John P. Volpe

Fugitive Salmon: Assessing the Risks of Escaped Fish from Net-Pen Aquaculture

Abstract The farming of salmon and other marine finfish in open net pens continues to increase along the worlds coastlines as the aquaculture industry expands to meet human demand. Farm fish are known to escape from pens in all salmon aquaculture areas. Their escape into the wild can result in interbreeding and competition with wild salmon and can facilitate the spread of pathogens, thereby placing more pressure on already dwindling wild populations. Here we assess the ecological, genetic, and socioeconomic impacts of farm salmon escapes, using a risk-assessment framework. We show that risks of damage to wild salmon populations, ecosystems, and society are large when salmon are farmed in their native range, when large numbers of salmon are farmed relative to the size of wild populations, and when exotic pathogens are introduced. We then evaluate the policy and management options for reducing risks and discuss the implications for farming other types of marine finfish.

Transmission dynamics of parasitic sea lice from farm to wild salmon

Marine salmon farming has been correlated with parasitic sea lice infestations and concurrent declines of wild salmonids. Here, we report a quantitative analysis of how a single salmon farm altered the natural transmission dynamics of sea lice to juvenile Pacific salmon. We studied infections of sea lice (Lepeophtheirus salmonis and Caligus clemensi ) on juvenile pink salmon (Oncorhynchus gorbuscha) and chum salmon (Oncorhynchus keta) as they passed an isolated salmon farm during their seaward migration down two long and narrow corridors. 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. The farm-produced cohort of lice parasitizing the wild juvenile hosts reached reproductive maturity and produced a second generation of lice that re-infected the juvenile salmon. This raises the infection pressure from the farm by an additional order of magnitude, with a composite infection pressure that exceeds ambient levels for 75 km of the two migration routes. Amplified sea lice infestations due to salmon farms are a potential limiting factor to wild salmonid conservation.

Epizootics of wild fish induced by farm fish

The continuing decline of ocean fisheries and rise of global fish consumption has driven aquaculture growth by 10% annually over the last decade. The association of fish farms with disease emergence in sympatric wild fish stocks remains one of the most controversial and unresolved threats aquaculture poses to coastal ecosystems and fisheries. We report a comprehensive analysis of the spread and impact of farm-origin parasites on the survival of wild fish populations. We mathematically coupled extensive data sets of native parasitic sea lice (Lepeophtheirus salmonis) transmission and pathogenicity on migratory wild juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon. 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. Although the migratory life cycles of Pacific salmon naturally separate adults from juveniles, fish farms provide L. salmonis novel access to juvenile hosts, in this case raising infection rates for at least the first ≈2.5 months of the salmons marine life (≈80 km of the migration route). Spatial segregation between juveniles and adults is common among temperate marine fishes, and as aquaculture continues its rapid growth, this disease mechanism may challenge the sustainability of coastal ecosystems and economies.

Fish Farms and Sea Lice Infestations of Wild Juvenile Salmon in the Broughton Archipelago—A Rebuttal to Brooks (2005)

Contrary to several recent studies, a review (Brooks, 2005) of sea lice (Lepeophtheirus salmonis) interactions between wild and farm salmon in the Broughton Archipelago, British Columbia, Canada, concluded that there is little potential for sea lice transmission from farm to wild salmon. In this rebuttal, we show that this conclusion was based on a flawed interpretation of how salinity affects louse development, a misunderstanding of how the timing of salinity changes corresponds to the timing of the juvenile salmon migration, models of larval dispersion that overestimate the transport of louse larvae, and a selective and misleading assessment of the literature. We analyze and extend the current models of larval dispersion and demonstrate the (perhaps counter-intuitive) result that sustained high abundances of infectious larvae should be expected near lice-infested salmon farms. We also highlight important studies overlooked in Brooks (2005) and clarify some misinterpretations. Counter to the conclusions in Brooks (2005), the modeling and empirical work to date on sea lice interactions between wild and farm salmon are consistent and point to a strong association between salmon farming and recurrent infestations of wild juvenile salmon in the Broughton Archipelago.

Fish farms, parasites, and predators: implications for salmon population dynamics

For some salmon populations, the individual and population effects of sea lice (Lepeophtheirus salmonis) transmission from sea cage salmon farms is probably mediated by predation, which is a primary natural source of mortality of juvenile salmon. We examined how sea lice infestation affects predation risk and mortality of juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon, and developed a mathematical model to assess the implications for population dynamics and conservation. A risk-taking experiment indicated that infected juvenile pink salmon accept a higher predation risk in order to obtain foraging opportunities. In a schooling experiment with juvenile chum salmon, infected individuals had increased nearest-neighbor distances and occupied peripheral positions in the school. Prey selection experiments with cutthroat trout (O. clarkii) predators indicated that infection reduces the ability of juvenile pink salmon to evade a predatory strike. Group predation experiments with coho salmon (O. kisutch) feeding on juvenile pink or chum salmon indicated that predators selectively consume infected prey. 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.

