But if the cormorant’s taste for alewives is seen by wildlife managers as a negative, placing them in competition with alien salmon, there is another alien species to be considered: the round goby.
The round goby is a small fish, 3 to 10 inches long, native to the Black and Caspian seas of Eurasia. It is a small, bulging-eyed, bottom-feeding, sculpin-like fish who is believed to have reached the Great Lakes via water taken from the Black or Caspian Sea into the bilges of transoceanic cargo ships and subsequently discharged into the Great Lakes. It is now well established in the Great Lakes in enormous populations, in some places covering the lake bottom at a density of 20 fish per square meter. In 2002 it was estimated that there were no fewer than 10 billion round gobies in the western basin of Lake Erie, smallest and most shallow of the Great Lakes.
The effect of such staggering numbers of this species is not fully known, but the concern is that they displace native fish at a crucial level of the food chain, and consume the eggs and young of many native fish, including both those who are of direct importance to commercial and sport fisheries, and those “forage fish” who such larger fish consume, as well as the smaller organisms who are at the bases of the food chain.
In the long run these characteristics of the round goby could limit the Great Lakes’ ability to support the species, since it is true that no predator can survive the loss of its own food supply. The concern is that irrevocable losses may occur in the meantime. Remember, the round goby has only recently arrived, first noticed in the late 1980s, and so it did not co-evolve with native organisms.2
Cormorants love them — easily obtained bite-sized meals of the perfect size in inexhaustible numbers — and while no one is suggesting that there are nearly enough cormorants in the Great Lakes to make a noticeable dent in the numbers of round gobies, it is both unfair and incorrect to assume that the presence of happily feeding cormorants in waters where round gobies occur translates into consumption of game or commercial fish species. They are very likely doing their best to reduce the number of an alien species that is, ironically, deleterious to the species of fish anglers and commercial fishers most covet.
We emphasize that the necessary strategy of all non-human predators is to obtain maximum benefit for minimum exertion, and round gobies provide a far better opportunity for cormorants to do so than the more active and less numerous species they are so often accused of depleting.
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The Agricultural Subsidy
The Missing Predator Argument
Semi-Science and Wildlife Management
So ... What Do Real Scientists Say?