ( Oncorhynchus keta) salmon captured at sites throughout the study area were examined for sea lice. The average number of sea lice per juvenile salmon declined by 96% over the study period. Such a substantial decline was not witnessed in similar samples from the nearby Broughton Archipelago. The decline could not be attributed to chance sampling, and only a small proportion of it was associated with environmental fluctuations.

Next-generation matrices for marine metapopulations: The case of sea lice on salmon farms.

Classifying habitat patches as sources or sinks and determining metapopulation persistence requires coupling connectivity between habitat patches with local demographic rates. While methods to calculate sources, sinks, and metapopulation persistence exist for discrete-time models, there is no method that is consistent across modeling frameworks. In this paper, we show how next-generation matrices, originally popularized in epidemiology to calculate new infections after one generation, can be used in an ecological context to calculate sources and sinks as well as metapopulation persistence in marine metapopulations. To demonstrate the utility of the method, we construct a next-generation matrix for a network of sea lice populations on salmon farms in the Broughton Archipelago, BC, an intensive salmon farming region on the west coast of Canada where certain salmon farms are currently being removed under an agreement between local First Nations and the provincial government. The column sums of the next-generation matrix can determine if a habitat patch is a source or a sink and the spectral radius of the next-generation matrix can determine the persistence of the metapopulation. With respect to salmon farms in the Broughton Archipelago, we identify the salmon farms which are acting as the largest sources of sea lice and show that in this region the most productive sea lice populations are also the most connected. The farms which are the largest sources of sea lice have not yet been removed from the Broughton Archipelago, and warming temperatures could lead to increased sea louse growth. Calculating sources, sinks, and persistence in marine metapopulations using the next-generation matrix is biologically intuitive, mathematically equivalent to previous methods, and consistent across different modeling frameworks.

Timing and probability of arrival for sea lice dispersing between salmon farms.

Sea lice are a threat to the health of both wild and farmed salmon and an economic burden for salmon farms. With a free-living larval stage, sea lice can disperse tens of kilometres in the ocean between salmon farms, leading to connected sea louse populations that are difficult to control in isolation. In this paper, we develop a simple analytical model for the dispersal of sea lice (Lepeophtheirus salmonis) between two salmon farms. From the model, we calculate the arrival time distribution of sea lice dispersing between farms, as well as the level of cross-infection of sea lice. We also use numerical flows from a hydrodynamic model, coupled with a particle tracking model, to directly calculate the arrival time of sea lice dispersing between two farms in the Broughton Archipelago, British Columbia, in order to fit our analytical model and find realistic parameter estimates. Using the parametrized analytical model, we show that there is often an intermediate interfarm spacing that maximizes the level of cross-infection between farms, and that increased temperatures will lead to increased levels of cross-infection.

Oceanic, Multispecies, Resilient Resistance: Whales, Noise Pollution, and Tiny House Warriors

