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correlation?

Posted by tideliner on 04 Mar 2011 at 22:00 GMT

How can you arbitrarily deem testing sites to be high and/or low exposure so proximate to each other in Georgia Strait? You said yourself that the tide can move nauplii up to 8km in a single day.Wouldn't nauplii from fish farm lice be broadcast in the same manner ?oscillating back and forth in the tide. These are high current areas with phenomenal tidal flow. The only test site that may come close to representing an upstream location for sampling was dismissed as an outlier, yet it had the highest counts The reason that you see the fish farms as the likely and probable source of the infection is that you aren't looking anywhere else.Georgia Strait in its entirety is likely a good breeding ground for sea lice.( it has salinity and temp. profiles to facilitate inital contact) Wild salmon have always been affected by sea lice long before the presence of salmon farms and freshwater farm- raised smolts are regularly infected by sea lice in all farming areas. There is a considerable source of sea lice on this coast other than the salmon farms. Always has been.
To suggest that the processing plant was a possible source of the infection and not discuss the much higher likelihood of the source being somewhere along the route between the Fraser and your testing site in your outlying sample location is irresponsible . The host-parasite relationship between L. Salmonis and our wild pacific salmon has always been there. They evolved together.
You can not down play salinity and temperature as factors. They over ride all other factors when it comes to initial contact between host and parasite. As a control, why choose the Skeena River area? Sure there are no fish farms but lower temp and salinity profiles more adequately explain the lesser levels of louse infection. furthermore the fish sampled as the control are genetically distinct from the Fraser fish. Doesn't it make sense to have a control in the Gulf Islands or anywhere along the east coast of Vancouver Island where you would easily find juvenile fraser river sockeye that you could reasonably assume hadn't been exposed to a fish farm . Thus eliminating the significant variables of salinity temp and genetic pre- dispositon to infection
If you displayed salinity, temp and fish size alongside the louse counts and included the outlier ; much different conclusions could be drawn from your research.Conclusions that would contravene both your hypotheses and premises.

No competing interests declared.

RE: correlation?

pricem replied to tideliner on 07 Mar 2011 at 20:11 GMT

We did not arbitrarily deem our collection sites for sockeye as high or low exposure; we used molecular genetic analyses to determine whether fish had either passed a salmon farm (i.e., were caught downstream of a farm and were therefore exposed to farm-origin pathogens) or not. The summer discharge of the Fraser River means that net movement of water through this region even within a tidal cycle is dominated by outflow; thus, the sites were characterized as upstream of farms or downstream of farms depending on where we caught fish. If there was effective mixing of the waters, and salmon farms were not the primary source for sea lice as the commentator suggests, we would not have observed the pattern of more heavily Caligus infected juveniles downstream of farms.

We did consider other sources for the lice beyond salmon farms, such as herring or other wild juvenile salmon. However, as we wrote in our paper, these alternative sources would need to be situated in such a way that matched the salmon farms in the region (i.e., more louse-infected herring or juvenile salmon downstream of salmon farms).

We used two control areas, one on the north coast where there are no salmon farms, and the other upstream of salmon farms in the Discovery Islands. This enabled us to compare differences in sea lice infection levels between exposure regimes. Our results, that juvenile sockeye downstream of salmon farms hosted more sea lice than those upstream of farms, could not be explained by differences in salinity or temperature, which we measured and analyzed. In other words, our paper did exactly what the commentator has suggested: we did consider “salinity and temperature alongside the louse counts and included the outlier site”, and it was simply not true that “much different conclusions could be drawn from your research.” The only consistently significant factor to explain the most abundant louse species infecting juvenile sockeye was the presence of salmon farms. 

No competing interests declared.