Tessa B. Francis, Northwest Fisheries Science Center, NOAA and Daniel E. Schindler, University of Washington.
Background/Question/Methods It is well established in stream ecosystems that riparian vegetation is associated with the flux of terrestrial insects to streams, which then become prey for fish. However, little is known about the importance of terrestrial insect subsidies to lake fishes, despite growing recognition of the connections between aquatic and riparian habitats of lakes. Further, though human development of lakeshores and watersheds is widespread, it is unknown whether lakeshore urbanization alters the magnitude of terrestrial fluxes to lakes. Because lakeshore development has been found to be negatively correlated with riparian vegetation, which is habitat for terrestrial invertebrates, we expected that shoreline urbanization would reduce the prevalence of terrestrial invertebrates in fish diets. We quantified the effects of lakeshore urbanization on terrestrial insect subsidies to fish at three scales: a focused comparison of annual patterns in 4 lakes in the U. S. Pacific Northwest, a one-time field survey of 28 Pacific Northwest lakes, and a literature survey of North American lakes.
Results/Conclusions At all geographical scales, terrestrial invertebrate subsidies to fish were negatively correlated with shoreline development. In the focused comparison, terrestrial insects comprised up to 100% (average = 18%) of fish diet mass in wholly undeveloped lakes, versus a maximum of 2% (average = 0.2%) of fish diet mass in highly developed lakes. Across all lakes, terrestrial insects were virtually absent from fish diets in lakes with greater than half of the shoreline developed. In addition, trout (Oncorhynchus spp.) in undeveloped lakes had on average 50% higher daily energy intake, and terrestrial prey represented as much as 50% of the total energy intake for trout in undeveloped lakes. Temporal variability of the terrestrial subsidy suggests that these inputs are distinctly pulsed. The lack of such pulses across all highly developed lakes indicates the absence or rarity of terrestrial insect subsidies in urban lakes. These results are the first systematic quantification of terrestrial insect subsidies to fish in lakes, and show that these inputs can be substantial and energetically important to fish.