Oceanic, Multispecies, Resilient ResistanceWhales, Noise Pollution, and Tiny House Warriors Leesa K. Fawcett (bio) and Morgan Johnson (bio) The Orcas Return In January 2021, northern resident Orcas returned to the Broughton Archipelago of British Columbia after twenty years of absence. This social group, known as the A5 pod, includes members, now pod matriarchs, who had been identified as youngsters decades earlier in the area. On this epic return the group brought a brand-new calf with them, pictured in Figure 1.1 The northern resident Orcas are classified as a species at risk; they live in a watery liminal zone between vulnerability, resilience, resistance, and extinction. Hal Whitehead and Luke Rendell write that, compared to land mammals, whales have nowhere to hide (either sonically or visually) in the water except with each other, so sociality, followed by culture, became hugely important.2 They convincingly discuss how this culture allows for intergenerational teachings: arguably, some of the pod members remembered how to return to this Pacific Ocean area, which was once rich with chinook salmon. Indigenous knowledge and western science suggest the whales left because the fish farming industry used deafening acoustic deterrent devices to harass sea lions and seals to scare them away from the caged salmon. These whales are contending with new life, old memories, more salmon, and a quieter ocean space to swim through. Ernest Alfred, a member of the Namgis First Nation, said "the return of the Northern residents is a sign that salmon stocks are rebounding."3 [End Page 111] Alfred has mobilized his own form of resistance to the drastic decline in salmon populations by protesting against the fish farming industry—he and others occupied a fish farm for 284 days.4 Scientist and activist Alexandra Morton, who has protested the fish farming industry for decades, says, "In ecosystem collapse you need to know where to plant your body."5 Three First Nations and the BC government have worked out an agreement to protect wild salmon and recognize Indigenous values and rights by removing seventeen fish farms from the Broughton Archipelago by 2023. Fish farming can be a noisy, polluting business. So far five fish farms have been decommissioned and five more will be decommissioned in 2022.6 The ocean commons will be quieter. We often find ourselves despairing between the vast assemblages of multispecies entanglements and our academic articulations of oceanic past, present, and futures. Throughout this research process we have been guided and inspired by the poetry of award-winning poet and the city of Halifax's eighth poet laureate, Sue Goyette, whose words evoke the smells, textures, sights, and tastes of an ocean world and the looming force of slow violence that has become part of it. As Rob Nixon posits, "[t]he narrative imaginings of writer-activists may thus offer us a different kind of witnessing: of sights unseen."7 In the context of multispecies knowledges, relentless capitalist takeover of the commons, and assemblages of resilience, Goyette's poem "fifty-five" from her collection Ocean poignantly and recursively maps our way through this paper. The epigraph that begins the following section is the start of her poem, reminding us about what we have forgotten. We weave fragments of Goyette's poem throughout this paper as an imaginative invitation to witness "sights unseen" and thoughts unthought. Resilience as Resistance to Violence Our elders insisted the ocean was still there.That we were born with a seed of it and when we spoke,its waves pressed against our words for a further shore.But we had let ourselves become subdivided and suburban,buckling our talk into seatbelts, mad always for safety.When had our schedules become the new mountains? —Sue Goyette, "fifty-five" from Ocean [End Page 112] Click for larger view View full resolution Fig. 1. A5 pod mother (A69, dubbed Midsummer) swims with her new calf (A126). The calf's grandmother and pod matriarch (A43, dubbed Ripple), who would have been a teenager the last time the pod frequented their regular hunting ground in the Broughton Archipelago. In the 1970s the A5 pod, like so many others, suffered attacks and captures as humans began to bring Orcas into aquariums. The...

Salmon lice in the Pacific Ocean show evidence of evolved resistance to parasiticide treatment

Parasitic salmon lice (Lepeophtheirus salmonis) threaten the economic and ecological sustainability of salmon farming, and their evolved resistance to treatment with emamectin benzoate (EMB) has been a major problem for salmon farming in the Atlantic Ocean. In contrast, the Pacific Ocean, where wild salmon are far more abundant, has not seen widespread evolution of EMB-resistant lice. Here, we use EMB bioassays and counts of lice on farms from the Broughton Archipelago, Canada—a core region of salmon farming in the Pacific—to show that EMB sensitivity has dramatically decreased since 2010, concurrent with marked decrease in the field efficacy of EMB treatments. Notably, these bioassay data were not made available through public reporting by industry or by the federal regulator, but rather through Indigenous-led agreements that created a legal obligation for salmon-farming companies to provide data to First Nations. Our results suggest that salmon lice in the Pacific Ocean have recently evolved substantial resistance to EMB, and that salmon-louse outbreaks on Pacific farms will therefore be more difficult to control in the coming years.

Infrastructure that Sings: Kwawaka'wakw Social Media for Wild Salmon in the Broughton Archipelago

Background: As an exercise in decolonizing infrastructural approaches to communications, this article applies the framework of “Two-Eyed Seeing” to the example of Kwawaka'wakw social media communications related to the occupation of fish farms in the Broughton Archipelago in 2017.Analysis: Kwawaka'wakw social media communications worked to enact a “living infrastructure,” upholding reciprocal relations with salmon and ocean waters, in support of the mutual flourishing of the waters, salmon, and people.Conclusions and implications: Kwawaka'wakw nations and their allies drew on social media to shift communicational channels beyond mainstream media. This supported Kwawaka'wakw in building relationships with broader publics, who helped pressure government to make change.

Validation of a sea lice dispersal model: principles from ecological agent-based models applied to aquatic epidemiology

Sea lice are one of the most economically costly and ecologically concerning problems facing the salmon farming industry. Here, we validated a coupled biological and physical model that simulated sea lice larvae dispersal from salmon farms in the Broughton Archipelago (BA), British Columbia, Canada. We employed a concept from ecological agent-based modeling known as ‘pattern matching’, which identifies similar emergent properties in both the simulated and observed data to confirm that the simulation contained sufficient complexity to recreate the emergent properties of the system. One emergent property from the biophysical simulations was the existence of sub-networks of farms. These were also identified in the observed sea lice count data in this study using a space-time scan statistic (SaTScan) to identify significant spatio-temporal clusters of farms. Despite finding support for our simulation in the observed data, which consisted of over a decade’s worth of monthly sea lice abundance counts from salmon farms in the BA, the validation was not entirely straightforward. The complexities associated with validating this biophysical dispersal simulation highlight the need to further develop validation techniques for agent-based models in general, and biophysical simulations in particular, which often result in patchiness in their dispersal fields. The methods utilised in this validation could be adopted as a template for other epidemiological dispersal models, particularly those related to aquaculture, which typically have robust disease monitoring data collection plans in place.

