Presentation: State-of-Knowledge Initiative for the Special Committee on Sustainable Aquaculture of the British Columbia Legislature

Delivered November, 2006.

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Salmon Farms, Sea Lice and Wild Pacific Salmon

[A full pdf copy for printing may be downloaded from the WAVES database.]

Since the first reported incidence of the sea lice Lepeophtheirus salmonis on juvenile pink salmon in June 2001⁸ and their association of this infection with proximity to salmon farms, this debate has involved four issues:

1. The prevalence and intensity of sea lice on juvenile pink and chum salmon in areas with salmon farming versus areas without.
2. The source of sea lice within the Broughton Archipelago (BA).
3. The impact of sea lice on individual juvenile pink and chum salmon.
4. The causes of variation in adult population size for pink and chum salmon within the Broughton region, and the contribution of sea lice infections to this variation.

(Sea lice in these discussions refer to two species L. salmonis and Caligus clemensi. Differences in host specificity and impacts on juvenile salmon make accounting for these species important. Further, while the focus has been on juvenile pink and chum due to their small size at sea entry, sea lice infect all six species of Pacific salmon).

Before observing extensive sea lice infection on juvenile pink salmon in the Broughton region, documentation of sea lice on juvenile salmon was very limited and when noted the infections commonly involved Caligus clemensi. 

Anecdotally, biologists that have worked extensively with juvenile pink and chum salmon described the occurrence as rare. Since 2001, three publications support the limited occurrence of L. salmonis on juvenile pink and chum salmon along the B.C. coast but outside of the BA.

In summary, the studies report prevalence of L. salmonis less than 5% with low infection intensity^{9, 11, 12}. (Prevalence is the % of fish sampled that carried at least one sea louse. Intensity is the number of lice an infected fish carried.)

Two of these studies involve extensive sampling in near-shore seawaters and offshore; and report increased infections offshore as salinity and size of the hosts increase. Two additional sampling programs along the west coast of Vancouver Island also report low infection of chum salmon (<10%), but involve inlets that include salmon farms (Clayoquot and Nootka Sounds).

While some of these programs may be limited in duration and area, the combined evidence indicates low incidence and infection levels of L. salmonis outside of the Broughton Archipelago, in areas with and without salmon farming. DFO is aware of monitoring in southern Johnstone Strait but have not been provided results of that work.

A method to determine the origin of sea lice (i.e., did the sea lice originate from a salmon farm or a natural host?) has not yet been developed. Methods to directly determine the origin have examined chemical composition of the lice or genetic differences between sources of lice. While differences between samples have been detected, no method is available yet that can be directly applied in the BA.

At this time, the origin of lice is being inferred from sampling programs conducted within the BA. The prominent papers in this discussion are Krkosek et al.^{5, 6. 7}.

These authors report results from intensive sampling surveys of juvenile wild salmon and sea lice conducted in the BA during 2003–2005. Based on the patterns of sea lice infections that the authors describe as “striking consistent”, they develop mathematical models that lead them to conclude that “farm-origin lice induced 9-95% mortality in several sympatric wild juvenile pink and chum salmon populations” in the BA⁷.

The compelling consistency in the patterns of lice infection reported by the authors, that is the foundation for their analysis, is not evident however in data obtained independently from extensive surveys conducted by DFO in the same areas and during the same time periods.

For example, in Krkosek et al.⁶, the patterns of sea lice infection for 2003 are indeed remarkably consistent for each development stage of lice and in each of the four data sets. The data obtained by DFO in the same time period and area (May 11-24, Knight Inlet) show a similar but not as pronounced pattern of infection.

However, the DFO data collected in many other weeks in the same area show much greater variability. The “strikingly consistent” patterns reported in Krkosek et al.^{6, 7} were not consistently observed in the more extensive DFO data (2003-2005, see highlighted text below).

At broader spatial levels though, there is strong agreement between researchers.

The average prevalence (proportion of fish infected) and intensity (number of lice per infected fish), and the species composition of sea lice observed on juvenile wild salmon are all similar for each year from 2003-2006 (A. Morton, personal communications), despite many differences in the dates and locations sampled, and the sampling methods.

Note: In March 2003 Fisheries and Oceans Canada (DFO) initiated extensive new scientific research to investigate the potential interactions of salmon farms, sea lice, and wild salmon to obtain sufficient independent information to support government policy decisions.

From 2003-2006 DFO conducted an extensive marine monitoring program to determine sea lice infections of wild juvenile salmon in the Broughton. Sampling was conducted at 100 -150 locations, using both beach seine and purse seine fishing gear, at weekly (2003), bi-weekly (2004), or monthly intervals (2005 and 2006) during the period from March to July (Table in presentation).

Samples of juvenile pink and chum salmon, and other species of fish (particularly sticklebacks) caught along with the juvenile salmon, were immediately frozen in individual bags and subsequently examined for sea lice and fish health at the DFO laboratory at the Pacific Biological Station in Nanaimo, B.C. The 2003 results have been reported in Jones and Nemec².

DFO results for 2003-2006 show that levels of sea lice infection, and the lice species involved, are highly variable between locations within a year and between years.

Results also show that each year the abundance of lice on salmon decreased as the fish grew, but the decline coincided with increased proportions of maturing lice. There was also a strong positive relationship each year between sea lice infection and water salinity.

We have also documented infection of three spine stickleback (Gasterosteus aculeatus) with L. salmonis each year³ at levels significantly higher than on juvenile salmon in the same locations in the BA.
 
