SPRING FORAGING DISTRIBUTION AND HABITAT SELECTION BY DOUBLE-CRESTED CORMORANTS ON THE PENOBSCOT RIVER, MAINE USA

Restoration of the Atlantic salmon (Salmo salar) to New England rivers requires, in part, an understanding of the use of habitat and prey by potential and known predators. We examined variation in habitat use b y Double-crested Cormorants (Phalacrocorax auritus), from April throug h June 1992-94 on ca. 288 km of the 2 primary tributaries and the main stem of the Penobscot River, Maine. Our objectives were to quantify te mporal and spatial variation in habitat use and test for selection of spring foraging habitats by cormorants. Cormorants forage during dayli ght hours only To determine the distribution of foraging birds we cond ucted aerial surveys at intervals of <8 days. Counts from individual a erial surveys were analyzed relative to 9 river sections and used to e xamine selection of foraging habitats, such as free-flowing areas and the headponds and tailraces of darns. We defined selection as the diff erence between the proportionate length of each river section relative to the total river km available and the observed proportion of birds using each section, All birds observed from the aircraft within a rive r section were assumed to be foraging within that river section. Cormo rants were first observed between 15 and 20 April in all 3 years. Indi viduals consistently selected (P < 0.05) against use of 2 large tribut aries and sections >105 km north of the mouth of the mainstem. However , from late April through early June of all years, birds selected (P < 0.05) 4 of the 5 mainstem dams and the estuarine portion of the river . Cormorant use of dams remained at or above expected levels until the second week of June in all years. We suggest that cormorant selection for foraging areas adjacent to clams reflected a higher availability of prey possibly due to delay and injury of migrating Atlantic salmon smelts. Cormorant selection of the estuarine portion of the Penobscot River was likely a response to a seasonally increasing availability an d abundance of estuarine and marine prey, in addition to resident fres hwater species and migrating smelts.