Food for fish dwindling on developed lakes

A pulse of midges swarms over Lake Malawi in Africa. Photo credit: The Daily Mail.

Freshwater fish often rely on terrestrial insects as a portion of their food supply. In lakes, the size and shape of the lake can determine how much the fish rely on terrestrial insects for food. But with humans’ love of lakefront property, the resulting development of lakeshores could have an impact on these insect subsidies.

Tessa Francis, a post-doctoral researcher at the National Oceanographic and Atmospheric Administration (NOAA) asked this question in her talk yesterday at the ESA Annual Meeting. She identified fish stomach contents over the course of a year in four Pacific Northwest lakes, surveyed fish in Pacific Northwest lakes — 28 of them — and compiled published data on fish populations in lakes across North America.

Francis found that at undeveloped lakes, insect outbreaks often happen in pulses, where insects emerge over a short time period. She remembers pulses of millions of flying ants at one undeveloped lake in British Columbia during the spring of her study season, but only days later the ants had all but disappeared.

In highly developed areas, however, these insect pulses vanish. This disparity was apparent in fish food availability: In the undeveloped 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. What’s more, Francis’ large-scale assessment of published data also showed this pattern at the regional and national scale.

This difference in food subsidy translates into fish behavior and nutrition. Francis found 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.”