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Simoom Sound, Canada

Krkosek M.,University of Otago | Connors B.M.,Salmon Coast Field Station | Connors B.M.,Simon Fraser University | Lewis M.A.,Center for Mathematical Biology | And 2 more authors.
American Naturalist

Allee effects are thought to mediate the dynamics of population colonization, particularly for invasive species. However, Allee effects acting on parasites have rarely been considered in the analogous process of infectious disease establishment and spread. We studied the colonization of uninfected wild juvenile Pacific salmon populations by ectoparasitic salmon lice (Lepeophtheirus salmonis) over a 4-year period. In a data set of 68,376 fish, we observed 85 occurrences of precopular pair formation among 1,259 preadult female and 613 adult male lice. The probability of pair formation was dependent on the local abundance of lice, but this mate limitation is likely offset somewhat by mate-searching dispersal of males among host fish. A mathematical model of macroparasite population dynamics that incorporates the empirical results suggests a high likelihood of a demographic Allee effect, which can cause the colonizing parasite populations to die out. These results may provide the first empirical evidence for Allee effects in a macroparasite. Furthermore, the data give a rare detailed view of Allee effects in colonization dynamics and suggest that Allee effects may dampen the spread of parasites in a coastal marine ecosystem. © 2012 by The University of Chicago. Source

Price M.H.H.,University of Victoria | Morton A.,Salmon Coast Field Station | Reynolds J.D.,Simon Fraser University
Canadian Journal of Fisheries and Aquatic Sciences

Salmon farms are spatially concentrated reservoirs of fish host populations that can disrupt natural salmonid host-parasite dynamics. Sea lice frequently infect farm salmon and parasitize sympatric wild juvenile salmonids, with negative impacts on survival in Europe and Pacific Canada. We examined louse parasitism of wild juvenile chum salmon (Oncorhynchus keta) and pink salmon (Oncorhynchus gorbuscha) from three salmon farming regions in British Columbia (Finlayson, Broughton Archipelago, and Georgia Strait). 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. Source

Williams J.,St. Francis Xavier University | Polk Z.C.S.,Salmon Coast Field Station | Smit L.A.,British Columbia Ministry of forests
Journal of Shellfish Research

In the spring of 2007, a small number of empty valves from the bay scallop, Argopecten irradians irradians (Lamarck), were observed at several locations in Pomquet Harbour, Nova Scotia, Canada. Snorkeling surveys at selected sites in the harbor were carried out in spring 2007, summer 2007, and spring 2008. Scallop densities were very low in all sampling times, ranging between 0.04 and 0.053 scallops/m2. The size distributions from 2007 suggested a single cohort of 0 + individuals, with one 1 + scallop. The 2008 samples were predominantly 1 + scallops, with five 0 + individuals. Four marked scallops from 2007 were collected alive in 2008, demonstrating their ability to overwinter. These scallops had an average daily growth rate of 0.08 mm/day from July 2007 to May 2008. A thermistor deployed in the same eelgrass beds indicated that the scallops survived extended periods of time with temperatures below 0°C. In 2013, four of the sites were surveyed again, and dramatic increases in density were recorded, averaging 1.713 scallops/m2. The bay scallops in Pomquet Harbour probably represent an incidental introduction, as larvae, from aquaculture operations ∼80 km to the north. Source

Neil Frazer L.,University of Hawaii at Manoa | Morton A.,Salmon Coast Field Station | Krkosek M.,University of Otago
Proceedings of the Royal Society B: Biological Sciences

Host density thresholds are a fundamental component of the population dynamics of pathogens, but empirical evidence and estimates are lacking. We studied host density thresholds in the dynamics of ectoparasitic sea lice (Lepeophtheirus salmonis) on salmon farms. Empirical examples include a 1994 epidemic in Atlantic Canada and a 2001 epidemic in Pacific Canada. A mathematical model suggests dynamics of lice are governed by a stable endemic equilibrium until the critical host density threshold drops owing to environmental change, or is exceeded by stocking, causing epidemics that require rapid harvest or treatment. Sensitivity analysis of the critical threshold suggests variation in dependence on biotic parameters and high sensitivity to temperature and salinity. We provide a method for estimating the critical threshold from parasite abundances at subcritical host densities and estimate the critical threshold and transmission coefficient for the two epidemics. Host density thresholds may be a fundamental component of disease dynamics in coastal seas where salmon farming occurs. © 2011 The Royal Society. Source

Krkosek M.,University of Toronto | Krkosek M.,University of Otago | Ashander J.,University of Alberta | Ashander J.,University of California at Davis | And 3 more authors.
American Naturalist

The exchange of native pathogens between wild and domesticated animals can lead to novel disease threats to wildlife. However, the dynamics of wild host-parasite systems exposed to a reservoir of domesticated hosts are not well understood. A simple mathematical model reveals that the spill-back of native parasites from domestic to wild hosts may cause a demographic Allee effect in the wild host population. A second model is tailored to the particulars of pink salmon (Oncorhynchus gorbuscha) and salmon lice (Lepeophtheirus salmonis), for which parasite spill-back is a conservation and fishery concern. In both models, parasite spill-back weakens the coupling of parasite and wild host abundance-particularly at low host abundance-causing parasites per host to increase as a wild host population declines. These findings show that parasites shared across host populations have effects analogous to those of generalist predators and can similarly cause an unstable equilibrium in a focal host population that separates persistence and extirpation. Allee effects in wildlife arising from parasite spill-back are likely to be most pronounced in systems where the magnitude of transmission from domestic to wild host populations is high because of high parasite abundance in domestic hosts, prolonged sympatry of domestic and wild hosts, a high transmission coefficient for parasites, long-lived parasite larvae, and proximity of domesticated populations to wildlife migration corridors. © 2013 by The University of Chicago. Source

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