Sea lice and salmon population dynamics: effects of exposure time for migratory fish

The ecological impact of parasite transmission from fish farms is probably mediated by the migration of wild fishes, which determines the period of exposure to parasites. 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. The duration of parasite exposure is probably critical to salmon–louse population dynamics, and should therefore be accommodated in coastal planning and management where fish farms are situated on wild fish migration routes.

Body mass explains characteristic scales of habitat selection in terrestrial mammals.

Niche theory in its various forms is based on those environmental factors that permit species persistence, but less work has focused on defining the extent, or size, of a species’ environment: the area that explains a species’ presence at a point in space. We proposed that this habitat extent is identifiable from a characteristic scale of habitat selection, the spatial scale at which habitat best explains species’ occurrence. We hypothesized that this scale is predicted by body size. We tested this hypothesis on 12 sympatric terrestrial mammal species in the Canadian Rocky Mountains. For each species, habitat models varied across the 20 spatial scales tested. For six species, we found a characteristic scale; this scale was explained by species’ body mass in a quadratic relationship. Habitat measured at large scales best-predicted habitat selection in both large and small species, and small scales predict habitat extent in medium-sized species. The relationship between body size and habitat selection scale implies evolutionary adaptation to landscape heterogeneity as the driver of scale-dependent habitat selection.

Nonlethal assessment of juvenile pink and chum salmon for parasitic sea lice infections and fish health

Abstract Industrial salmon farming has been correlated with infestations of parasitic sea lice Lepeophtheirus salmonis in adjacent wild juvenile salmonids and declines of sympatric wild salmonid populations. Prohibitively large financial, human, and logistical resource requirements prevent the implementation of long-term, large-scale monitoring programs to assess the effect of farms on wild salmonids. We report a novel nonlethal sampling procedure for quantifying louse abundances and measures of fish health on wild juvenile pink salmon Oncorhynchus gorbuscha and chum salmon O. keta during their early marine life history phase. The method significantly reduces the resource requirements of sampling programs and provides a desirable nonlethal alternative for studying depressed or threatened populations. The simplicity of the protocol facilitates public participation, further decreasing costs while increasing the potential spatiotemporal coverage and resolution of future research–monitoring programs.

Effects of Water Temperature on Interspecific Competition between Juvenile Bull Trout and Brook Trout in an Artificial Stream

Abstract The distribution and abundance of bull trout Salvelinus confluentus is influenced by a number of factors, including the presence of nonnative brook trout S. fontinalis, and populations are in decline across the native range. Where sympatric, bull trout are more likely to persist in higher-elevation, cooler streams. To test the hypothesis that competition between juveniles of these species is independent of water temperature, behavioral observations were made of each species in artificial streams. Two temperature (8°C and 15°C) and three density (4.1, 8.2, and 4.1 fish of each species/m2) treatments were used. The results support the contention that bull trout are competitively disadvantaged in warmer water. In warm water, bull trout significantly increased intraspecific agonism. Temperature and density did not influence intraspecific brook trout behavior; however, brook trout tended to increase interspecific agonism in warm water. In mixed-species treatments, brook trout agonism was positively as...

Native eelgrass (Zostera marina L.) survival and growth adjacent to non-native oysters (Crassostrea gigas Thunberg) in the Strait of Georgia, British Columbia.

Abstract We investigated effects of the introduced Pacific oyster (Crassostrea gigas) on native eelgrass (Zostera marina) health on Cortes Island, British Columbia, Canada. Oysters physically alter their environment by increasing habitat complexity and altering water flow, and possibly by causing sulphide to accumulate in the sediment. Sulphide is toxic to eelgrass, and the current decline of eelgrass around Cortes Island may be a consequence of oyster population growth. While oysters and eelgrass coexist at a regional scale, eelgrass is typically absent directly seaward of oyster beds (the “below-oyster zone”) on Cortes Island. In a controlled experiment, we transplanted eelgrass plugs to below-oyster plots to determine whether this habitat is suitable for eelgrass growth. Shoot and leaf number were significantly greater over time in eelgrass-bed transplants than in below-oyster transplants. These results indicate that the below-oyster zone is unsuitable for eelgrass growth; if a causal link exists between oyster presence in the high intertidal zone and eelgrass absence directly seaward, then expansion of feral and farmed oyster beds will result in further eelgrass loss on Cortes Island.