Scientists removed 70% of a lake’s logs, and yellow perch nearly vanished within two years

Underwater logs may look like debris, but a Wisconsin experiment showed how quickly freshwater ecosystems can change when that structure disappears. Scientists removed more than 70% of the coarse woody habitat from one basin of Little Rock Lake. Within two years, yellow perch, once the lake’s most abundant fish, had fallen to very low numbers.

The study took place at the 18-hectare, undeveloped and unfished lake in Vilas County. A large curtain divides it into two similar basins, preventing fish and water from moving between them. That arrangement gave researchers a rare whole-lake laboratory: they removed woody debris from the shallow shoreline of one basin in 2002, while leaving the other basin unchanged as a reference.

Fallen trees and branches create hiding, feeding and sheltering places for fish. They also increase the range of habitats available underwater. Before the experiment, the two basins had broadly similar food webs dominated by aquatic prey. After the removal, however, their fish communities began to diverge.

The sharpest change involved yellow perch. Researchers linked the decline to increased predation and little or no successful recruitment of young fish. In the reference basin, perch produced several generations of young in successive years and rebuilt their population.

Largemouth bass changed as well. In the basin without the logs, they ate less fish and relied more on prey from land. Their growth also slowed compared with bass in the reference basin. The findings suggest that removing underwater structure can reshape predator-prey relationships across an entire ecosystem, rather than affecting only one species.