The relevance of larval biology on spatiotemporal patterns of pathogen connectivity among open-marine salmon farms

Warming waters are changing marine pathogen dispersal patterns and infectivity worldwide. Coupled biological–physical modelling has been used in many systems to determine the connectivity of metapopulations via infectious disease particles. Here we model the connectivity of sea lice larvae (Lepeophtheirus salmonis) among salmon farms in the Broughton Archipelago, British Columbia, Canada, using a coupled biological–physical model. The physical model simulated pathogen dispersal, while the biological component influenced the survival and developmental rates of the sea lice. Model results predicted high temporal variability in connectivity strength among farms, an emergent effect from the interacting parts of the simulation (dispersion versus survival and development). Drivers of temporal variability were disentangled using generalized additive modeling, which revealed the variability was most strongly impacted by the spring freshet, which can act as a natural aid for sea lice control in the Broughton Archipelago. Our results suggest that farm management strategies can benefit by accounting for short-term spikes in regional pathogen connectivity among farms. Additionally, future scenarios of a warming climate with reduced snowpack can make sea lice control more challenging.

Estimation of spatiotemporal transmission dynamics and analysis of management scenarios for sea lice of farmed and wild salmon

Parasite transmission between farmed and wild salmon affects the sustainability of salmon aquaculture in Pacific Canada. Understanding and managing parasites in aquaculture is challenged by spatial and temporal variation in transmission dynamics. We developed a mechanistic model that connects sea louse (Lepeoptheirus salmonis) outbreak and control on farmed salmon (Salmo salar) to spatiotemporal dynamics of sea lice on migrating wild juvenile salmon (Oncorhynchus keta and Oncorhynchus gorbuscha). We fitted the model to time series of sea lice on farmed salmon and spatial surveys of juvenile wild salmon in the Broughton Archipelago. We used the parameterized model to evaluate alternative management scenarios based on the resulting sea louse infestations and predicted mortality of wild salmon. Early and coordinated management of sea lice on salmon farms was most effective for controlling outbreaks in wild salmon, while uncoordinated treatments led to a resurgence of sea lice on salmon farms during the juvenile salmon migration. This study highlights the importance of incorporating spatiotemporal variability when considering infectious disease dynamics shared by farmed and wild hosts, particularly when migratory wildlife are involved.

The Use of Kernel Density Estimation With a Bio-Physical Model Provides a Method to Quantify Connectivity Among Salmon Farms: Spatial Planning and Management With Epidemiological Relevance.

Connectivity in an aquatic setting is determined by a combination of hydrodynamic circulation and the biology of the organisms driving linkages. These complex processes can be simulated in coupled biological-physical models. The physical model refers to an underlying circulation model defined by spatially-explicit nodes, often incorporating a particle-tracking model. The particles can then be given biological parameters or behaviors (such as maturity and/or survivability rates, diel vertical migrations, avoidance, or seeking behaviors). The output of the bio-physical models can then be used to quantify connectivity among the nodes emitting and/or receiving the particles. Here we propose a method that makes use of kernel density estimation (KDE) on the output of a particle-tracking model, to quantify the infection or infestation pressure (IP) that each node causes on the surrounding area. Because IP is the product of both exposure time and the concentration of infectious agent particles, using KDE (which also combine elements of time and space), more accurately captures IP. This method is especially useful for those interested in infectious agent networks, a situation where IP is a superior measure of connectivity than the probability of particles from each node reaching other nodes. Here we illustrate the method by modeling the connectivity of salmon farms via sea lice larvae in the Broughton Archipelago, British Columbia, Canada. Analysis revealed evidence of two sub-networks of farms connected via a single farm, and evidence that the highest IP from a given emitting farm was often tens of kilometers or more away from that farm. We also classified farms as net emitters, receivers, or balanced, based on their structural role within the network. By better understanding how these salmon farms are connected to each other via their sea lice larvae, we can effectively focus management efforts to minimize the spread of sea lice between farms, advise on future site locations and coordinated treatment efforts, and minimize any impact of farms on juvenile wild salmon. The method has wide applicability for any system where capturing infectious agent networks can provide useful guidance for management or preventative planning decisions.