The impact of sea lice on individual Pacific salmon was initially inferred from research conducted in Europe. However, recent research conducted within the Broughton^{7, 10} and at the Pacific Biological Station4 (S. Jones, unpublished data) have now assessed pink and chum salmon mortality and infection levels.

Morton and Routledge10 reported declining health and high mortality rates of wild juvenile pink salmon that were infected with sea lice in the wild and held in marine enclosures in the Broughton. However, similar experiments conducted by DFO in the laboratory in 2005 and 2006 showed no decline in health of juvenile salmon and no mortality of juvenile pink and chum⁴.

Differences between field and laboratory studies are not uncommon in biological sciences. The background conditions for fish sampled in the wild are obviously less controlled than in a laboratory, but which assessment of impact due to sea lice is the more realistic? These differences are a significant issue that remains to be resolved.

Assessing a potential impact on pink and chum populations is a demanding challenge. As described in the Section about the biology of pink salmon, the challenge is to attribute a change in adult returns to a specific source of mortality when we know that natural mortality is extensive and inherently variable.

Since 2003 DFO has increased the monitoring of adult pink and chum salmon that returned to the main rivers and streams to more accurately assess changes in abundance.

DFO has also implemented one program to monitor pink fry emigrating from the Glendale River in an attempt to estimate the rate of marine survival (juvenile to adult return).

The results currently do not confirm or support a direct association between sea lice infection levels and subsequent adult returns.

Returns of pink salmon since 2000 have shown extreme variation and the only scientific publication on this topic reports exceptional marine survival of pink salmon that returned in 2004¹.

DFO and the Pacific Salmon Forum are presently examining how to improve monitoring of pink and chum in the BA and the timing of changes in marine survival rates.

References

1. Beamish, R.J., S. Jones, C. Neville, R. Sweeting, G. Karreman, S. Saksida, and E. Gordon. 2006. Exceptional
    marine survival of pink salmon that entered the marine environment in 2003 suggests that farmed Atlantic
    salmon and Pacific salmon can coexist successfully in a marine ecosystem on the Pacific coast of Canada.
    ICES J. Mar. Sci. 63: 1326-1337.

2. Jones, S. R. M. and A. Nemec. 2004. Pink Salmon Action Plan: sea lice on juvenile salmon and some non
    salmonid species in the Broughton Archipelago in 2003. Canadian Science Advisory Secretariat Research
    Document 2004/105. Fisheries and Oceans Canada.

3. Jones SRM, Prosperi-Porta G, Kim E, Callow P, and Hargreaves NB. 2006a. The occurrence of Lepeophtheirus
    salmonis and Caligus clemensi (Copepoda: Caligidae) on threespine stickleback Gasterosteus aculeatus in
    coastal British Columbia. J. Parasitol. 92: 473-480.

4. Jones, S., E. Kim, and S. Dawe. 2006b. Experimental infections with Lepeophtheirus salmonis (Kroyer) on
    threespine sticklebacks, Gasterosteus aculeatus L., and juvenile Pacific salmon Oncorhynchus spp. J. Fish
    Diseases 29: 489-495.

5. Krkosek, M., A. Morton, and J.P.Volpe. 2005a. Nonlethal assessment of juvenile pink and chum salmon for
    parasitic sea lice infections and fish health. Trans. Am. Fish. Soc. 134: 711-716.

6. Krkosek, M., M. A. Lewis, and J. P. Volpe. 2005b. Transmission dynamics of parasitic sea lice from farm to
    wild salmon. Proc. R. Soc. Lond Ser B. 272:689-696.

7. Krkosek, M., M. A. Lewis, A. Morton, L. N. Frazer and J. P. Volpe. 2006. Epizootics of wild fish induced by
    farm fish. Proc. Natl. Acad. Sci. USA 103:15506-15510.

8. Morton, A.B. and R. Williams. 2003. First report of a sea louse, Lepeophtheirus salmonis, infestation on
    juvenile pink salmon, Oncorhynchus gorbuscha, in nearshore habitat. Can. Field-Nat., 117(4): 634-641.

9. Morton, A., Routledge, R., Peet, C. and Ladwig, A. 2004. Sea lice (Lepeophtheirus salmonis) infection rates
    on juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon in the near shore marine
    environment of British Columbia, Canada. Can. J. Fish. Aquat. Sci. 61, 147-157.

10. Morton, A. and R. Routledge. 2005. Mortality rates for juvenile Pink Oncorhynchus gorbuscha and Chum O.
    keta salmon infested with sea lice Lepeophtheirus salmonis in the Broughthon Archipelago. Alaska Fish. Res.
    Bull. 11:146-152.

11. TNAR (Tsimshian Nation Aquatic Resources). 2005. North coast marine baseline survey and sea lice
    research program 2004. 57p. Tsimshian Tribal Council, 138 1st Ave. W., Prince Rupert, B.C. V8J 1A8.

12. Wertheimer, A.C., et al. 2003. Sea lice infection of juvenile salmon in marine waters of the northern region
    of southeast Alaska, May-August 2003. (NPAFC Doc. 706) 13 p. Auke Bay Lab., Alaska Fish. Sci. Cen., Nat.
    Mar. Fish. Serv., NOAA, U.S. Dept. Commerce, 11305 Glacier Highway, Juneau, AK 99801-8626, U.S.A.