E-Letter responses to:

reports: Martin Krkosek, Jennifer S. Ford, Alexandra Morton, Subhash Lele, Ransom A. Myers, and Mark A. Lewis Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon Science 2007; 318: 1772-1775 [Abstract] [Full text] [PDF]

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Published E-Letter responses:

Response to M. F. Tlusty's E-Letter

Martin Krkošek, Jennifer S. Ford, Alexandra Morton, Subhash Lele, Mark A. Lewis   (27 August 2008)

Opportunities to Improve Aquaculture and Wild Fisheries

Michael F. Tlusty   (10 July 2008)

Response to M. F. Tlusty's E-Letter 27 August 2008
Martin Krkošek Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, AB, Canada, Jennifer S. Ford, Alexandra Morton, Subhash Lele, Mark A. Lewis Respond to this E-Letter: Re: Response to M. F. Tlusty's E-Letter	We appreciate that many factors affect salmon population dynamics (1, 2) as well as sea lice transmission and pathogenicity (3, 4). In our Report (5) we tested for the effect of salmon lice infestations on pink salmon population dynamics and controlled for other factors by using a comparative approach. Pink salmon population dynamics in areas exposed and unexposed to salmon farms were nearly identical before lice infestations began in the exposed area. During the infestations exposed populations declined significantly, whereas unexposed populations remained productive. Because the two areas share the many factors that affect pink salmon population dynamics—evidenced by their synchronous fluctuations (6)—but differ in lice infestations (7–12), the infestations likely caused the difference in pink salmon population dynamics between the two areas. Tlusty suggests factors other than farms may have caused the infestations in the exposed area. The stickleback hypothesis is easily rejected because early life stages of sea lice dominate infestations of juvenile wild salmon near salmon farms (7, 9–12), whereas lice on stickleback do not survive to reproductive age (13, 14). Also, stickleback are widespread in British Columbia, whereas sea lice infestations on juvenile salmon have only occurred near salmon farms. Bacterial loads in the Broughton Archipelago may be higher than in unexposed areas, possibly also due to salmon farms, but there is no evidence of increased bacteria predisposing juvenile Broughton pink salmon to lice, despite research programs conducting comprehensive pathology screening. Temperature may have increased, but it did so similarly in both exposed and unexposed areas (15). We cannot identify anything about the "quality" of Broughton pink salmon that may underlie the infestations. These factors suggested by Tlusty do not correlate well with the lice infestations and don't suggest causal processes. The evidence that salmon farms caused the infestations and pink salmon population declines in the Broughton Archipelago is extensive. Where there are no salmon farms, louse abundance is low on pink salmon during early marine life because the vast majority of wild adult salmon that carry the parasite are offshore (1, 7, 8). Salmon farms produce large quantities of naupliar and copepodid lice during the early marine life of wild pink salmon in the Broughton Archipelago (16), and many studies have documented the transmission from farm to wild juvenile salmon (7, 9–12). Lice are pathogenic to juvenile pink salmon during early marine life (10, 17) and the mortality rate of infected pink salmon is very similar when estimated from small-scale experiments (10), and analyzing multi-year multi-population escapement and infestation data (5). These quantitative mechanistic linkages form a consilience of scientific evidence that indicates causal processes are underway, not spurious correlation. Martin Krkošek Centre for Mathematical Biology, Department of Mathematical and Statistical Sciences, and Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada. Jennifer S. Ford‡ Biology Department, Dalhousie University, Halifax, NS, Canada. Alexandra Morton Salmon Coast Field Station, Simoom Sound, BC, Canada. Subhash Lele Centre for Mathematical Biology, Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, AB, Canada. Mark A. Lewis Centre for Mathematical Biology, Department of Mathematical and Statistical Sciences, and Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada. ‡present address: Ecology Action Centre, Halifax, NS, Canada. Refererences 1. C. Groot, L. Margolis, Pacific Salmon Life Histories (UBC Press, Vancouver, 1991). 