Planktonic and parasitic stages of sea lice (Lepeophtheirus salmonis and Caligus clemensi) at a commercial Atlantic salmon (Salmo salar) farm in British Columbia, Canada

Planktonic and parasitic sea lice (Lepeophtheirus salmonis and Caligus clemensi) were examined at a commercial Atlantic salmon (Salmo salar) farm near the Broughton Archipelago, British Columbia (BC) from December 2012 through March 2014. Parasitic sea lice were counted on a minimum of 20 fish in three pens per month. Surface seawater salinity ranged from 19 to 35PSU. Parasite abundance was highest in winter, peaking in January 2013 at 6.5 ind fish−1 (motiles were 14.3% L. salmonis, 85.7% C. clemensi) and February 2014 at 3.3 ind fish−1 (motiles were 96.5% L. salmonis, 3.5% C. clemensi). SLICE® (emamectin benzoate) administered in both winters rapidly reduced parasitic sea lice numbers. Monthly parasite abundance was significantly correlated with the density of sea lice larvae (nauplius and copepodid stages) (r2=0.65, p=0.01) suspended in the water column of the salmon pens. Larval density was calculated monthly via triplicate vertical plankton hauls within six pens, as well as at a reference site approximately 150m away. Larval density at the farm peaked in January 2013 (mean±SE: 1.3±0.6indm−3) and January 2014 (1.0±0.3indm−3). Sea lice nauplii were found in all samples at the reference site in densities similar to those observed within pens. Overall, the majority (87.8%) of sea lice in the plankton samples were nauplii, with copepodids comprising 5.2% and motile stages 1.8%. Surprisingly, the remaining 5.2% of planktonic sea lice were chalimus stages of C. clemensi, and were found both before and after SLICE® administration. For comparison, L. salmonis nauplius densities were estimated from established relationships between water temperature and salinity, the number of female lice present, and the approximate number of fish on site. These estimated densities were significantly (t=3.41, p=0.009) higher than actual nauplius densities observed at the farm, suggesting that sea lice larvae quickly dispersed from the farm after hatching. This study provides information on all sea lice stages present throughout an Atlantic salmon production cycle in BC (through two SLICE® treatments) and the first report of planktonic chalimus stages of C. clemensi, a commercially-relevant though relatively under-studied sea louse species.

Assessment of circulation and inter-basin transport in the Salish Sea including Johnstone Strait and Discovery Islands pathways

The Salish Sea consisting of Puget Sound and Georgia Basin in U.S and Canadian waters has been the subject of several independent data collection and modeling studies. However, these interconnected basins and their hydrodynamic interactions have not received attention as a contiguous unit. The Strait of Juan de Fuca is the primary pathway through which Pacific Ocean water enters the Salish Sea but the role played by Johnstone Strait and the complex channels northeast of Vancouver Island, connecting the Salish Sea and the Pacific Ocean, on overall Salish Sea circulation has not been characterized. In this paper we present a modeling-based assessment of the two-layer circulation and transport through the multiple interconnected sub-basins within the Salish Sea including the effect of exchange via Johnstone Strait and Discovery Islands. The Salish Sea Model previously developed using the finite volume community ocean model (FVCOM) was expanded over the continental shelf for this assessment encircling Vancouver Island, including Discovery Islands, Johnstone Strait, Broughton Archipelago and the associated waterways. A computational technique was developed to allow summation of volume fluxes across arbitrary transects through unstructured finite volume cells. Tidally averaged volume fluxes were computed at multiple transects. The results were used to validate the classic model of Circulation in Embracing Sills for Puget Sound and to provide quantitative estimates of the lateral distribution of tidally averaged transport through the system. Sensitivity tests with and without exchanges through Johnstone Strait demonstrate that it is a pathway for Georgia Basin runoff and Fraser River water to exit the Salish Sea and for Pacific Ocean inflow. However the relative impact of this exchange on circulation and flushing in Puget Sound Basin is small.

