Double-crested
cormorants can never deplete fishThis is a copy of an article in the Toronto Star by Barry Kent MacKay - July 30, 2000
Double-crested
cormorants can never deplete fish
This is a copy of an article in the
Toronto Star by Barry Kent MacKay - July 30, 2000
At the Ontario Ministry of Natural Resources and
the Canadian Wildlife Service, biologists understand that colonial water birds
do not deplete native fish stocks.
The political masters of those biologists, however, are under pressure from a
less knowledgeable but powerful sport and commercial fishing lobby to kill many
of the double-crested cormorants whose numbers are currently increasing in the
Great Lakes.
As reported here recently, a five-year provincial study of cormorants is
underway that is unlikely to add anything to what we already know about the
relationship between cormorants and fish. To the simple-minded and
biologically illiterate, the equation goes something like this: cormorants eat
fish, therefore too many cormorants will eat too many fish.
But it cannot happen.
Cormorants don't endlessly hold their breath and patrol all corners of the
sub-aquatic environment. In fact, their plumage is only lightly water
repellent. They simply can't endlessly swim but must spend time perched
above water, usually on a snag, rock or piling, holding out their wings to dry.
The process of diving underwater, fighting natural buoyancy, and pursuing and
capturing prey all take energy. There must be enough fish to allow the
calories consumed to balance those expended. Long before fish stocks are
depleted, the cormorants ( like other such natural predators ) reach a point of
diminishing returns. Thus a large number of cormorants is a good sign, not
a bad sign, clearly indicating a large number of fish.
While cormorants lay two to seven eggs ( usually three or four ) at a time, fish
lay thousands. Fish have what biologists call a high recruitment
potential.
Cormorants must share the fish they do catch with dependent, immobile
young. Not only must there be enough fish to maintain the cormorant and
its family, but those fish must also be located near the nesting colony. I
again emphasize that that cannot happen unless there are significant numbers of
fish ( who lay tens to thousands of more eggs, on average, than cormorants ).
Cormorants tend not to go after the large commercial and game fish. A
large fish provides more nutriment than a small one, but often at a greater
expenditure of energy to the cormorant. The truly large game and
commercial fish, which are also prolific egg-layers, are simply too big for a
cormorant to swallow.
Compared to predatory fish, not to mention human fishers with their gill nets,
lures, high-tech fish finders and various other paraphernalia, cormorants are
inefficient fish catchers.
Ontario is blessed with a huge volume of water, filled with a wide variety of
naturally occurring fish. That's not enough, so still more fish, many not
native, are dumped into the water for the benefit of sports fishers. And
still the cormorants are supposed to be "controlled" because of
irrational fears that somehow cormorants will do the impossible and reduce fish
stocks.
It never has happened because it can't happen.
Flock of Double-crested cormorants taking off south of Cove Island, near Tobermory.