Effects of Habitat Volume and Fathead Minnow Introduction on Larval Survival of Two Endangered Sucker Species in Upper Klamath Lake, Oregon

Abstract

We used laboratory and field studies to evaluate the degree to which the abundance of introduced fathead minnow Pimephales promelas and volume of nursery habitat were limiting factors for survival of larval shortnose suckers Chasmistes brevirostris and Lost River suckers Deltistes luxatus in Upper Klamath Lake. In 20-h experiments in which we manipulated water depth and vegetation, fathead minnow fed on sucker larvae, but deeper water and the presence of vegetation reduced predation, suggesting that predator and prey densities and cover influenced predation. Older larvae had higher survival than younger larvae in shallow water, and vegetation increased survival for all larvae in shallow water. We also used larval production estimates from 11 years of field surveys to calculate annual larval sucker survival. Average annual larval sucker survival from 10 to 15 mm total length was about 18% (range, 4.4-48.0%). The shoreline abundance of adult fathead minnow had a negative relationship with—and appeared to set an upper limit on—annual larval sucker survival, which was consistent with the density relationship found in laboratory studies. In addition, the average shoreline abundance of fathead minnow was about three times that of any other species and had negative relationships with the abundance of all other species and lake elevation. Recent management of Upper Klamath Lake has kept June lake elevations near the 1981-2004 mean, but it is unclear whether fluctuations in the lake's fishes are due to lake management, to larger-scale processes such as climate, to biotic interactions resulting from environmental changes, or to a combination of those factors. This study illustrates the need to better understand the biological trade-offs with management-manipulated variables such as lake elevation. In addition, to manage the competing demands on Upper Klamath Lake, it will be important to recognize that high survival of larval suckers will be less frequent than in the period preceding fathead minnow presence.