Lake Ontario Model Affirms State's Decision to Stock Fewer Game Fish

BUFFALO, N.Y. -- Owners of charter-boat fishing companies on Lake Ontario loudly protested the decision by New York State in 1993 to cut by 50 percent the amount of gamefish stocked in the lake.

A new mathematical model of the lake developed by University at Buffalo researchers shows that the state's action made good scientific and economic sense.

The model was developed to investigate the potential for a decline in alewife, the main fish on which Lake Ontario salmon feed, should stocking levels of gamefish get too high.

According to the model, if the stocking of salmon and trout had continued at a high level and the levels of certain nutrients, such as phosphorus, in the lake, remained low, the collapse of the whole fishery would be a long-term possibility, especially if there were severe winters. "If the alewife population declines too much, the lake could lose its main food supply for gamefish and, if that happens, these fish won't grow because they won't have anything to eat," explained Joseph V. DePinto, Ph.D., director of the UB Great Lakes Program and principal author of the study.

A paper describing the model by DePinto and Rajeev Jain, UB doctoral student, is published in the current issue of the Journal of Aquatic Ecosystem Health.

The model developed by DePinto and Jain provides scientists with a way of understanding how water quality is affected by the complex interactions among nutrient loads and the levels of fish stocking and harvesting in the lake.

By taking what the researchers describe as a more holistic approach to depicting an aquatic ecosystem, this type of model may prove to be a useful tool in predicting how other systems respond to various environmental changes.

"This kind of approach recognizes that humans are just another part of an ecosystem and that long-term ecosystem health cannot be maintained merely by considering human needs," said DePinto.

The model takes into account another potential cause for a decline in alewife: the reduced loading of phosphorus, a key nutrient for the growth of algae. The algae constitute the base of the food chain in the lake, converting solar energy to food for organisms all the way up the food chain.

Under the restrictions on phosphates in the lake recommended under the Great Lakes Water Quality Agreement, phosphorus loading to Lake Ontario decreased by approximately 50 percent between 1970 and 1985. Over the same period, gamefish stocking to the lake increased by more than 400 percent.

In 1993, in an effort to avoid a total collapse of the ecological balance in the system, the New York State Department of Environmental Conservation and the Ontario Ministry of Environment cut back by 50 percent the amount of salmon and trout they stock in the lake, to reduce predation on alewife.

However, the charter-boat fishing companies, who rely on a lake that is well-stocked with game fish like salmon and trout, contended that it was not the overstocking of alewife predators that had led to the decline.

"Both sides were guessing," said DePinto. "The question was, whose guess was more educated?"

To find out, DePinto and Jain have been working to integrate available information on fish in Lake Ontario into a modeling framework. The project was funded by New York Sea Grant and the New York State Legislature.

"The state cut the stocking levels because of concern that by keeping them as high as they were through the early 1990s, they might be in danger of wiping out the fish the salmon and trout feed on," said DePinto. "Our model suggests that was a wise decision."

DePinto added that the model also suggests that stocking more gamefish would result in diminishing returns since there would be less fish in the lake for the extra gamefish to eat, resulting in poor growth rates and increased mortality.

"The bottom line is that this level of stocking is pretty consistent with what we call the carrying capacity of the lake and stocking much more than this won't help," he said.