Raincoast biologist Michael Price is lead author on the recently released peer-reviewed paper Evidence of farm-induced parasite infestations on wild juvenile salmon in multiple regions of coastal British Columbia, Canada. Co-authored by Alexandra Morton and John Reynolds, and published in the Canadian Journal of Fisheries and Aquatic Sciences, the paper suggests that salmon farms in multiple regions elevate levels of sea lice on wild juvenile salmon. Below, Michael addresses some of the criticism leveled against the paper from the fish farm industry.

Positive Aquaculture Awareness – November 9, 2010

Criticism: “What we have here is a deliberate failure to communicate. When you look closely, the authors make their data say all sorts of things that it doesn’t really support. And at the end of the day, they are repeating the same tired old unproven mantra (“there are lots of sea lice on fish around salmon farms, therefore they are coming from salmon farms, therefore we must put all salmon farms on land”). Any Logic 101 student could point out the flaws in that argument. Correlation does not equal causation.”

Response: Because independent scientists in British Columbia do not have access to salmon farms or farm data, and are thus restricted to field-based correlative analyses to test the effect of salmon farms on sea lice infestations, evidence becomes compelling only after extensive replication. Associations between wild juvenile salmon and farm-induced sea louse infestations have been repeatedly documented in the Broughton Archipelago (Morton et al. 2005; Krkosek et al. 2005a, 2006) and the Georgia Strait (Morton et al. 2008).

BC Salmon Farmers Association – November 10, 2010

Criticism: “The trouble is that their overly-simplistic analysis has significant gaps, reuses questionable methodology and is based on flawed assumptions. For example, the ‘control site’ – the area with no farms where they collect samples to compare with fish from near farms – has significantly lower salinity levels than the other locations. Since sea lice levels are naturally lower where salinity is reduced, this makes it a poor comparison.”

Response: Salinity readings were lower on average in Bella Bella where there are no farms, but were within the tolerable range for both sea louse species. It is true that this may have accounted for the exceptionally low lice levels on juveniles in the Bella Bella region. However, we also compared lice levels within each salmon farm region between sites of Low Exposure to farms and High Exposure to farms. Juvenile salmon assessed at High Exposure sites consistently hosted significantly higher lice levels than juveniles at Low Exposure sites in all 3 salmon farms regions, even though salinity values were similar at all sites. This suggests that salmon farms are a contributing factor to louse infection levels.

We used a sophisticated statistical analysis to further test whether salinity was a factor in the prevalence of sea lice on juvenile salmon; we also tested temperature and fish host size. However, the only significant factor to explain the data was Exposure to salmon farms. Variation in salinity, temperature, and host size that we recorded throughout our 4 study regions did little to explain sea lice infection levels on juvenile salmon.

Fish Farming Xpert – November 11, 2010

Criticism: “The number of sea lice in the natural environment is affected by the salinity of the ocean. There is a genetic difference between sea lice in the Pacific Ocean and the Atlantic Ocean. Wild salmon in BC have a natural ability to shed sea lice once the fish have reached a certain size. These are all important points proven in the expansive body of sea lice research that need to be considered when reviewing the latest paper from anti-salmon farm campaigner Alexandra Morton, Michael Price and John Reynolds.”

Response: Salinity is one factor among many that may influence the number of sea lice in coastal areas; temperature of the ocean is another. However, a more influential factor is the presence and abundance of host fish for lice to successfully reproduce and complete their life cycle. Salmon farms host millions of domesticated hosts in crowded conditions for long periods of time – a classic scenario for pathogen proliferation, and sea lice are one such pathogen that proliferates on salmon farms.

Wild salmon in BC may have a natural ability to shed sea lice if an individual is large enough, if the numbers of sea lice are few, and the sustained infection pressure is short. However, we assessed numerous juveniles within the vulnerable size of direct mortality to sea lice infection, and the period of infection pressure was sustained for more than two months. These conditions have been shown to place juveniles at high risk of direct mortality to sea lice infection. Of greater concern, however, is the likelihood of indirect effects. Juvenile salmon hosting lice are more at risk to predation than juveniles not hosting lice, and this indirect mortality effect may be of greater consequence to salmon populations than direct effects.