Closing the net on foreign fish

A Canadian ecology team has a warning for fishery managers: Beware of trying to "enhance" a fishery by introducing alien fish into freshwater lakes. They can disrupt established food chains in subtle ways.

In fact, the three ecologists say, this worldwide practice probably should be curbed. M. Jake Vander Zander, with the University of California at Davis, and John Casselman and Joseph Rasmussan, with the Ontario Ministry of Natural Resources in Picton, only studied a few Canadian lakes. Yet their research has broad implications.

So too does the tool they used to gain their insight. Elements come in different forms called isotopes that have slightly different weights. The ecologists worked with two forms of carbon and two of nitrogen. The ratio of the two carbon forms - carbon 13 and 12 - in a fish's body reflect the fish's food source. The ratio of nitrogen 15 to nitrogen 14 reflects where an animal is in the food chain. It shows whether a fish is a top predator dining on other fish, in the middle of the eat-and-be-eaten food chain, or near the bottom of the chain, grazing on tiny animals called zooplankton.

Some of the lakes in the study had been stocked with nonnative small-mouth bass and rock bass. Other lakes, used for reference, are still in pristine condition. In all the lakes, the native top predator is, or had been, trout.

Reporting in Nature magazine, the ecologists say their findings "indicated that, on average, the lake trout diet in the pristine lakes was 62 percent fish while in the invaded lakes it had dropped to 22 percent." The bass apparently were cleaning out the trout food fish, forcing trout to move down the food chain toward a plankton-based diet. It's as though a stranger walked into a steak house, snatched all the meat, and left the restaurant to serve cattle feed. Just as the steak house would lose customers, so the affected lakes lost some of their trout.

The problem isn't clearcut. In some instances, lake ecology is so structured that trout and bass don't compete for the same food fish. But where there is competition, nonnative invasive predators make native predators suffer. The ecologists conclude that "protecting native fish populations will often necessitate halting the intentional introduction of bass and other predators, and limiting the use of live bate by anglers." Bait fish often escape and invade a lake.

Isotope ratios can help pin down human impacts on natural food chains. They can untangle an otherwise confusing web of food sources.

Paul Stapp explains that such findings "show the novel information you can get from isotope studies." For example, isotope analysis of organic remains can trace food webs of ancient human habitations. This has "huge implications for archaeology," he says.

(c) Copyright 1999. The Christian Science Publishing Society