Recent failure to control sea louse outbreaks on salmon in the Broughton Archipelago, British Columbia

The advent and growth of salmon farming has changed the epidemiology of some salmon diseases. In 2015, in the salmon-farming region of the Broughton Archipelago, British Columbia, an outbreak of native ectoparasitic copepods (sea lice; Lepeophtheirus salmonis) recurred in wild juvenile salmon after a decade of effective control. We draw on a 15-year data set of sea lice on wild and farmed salmon in the area to assess the evidence for four factors that may explain the recent outbreak: (i) poorly timed parasiticide treatments of farmed salmon relative to wild salmon migration, (ii) evolution of resistance to parasiticide treatments in sea lice, (iii) anomalous environmental conditions promoting louse population growth, and (iv) a high influx of lice with an abundant pink salmon (Oncorhynchus gorbuscha) return in 2014. We propose that a combination of poorly timed treatments and warm environmental conditions likely explains the outbreak. Where wild salmon conservation is a concern, a more effective approach to managing sea lice on wild and farmed salmon could incorporate the out-migration timing of wild juvenile salmon and information on environmental conditions.

Sea-louse parasites on juvenile wild salmon in the Broughton Archipelago, BritishColumbia, Canada.

The global expansion of aquaculture has changed the structure of fish populations in coastal environments, with implications for disease dynamics. In Pacific Canada, farmed salmon act as reservoir hosts for parasites and pathogens, including sea lice (Lepeophtheirus salmonis and Caligus clemensi) that can transmit to migrating wild salmon. Assessing the impact of salmon farms on wild salmon requires regular monitoring of sea-louse infections on both farmed and wild fish. Since 2001, we have collected juvenile pink (Oncorhynchus gorbuscha) and chum (O.keta) salmon annually at three sites in the Broughton Archipelago in British Columbia, Canada, during the annual juvenile salmon migration from fresh water to the open ocean. From sampled fish, we recorded counts of parasitic copepodid-, chalimus-, and motile-stage sea lice. We report louse abundances as well as supplementary observations of fish size, development, and health.

Suitability of remotely-sensed sea surface temperature for aquaculture research: comparison with in situ records from salmon farms in British Columbia (BC), Canada

Suitability of remotely-sensed sea surface temperature for aquaculture research: comparison with in situ records from salmon farms in British Columbia (BC), Canada

Spatial patterns of sea lice infection among wild and captive salmon in western Canada

Context Parasite transmission between captive and wild fish is mediated by spatial, abiotic, biotic, and management factors. More effective population management and conservation strategies can result from multivariable assessments of factors associated with spatial dynamics of parasite spillover. Objective Our study characterised spatial patterns of sea lice (Lepeophtheirus salmonis, Caligus clemensi) infection on out-migrating chum (Oncorhynchus keta) and pink (O. gorbuscha) salmon in an area with Atlantic salmon (Salmo salar) farming. Methods A multivariable statistical model for sea louse parasitism of out-migrating chum and pink salmon was developed from 166,316 wild salmon sampled in the Broughton Archipelago, British Columbia, Canada from 2003 to 2012. We assessed for factors hypothesized to influence sea lice infection levels, at the non-motile life stage, including spatial scales of infection sources. Results 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. Except for 2004, sea lice infection on farms were typicallywell below the regulatory level(3 motiles per fish). Average intensity of non-motile infections observed on the wild fish were 6.36 (SD = 9.98) in 2004 compared to 1.66 (SD = 1.25) for the other years. Conclusions Accuracy of future model estimates will benefit by including hydrodynamic data accounting for anisotropic spread of sea lice from sources. Multivariable statistical modelling over long time series data strengthens understanding of factors impacting wild juvenile salmon infection levels and informs spatial patterns of aquatic epidemiology.

Space–time cluster analysis of sea lice infestation (Caligus clemensi and Lepeophtheirus salmonis) on wild juvenile Pacific salmon in the Broughton Archipelago of Canada

Sea lice infestation levels on wild chum and pink salmon in the Broughton Archipelago region are known to vary spatially and temporally; however, the locations of areas associated with a high infestation level had not been investigated yet. In the present study, the multivariate spatial scan statistic based on a Poisson model was used to assess spatial clustering of elevated sea lice (Caligus clemensi and Lepeophtheirus salmonis) infestation levels on wild chum and pink salmon sampled between March and July of 2004 to 2012 in the Broughton Archipelago and Knight Inlet regions of British Columbia, Canada. Three covariates, seine type (beach and purse seining), fish size, and year effect, were used to provide adjustment within the analyses. The analyses were carried out across the five months/datasets and between two fish species to assess the consistency of the identified clusters. Sea lice stages were explored separately for the early life stages (non-motile) and the late life stages of sea lice (motile). Spatial patterns in fish migration were also explored using monthly plots showing the average number of each fish species captured per sampling site. The results revealed three clusters for non-motile C. clemensi, two clusters for non-motile L. salmonis, and one cluster for the motile stage in each of the sea lice species. In general, the location and timing of clusters detected for both fish species were similar. Early in the season, the clusters of elevated sea lice infestation levels on wild fish are detected in areas closer to the rivers, with decreasing relative risks as the season progresses. Clusters were detected further from the estuaries later in the season, accompanied by increasing relative risks. In addition, the plots for fish migration exhibit similar patterns for both fish species in that, as expected, the juveniles move from the rivers toward the open ocean as the season progresses The identification of space–time clustering of infestation on wild fish from this study can help in targeting investigations of factors associated with these infestations and thereby support the development of more effective sea lice control measures.

