Killing the competition: Cormorants

Tuesday, March 17, 2009 at 2:11 PM Bookmark and Share
Today I came across a nice overview of the current problems surrounding Cormorants and the push to kill off large numbers to maintain or increase fishing yields. The article appeared as this months Natural History Magazine's featured article "To Kill a Cormorant" by Richard J. King.


If you haven't heard of this little conflict before, check out this New York Times article from 1998: The Slaughter of Cormorants in Angler Country. For a closer look at a Cormorant, click here, here, and especially here.

Why the conflict? The main causes are aesthetics and good ol' interspecific competition for resources - one of the big claims is that Double-crested Cormorants deplete populations of game fish. The validity of this claim is somewhat of a mixed bag: Cormorants indeed cause declines in commercial fish farms where fish are accessible and at high densities. In wild settings however, evidence is weak and their impact on fish populations seems to depend on the location and the fish species in question.

Competition for fish aside, Cormorants can also trash the small islands they decide to turn into breeding sites via a build-up of guano, which like most bird droppings is high in uric acid (the white stuff). This kills off vegetation, giving the combined result that these islands are as unpleasant to look at as it is to be downwind of them. As Richard King points out, historically those islands may have actually hosted Cormorants and their guano more so than the vegetation that took their place when Cormorant numbers dipped during the last couple of centuries. They can also compete for nest sites with various species of Herons and Egrets. This all adds up to a big public image problem - people don't like things that ruin scenic landscapes and push out more valued species.

So do Cormorants need to be controlled? Practically speaking, probably yes - but only in a few places, and hopefully with smarter controls than poorly regulated mass culling.

Are we going to avoid continuing on a "witch hunt" blaming these birds for our fisheries problems, or instead use good science to get at the root of this conflict? With the usual amount of patience and compromise, it seems a workable science based solution is quite possible.

In their Notice of Intent to Prepare a National Management Plan for Double-Crested Cormorants the US Fish and Wildlife Service (USFWS) mentions a court challenge against the USFWS for issuing permits to oil eggs in up to 10,500 Cormorant nests at two locations in the north east. Referring to that legal action, they go on to say that:
... the action highlights the need for scientific inquiry into the nature of the problems caused by double-crested Cormorants and an assessment of the utility of management actions most likely to resolve resulting conflicts.
By studying fish populations and the role Cormorants play in regulating them, along with the other problems caused by Cormorants, regulatory agencies can make effective and efficient management plans where necessary. Just as importantly, if not more so, the results of these studies will hopefully help the public to distinguish fact from fiction when it comes to Cormorants - good for the birds, and good for the people.

Related links:
  • The USFWS Double-crested Cormorant page. Includes their final Environmental Impact Statement, which is worth skimming.
  • EPA website search results.
  • Derby and Lovvorn, Canadian Journal of Acquatic Science, 1997. Some might take this as evidence against Cormorants - but based only on the abstract it appears to conclude little more than They ate a lot of the fish we put in the river. This, is what you'd expect: Cormorants eat fish. This may be a good study to look at stocking efficiency, but for our purposes the important question is what is the impact on the fish population?
  • That question is addressed, for example, in Engström, Ecography, 2009. The result? Despite large rates of consumption by Cormorants, there was no significant effect on the fish population.
  • To contrast, here is some commentary on a study showing Cormorants can impact some fish populations (Perch, in parts of Lake Huron).
  • Google scholar results for "cormorant fish".

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