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Corrolation ≠ causation

Posted by pbio on 07 May 2009 at 22:22 GMT

Author: Dallas Weaver, Ph.D.
Position: semi-retired
Submitted Date: February 21, 2008
Published Date: February 22, 2008
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

The issue of impacts of aquaculture salmon on wild stocks may be a little more complex than implied in this article by Ford and Myers. Their case against aquaculture is based upon correlations, which are not equal to causation, unless we truly understand all the variables. In particular, the authors state than anthropogenic impacts on the control and subject sites are comparable. However, from the viewpoint of the aquaculturist, logistics is a major cost factor and there is a very strong incentive to be as close as possible to harbors, rail and truck facilities, processing facility, airports, support services, etc. This would imply that the controls may be very different in a variety of anthropogenic impacts than the controls ranging from water pollution to seal attraction to harbors with shelter and occasional free food in the form of fishermen’s by-catch.

Meanwhile, the last several decades have seen a large human migration to the coastal areas in most of the world and probably most of the sites studied. The observation that there is a general decrease in both the controls and test sites and that these decreases don’t correspond very well to the time of very rapid salmon farming expansion could raise questions about salmon farming being a cause rather than part of a larger demographic trend.

The fact that the authors stated that the impacts of the salmon farms are apparently non-linear with tonnage and decrease with increasing tonnage would also point to a more general cause. This non-linearity was attributed to the aquaculturists getting better, which is probably true in some respects, but they still have a strong incentive to maximize the yield. In considering disease issues in aquaculture, the density of farms is a critical variable, where the potential for major disease problems always increases with the tonnage and farm density. Witness the present problems in Chile with ISA, which is most heavily hitting areas with high tonnage (Marine Harvest cutting 1000 employees Another example would be the collapse of shrimp farming in Taiwan and then later in China – now recovered at lower farm density. From the viewpoint of the fish farmer, the disease threat is from the wild fish to his farm, where he has the cost of treating or depopulating.

I think we need to fully understand the decreases that are occurring at the control sites before we think about claiming or implying causation by aquaculture.

Competing interests declared: No, I am not involved in salmon farming, but I do consult on water chemistry, treatment issues in shrimp aquaculture and fish hatcheries -- no salmon. I am also involved in aquaculture diseases as chair of the Aquaculture Disease Committee in California and sit on several boards and committees involved in aquaculture. This whole area of aquaculture impacts has more than it's share of "advocacy science" and fund raising opportunities.