Cormorants down to almost half of peak numbers

Spiel · June 13, 2009

Cormorants down to almost half of peak numbers

June 10, 2009

LAKE HURON-Cormorants may still be public enemy number one for many on Manitoulin, but their number has dropped dramatically over the past decade, according to the Ministry of Natural Resources (MNR).

"In 2000, the peak count on Lake Huron was 33,914 nests," said Patrick Hubert, avian biologist with the MNR. "In 2008, the total was down to 19,421 nests."

That means the number of cormorants on Huron has dropped by 43 percent-nearly by half-in less than a decade. And while results of this year's colony census are not yet available, Mr. Hubert doesn't anticipate a new spike in the population.

"I'm not saying it could never happen," he said. "You could get an invasion of food species that would support cormorants, but based on the food sources that exist, they seem to be stabilizing."

The ministry attributes the decline primarily to a collapse in alewives, an imported species that has formed a big part of the cormorants' diet. Mr. Hubert said harsh winters and salmon predation appear to be the two main factors in the alewife crash.

Cormorants are scarcer across Lake Huron, according to the biologist. "The decline has been in all three areas of the lake-Georgian Bay, the North Channel and the main body."

This represents the most glaring cormorant dip in the province, but numbers are also generally levelling off across the basin. "They've at least stabilized, if not declined, on the lower lakes as well," he said. "The population has probably reached its carrying capacity on the Great Lakes."

An exception to the trend would be Lake Superior, where numbers have actually risen in recent times, although Mr. Hubert characterized this increase as "minor," and not enough to explain the dwindling number of birds in our area. Superior's cormorant accretion "is not an order of magnitude that is consistent with the decline in Lake Huron," he said.

Angling groups feel the species is just as plentiful as ever, however, and has simply spread to other feeding grounds, whether that's neighbouring Great Lakes or various inland waterbodies. "We believe their nesting habitat is decreasing, so they have had to move on to other areas," said Rob Pineo, a forestry and wildlife biologist with the Ontario Federation of Anglers and Hunters (OFAH), in the recent edition of Ontario Out Of Doors.

If so, the MNR has little solid evidence of the diaspora. Banding of cormorants has been carried out in previous years at various nesting colonies on Lake Huron-400 birds were identified with pink-and-white leg bands in 2006, for instance, at a number of nesting sites in the North Channel and Georgian Bay-but none of these has been reported on Lake Superior, and few have shown up elsewhere in the province.

Some have been observed in wintering grounds in the US, but on this side of the border they don't seem to have dispersed in any appreciable way. "We haven't heard of our banded birds showing up in other areas in large numbers," said Mr. Hubert.

A few years ago, "there was a lot of concern about movement inland," he said. "But we've heard a lot less about that over the past couple of years. What we've heard, anecdotally, is that they may move in for a couple years, and nest, and then move on."

While some of Huron's birds have no doubt sought greener pastures-or bluer pools, as the case may be-the biologist said there could be other factors. "It's very difficult to say what's happened to those birds," he said. "It could be attributable to survival during the breeding season and reproductive output as well."

And while some may have found homes on inland lakes-be that on Manitoulin, or the provincial mainland-Mr. Hubert believes they prefer the bigger water, and will only reside temporarily on a smaller waterbody. "They show less fidelity to the inland lakes," he said.

In part, this is because there tends to be more human activity at such sites as compared to the remote rocky islands of Lake Huron. "They're a pretty wary bird, and if disturbed enough times, particularly with ground-nesting colonies, they won't stay around," he said.

Some of Huron's birds were removed a couple of years ago through vigilante culling, according to observations made by MNR personnel during nest counts undertaken in 2007. Between the birds that were shot or clubbed, and the eggs that would have hatched into birds had they not been crushed, it was estimated that about 3,000 cormorants were taken out in this fashion.

This is still a fraction of the number that has disappeared since the year 2000, however. By last year, there were 15,000 fewer nests at known colony sites, meaning 30,000 fewer breeding adults, so illegal culling would only account for a tenth of the decline-if that, as the nest counts don't take into account non-breeding juveniles.

Mr. Hubert said there was considerably less evidence of human disruption apparent at nest sites during last year's survey of colonies in Georgian Bay and the North Channel. "It appears that it's not the problem it was a couple of years ago," he said.

The ministry is continuing to visit these roosting and breeding areas to track fluctuations in the population, with another round of nest visits occurring at present on Lake Huron, although it's too early to provide results. "We're not done counting for this year," he said. "Hopefully in another couple of months we'll have an idea of what the numbers look like."

Apart from giving a picture of cormorant density, the colony assessments also provide researchers with a sense of the birds' dietary habits, based on regurgitant and other fishy remains that can be observed around the nesting areas. It doesn't provide a complete picture, as the smaller fish tend to be fully digested, but the MNR can glean some useful information about what the species is currently eating.

"It seems to vary from year to year, even within a colony," said Mr. Hubert. "In some areas they are eating a lot of gobies (an invasive species), while in other areas it's crayfish, suckers, rock bass, and the odd sport fish. Last year, an alewife was seen once in a while, but they're mostly absent now."

The birds' almost indiscriminate taste "is very consistent with what the literature says about their diet," said Mr. Hubert. "They're very opportunistic."

Pointing out the various species that find their way into a cormorant's belly will inevitably draw a mixed response, he suggested. The mention of gobies and crayfish will add fuel to the argument of cormorant defenders that the species is mainly dining on invasive and non-sport species, while evidence of smallmouth bass or perch will come as confirmation to anglers that the birds are wreaking havoc on the sport fishery.

"It's such a charged issue," said Mr. Hubert. "People see what they want."