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Cormorants: The Agricultural Subsidy

Published 01/25/11
By Barry Kent MacKay, Senior Program Associate

Double-breasted cormorant
(Painting by Barry Kent MacKay)

Whether we are talking about white-tailed deer, Canada geese, snow geese, double-crested cormorants, blackbirds, raccoons, crows, coyotes or any other native species of wildlife accused of being “too” common, one of the favorite reasons wildlife managers like to refer to, in order to explain why there has been an unprecedented increase in population size, is often called the “agricultural subsidy.”

Put simply, the argument goes that since pre-colonial times the carrying capacity of environments in North America has greatly increased for the targeted species, usually because of the presence of nutriment-rich farmland. This farmland features plants that have far greater nutriment value per unit of land than did the plants that existed on the continent in past centuries, in part because selective breeding of such species as corn has produced a far greater nutriment value per unit of land occupied than was present in ancestral strains of the crop, but also because many of the crops didn’t even exist in North America prior being brought here by pioneer farmers.

The argument is favored because it can be addressed in scientific terms with comparisons of caloric yield between current grains, for example, and earlier strains, and between acreage available now compared to an earlier time. Canada geese not only benefit from the availability or more nourishing grains, such as corn, than their distant ancestors had access to, but also because they are grazers, and turf grass provides a vast amount of food that was not available in earlier centuries.

Of course cormorants are not grain eaters, nor grazers of grass. But there are other forms of agriculture and the one that is of potential benefit to cormorants is aquaculture, the production of farmed fish. Aquaculture is, particularly in Asia, a relatively ancient form of agriculture to be sure, but really only started to gain popularity in North America in the closing decades of the 20th century, somewhat coinciding with a subsequent increase in double-crested cormorants, particularly in eastern North America.

In the Southeastern United States, farmers have found that fields that are largely depleted of the nutriment value required for plant crops can be surrounded by broad-topped levees, filled with water, and stocked with catfish and/or other marketable aquatic species. Fish farmers can produce from 4,000 to 8,000 pounds of fish per acre of such ponds, or sometimes more. The enterprise, like any agriculture, carries inherent risks and vagaries, including competition from foreign producers, disease, declines in water quality, equipment or power failures, freak weather events, and so on. Among the problems encountered by fish farmers are predators, including egrets and other herons, anhingas, grebes, pelicans, ospreys, mergansers and, of course, cormorants.

Wildlife managers argue that the fish in farms constitute an “agricultural subsidy.” The argument is that the fish farms have greatly enhanced the carrying capacity of cormorants wintering in the south over what it would have been in primal times. Young cormorants who are inexperienced at catching fish, we are told, would be more inclined to starve but for the easy pickings to be had at fish farms.

This sort of theory is beloved by wildlife managers as it serves the need to find a reason to reduce cormorants, thus placating the anti-cormorant lobby.

There are two problems with the idea that fish farms have led to an unprecedented increase in double-crested cormorants. First and foremost, the evidence is that there has been no such increase in double-crested cormorants over primal numbers.

How can that be? The problem is that cormorants are vulnerable and twice since European colonization of North America their numbers have greatly declined. Cormorant detractors ignore the first, largely undocumented, decline. If you compare current numbers of cormorants, or anything else, to the numbers that existed after populations have been depleted, of course you will see an increase. If you take 1,000 people, reduce that number to 100, and a generation later you can count 300 people, you can claim that there the population has tripled its size, but not that there are more people than ever before, only that there are more people than there were when their numbers had been depleted.

In 2006 scientists (not wildlife managers) published an important, peer-reviewed paper, “Historic Populations of the Double-crested Cormorant (Phalacrocorax auritus): Implications for Conservation and Management in the 21st Century,” by Linda R. Wires and Francesca J. Cuthbert of the University of Minnesota, in the journal Waterbirds, Volume 29. What Wires and Cuthbert found was that flocks much larger than any seen in modern times were recorded before constant persecution by humans wiped out so many birds and breeding colonies. This was long before DDT was introduced into the environment and began to wreak havoc on many fish-eating bird species, including the double-crested cormorant. For example, in 1891 a flock of migrating cormorants that was described as being four miles long and one and a half miles wide was seen in Minnesota. And as late as 1926, a flock estimated to include between 100,000 and 1 million birds was seen migrating up the Mississippi River. We know this thanks to the work of Wires and Cuthbert digging up old records. We also know that no such numbers currently exist, ergo, the species was once more common than it now is.

We don’t know for sure where members of those huge flocks of cormorants nested, although it was presumably in the lakes and sloughs of the prairie provinces, and possibly — particularly in the case of the Mississippi birds — some may have nested in Ontario. Flocks of these magnitudes have long since disappeared, but the question arises of how they existed in the absence of the aquaculture and fish ponds and impoundments now deemed by wildlife managers to be essential in supporting current, if smaller, numbers of the same species. There is no record of nesting colonies that would have accommodated such large numbers of cormorants, and yet they were somewhere, and subsequently greatly reduced in number or totally eliminated in advance of competent chroniclers recording them on their breeding grounds. These old records demonstrate that cormorants existed in numbers greater than now, and were reduced in number before there was accurate chronicling of their breeding numbers.

How did wildlife managers react to the news that they had been wrong in claiming that the “agricultural subsidy” had artificially enhanced cormorant numbers? They generally ignored it and kept making the claim. Science does not ignore inconvenient facts, but management too often does.

The other problem with the basic premise that cormorants are at all-time high numbers is that no effort has been tried to prove what, when considered thoroughly, might seem to be an absurd premise: that somehow the environment’s ability to sustain cormorants — its carrying capacity for the species — is made greater as a result of fish farms and water impoundments than it was in earlier centuries, notwithstanding that observers saw as many or more cormorants back then than we see now, at least in much of the continent. There is no proof that the vast and well-documented degradation of the wetland and coastal environment in the wintering range of cormorants since colonial times would not destroy more fish biomass than is compensated for by the advent of aquaculture in the Southeastern United States, and fish ponds elsewhere. It seems likely that neither contention can be proved, one way or the other. Wildlife managers do not even make the effort.

Introduction: Why Cormorants Can't Kill All Fish

Wildlife Management North and South of the Border

Energy Transference, Basic Physics and the Technology Prosthesis

Back in the Real World

Cormorants: Food Chains and Basic Ecological Principles

The Alewife, Alien Salmon and Trout, and the Double-Crested Cormorant

The Round Goby

The Difference Between Science and Management

NEXT » The Missing Predator Argument

Limiting Factors

Semi-Science and Wildlife Management

So ... What Do Real Scientists Say?

Footnotes

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