Freshwater fish, especially stream fish, rely on terrestrial insects as a portion of their food supply. But little is known about their importance to fish in lakes, where the size and shape of a lake can determine how much its fish rely on shoreline food sources.
Tessa Francis, a post-doctoral researcher at the National Oceanographic and Atmospheric Administration (NOAA), wanted to know if the urbanization of lakeshores affects the amount of terrestrial insects available as food to the lake's fish inhabitants. To answer her question, she analyzed fish stomach contents over the course of a year in four Pacific Northwest lakes, surveyed fish in 28 Pacific Northwest lakes and compiled published data on fish populations in lakes across North America.
At undeveloped lakes, insect outbreaks often happen in pulses, in which insects emerge over a short time period, but Francis and her team found no signs of these pulses in highly developed areas. This disparity was apparent in fish food availability: In the four lakes, terrestrial insects comprised up to 100 percent of the diet of fish in undeveloped lakes, in contrast to a maximum of 2 percent in developed lakes, a pattern that was also apparent at the regional and national scale.
Francis' research also showed that trout in developed lakes had a 50 percent lower daily intake of energy. Lower energy intake can slow growth and compromise fish reproduction, she says, which will ultimately lead to population declines. But she emphasizes that even a small amount of shoreline vegetation can serve as insect habitat.
"Our shorelines need to remain as intact as possible, with a mix of trees and shrubs," she says. "But we may not need a dense, native forest. There likely are designs that are compatible with both lakeshore development and sustaining lake food webs."
This research was presented at the Ecological Society of America's Annual Meeting on August 3, 2009.
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