2. T. P. Quinn, The Behavior and Ecology of Pacific Salmon and Trout (University of Washington Press, Seattle, 2005). 3. M. J. Costello, Trends Parasitol. 22, 475 (2006). 4. K. Boxaspen, ICES J. Mar. Sci. 63, 1304 (2006). 5. M. Krkošek et al., Science 318, 1772 (2007). 6. B. J. Pyper, F. J. Mueter, R. M. Peterman, D. J. Blackbourn, C. C. Wood, Can. J. Fish. Aquat. Sci. 58, 1501 (2001). 7. A. Morton, R. Routledge, C. Peet, A. Ladwig, Can. J. Fish. Aquat. Sci. 61, 147 (2004). 8. M. Krkošek et al., P. Roy. Soc. Lond. B 274, 3141 (2007). 9. M. Krkošek, M. A. Lewis, J. P. Volpe, P. Roy. Soc. Lond. B. 272, 689 (2005). 10. M. Krkošek, M. A. Lewis, A. Morton, L. N. Frazer, J. P. Volpe, Proc. Natl. Acad. Sci. USA 103, 15506 (2006). 11. A. B. Morton, R. Williams, Can. Field Nat. 117, 634 (2003). 12. A. Morton, R. D. Routledge, R. Williams, N. Am. J. Fish. Manage. 25, 811 (2005). 13. S. Jones, E. Kim, S. Dawe, J. Fish Dis. 29, 489 (2006). 14. S. R. M. Jones, G. Prosperi-Porta, E. Kim, P. Callow, N. B. Hargreaves, J. Parasitol. 92, 473 (2006). 15. Environment Canada 2005, www.ecoinfo.org/env_ind/region/shellfish/shellfish_e.cfm (Accessed 2 January 2008). 16. C. Orr, N. Am. J. Fish. Manage. 27, 187 (2007). 17. A. Morton, R. Routledge, Alaska Fisheries Research Bulletin 11, 146 (2005).
Opportunities to Improve Aquaculture and Wild Fisheries 10 July 2008
Michael F. Tlusty, Research New England Aquarium, Central Wharf, Boston, MA 02110, USA Respond to this E-Letter: Re: Opportunities to Improve Aquaculture and Wild Fisheries	The recent Report by M. Krkosek et al. ("Declining wild salmon populations in relation to parasites from farm salmon," 14 December 2007, p. 1772) attempts to place causation behind declines in pink salmon runs in the Broughton Archipelago directly on the salmon aquaculture. Their evidence is based on sea lice infestations of juvenile salmon in the archipelago, and subsequent decline in salmon population growth. Salmon runs outside the archipelago do not exhibit declines. Pathogen and disease conditions are conceptually modeled as three overlapping circles, where a disease condition depends on appropriate states of the environment, pathogen, and the host. Krkosek et al. argue that salmon farms provide a suitable environment in which sea lice multiply to infect passing salmon. While their argument depends on density-dependent functions, the increase in the number of farms precedes the increased mortality due to sea lice infestation. Such a singular focus on sea lice from aquaculture negatively impacting wild salmon may draw attention away from other significant factors that may also negatively impact wild salmon. Threespine stickleback (Gasterosteus aculeatus) can act as a host, and infection rates on stickleback were high in 2003 to 2005 (1), overlapping the significant sea lice infestation on wild salmon. There was geographic disparity between Krkosek et al.'s exposed and unexposed populations, and bacterial loading in the region of the exposed fish was also high since 2001, as indicated by increased shellfish area closures (2). In the same time period, the average water temperature was also greater (3). All could make the sea lice potentially more infective. There is also the potential that sea lice are genetically distinct, although low differentiation rates make this unlikely (4). Finally, there is the quality of the host. Multiple factors may render certain runs more susceptible to sea lice infection. By focusing solely on the link to aquaculture, and correlative as opposed to causative factors, other significant alternate hypotheses that may have wider ecosystem implications are being overlooked. To fully understand the relationship between aquaculture and wild fish runs, opportunities exist for scientists, conservation organizations and industry to work together on collaborative research projects which, taken together, could yield significant steps to improve both aquaculture and wild fisheries. Michael F. Tlusty New England Aquarium, Central Wharf, Boston, MA 02110, USA. References 1. S. R. M. Jones, G. Prosperi-Porta, E. Kim, P. Callow, N.B. Hargreaves, J. Parasitol. 92, 473 (2006). 2. Environment Canada 2005 (www.ecoinfo.org/env_ind/region/shellfish/shellfish_e.cfm) Accessed 15 December 2007. 3. D. Masson, P. F. Cummins, Cont. Shelf Res. 27, 634 (2007). 4. C. D. Todd, A. M. Walker, M. G. Ritchie, J. A. Graves, A. F. Walker, Can. J. Fish. Aquat. Sci. 61, 1176 (2004).