Sea lice infestations on juvenile chum and pink salmon in the Broughton Archipelago, Canada, from 2003 to 2012

Juvenile pink salmon Oncorhynchus gorbuscha and chum salmon O. keta were sampled by beach or purse seine to assess levels of sea lice infestation in the Knight Inlet and Broughton Archipelago regions of coastal British Columbia, Canada, during the months of March to July from 2003 to 2012. Beach seine data were analyzed for sea lice infestation that was described in terms of prevalence, abundance, intensity, and intensity per unit length. The median annual prevalence for chum was 30%, ranging from 14% (in 2008 and 2009) to 73% (in 2004), while for pink salmon, the median was 27% and ranged from 10% (in 2011) to 68% (in 2004). Annual abundance varied from 0.2 to 5 sea lice per fish with a median of 0.47 for chum and from 0.1 to 3 lice (median 0.42) for pink salmon. Annual infestation followed broadly similar trends for both chum and pink salmon. However, the abundance and intensity of Lepeophtheirus salmonis and Caligus clemensi, the 2 main sea lice species of interest, were significantly greater on chum than on pink salmon in around half of the years studied. Logistic regression with random effect was used to model prevalence of sea lice infestation for the combined beach and purse seine data. The model suggested inter-annual variation as well as a spatial clustering effect on the prevalence of sea lice infestation in both chum and pink salmon. Fish length had an effect on prevalence, although the nature of this effect differed according to host species.

Modeling Parasite Dynamics on Farmed Salmon for Precautionary Conservation Management of Wild Salmon

Conservation management of wild fish may include fish health management in sympatric populations of domesticated fish in aquaculture. We developed a mathematical model for the population dynamics of parasitic sea lice (Lepeophtheirus salmonis) on domesticated populations of Atlantic salmon (Salmo salar) in the Broughton Archipelago region of British Columbia. The model was fit to a seven-year dataset of monthly sea louse counts on farms in the area to estimate population growth rates in relation to abiotic factors (temperature and salinity), local host density (measured as cohort surface area), and the use of a parasiticide, emamectin benzoate, on farms. We then used the model to evaluate management scenarios in relation to policy guidelines that seek to keep motile louse abundance below an average three per farmed salmon during the March–June juvenile wild Pacific salmon (Oncorhynchus spp.) migration. Abiotic factors mediated the duration of effectiveness of parasiticide treatments, and results suggest treatment of farmed salmon conducted in January or early February minimized average louse abundance per farmed salmon during the juvenile wild salmon migration. Adapting the management of parasites on farmed salmon according to migrations of wild salmon may therefore provide a precautionary approach to conserving wild salmon populations in salmon farming regions.

Physiological consequences of the salmon louse (Lepeophtheirus salmonis) on juvenile pink salmon (Oncorhynchus gorbuscha): implications for wild salmon ecology and management, and for salmon aquaculture

Pink salmon, Oncorhynchus gorbuscha, are the most abundant wild salmon species and are thought of as an indicator of ecosystem health. The salmon louse, Lepeophtheirus salmonis, is endemic to pink salmon habitat but these ectoparasites have been implicated in reducing local pink salmon populations in the Broughton Archipelago, British Columbia. This allegation arose largely because juvenile pink salmon migrate past commercial open net salmon farms, which are known to incubate the salmon louse. Juvenile pink salmon are thought to be especially sensitive to this ectoparasite because they enter the sea at such a small size (approx. 0.2 g). Here, we describe how 'no effect' thresholds for salmon louse sublethal impacts on juvenile pink salmon were determined using physiological principles. These data were accepted by environmental managers and are being used to minimize the impact of salmon aquaculture on wild pink salmon populations.