Fish Gotta Swim

Yellow perch (Division of Public Affairs/U.S. Fish and Wildlife Service)

Is a lake an island? Fisheries managers have tended in the past to think so, treating each lake as a closed system. Man may move freshwater fish around, stocking some here and removing others there, but the fish themselves were judged to be mostly sedentary. Now many ichthyologists are expressing second thoughts, and suggest that, for their finny subjects, lakeshores do not a prison make.

“Fish Movement Among Lakes: Are Lakes Isolated?” is the title of a recent paper in Northeastern Naturalist (2008; Volume 15, Number 4) by Robert A. Daniels of the New York State Museum and several colleagues. Since 1995, these researchers have studied fish populations in five lakes linked by streams in the Adirondack Mountains. They netted, tagged, and released adult white suckers, brown bullheads, yellow perch, and other species, and also took advantage of data from local stocking programs that released largemouth bass (not native to the Adirondacks) and smallmouth bass.

Their results proved that a small (about one percent) but significant number of the tagged fish moved through streams from one lake to another. White suckers were the most mobile, making their way to new lakes in as few as 21 days after being tagged. But each of the species appeared in new lakes or in connecting streams. The researchers determined that their movements were not connected with any of the species’ habitual upstream migrations to feed or spawn.

Although this dispersal is a low-level phenomenon that probably has little effect on the population ecology of individual lakes, it poses new questions for biologists. Daniels and his colleagues point out that those regular movements can maintain the genetic similarity among populations throughout the watershed, but also spread diseases and parasites among the lakes.

“Dispersing fish can replenish declining or extirpated populations, or, as in the case of Largemouth Bass described here, can serve as the vanguard of an invasion of an exotic species,” they write. “Rather than be treated as isolated populations, fishes in lake communities may be better treated as a watershed-wide metapopulation.”

Whether we’re fishes or just folks, our world seems to get bigger all the time.