SPRING PREY USE BY DOUBLE-CRESTED CORMORANTS ON THE PENOBSCOT RIVER, MAINE, USA

We analyzed 2 sets of data for Double-crested Cormorant (Phalacrocorax auritus) stomach contents (including esophageal contents) that were c ollected from April through June of 1986-1988 (N = 580) and 1992-1993 (N = 200) on the Penobscot River, Maine. Our objectives were to examin e temporal and spatial variation in the spring diet and estimate the i mportance of Atlantic salmon (Salmo salar) smelts to the cormorant die t. We analyzed stomach contents relative to samples from 3 river secti ons: 5 mainstem dams collectively above the head of tide, and free-flo wing areas above and below the head of tide. Between years composition of taxa lists were compared (P = 0.05) relative to time and river sec tion. We estimated taxon importance for data collected during 1992-199 3 by ranking taxa according to 3 statistics: frequency of occurrence, mean percent volume, and numerical abundance. Data from 1986-88 were a nalyzed by frequency of occurrence only. Across the 3 river sections, the number of prey species recovered from cormorant stomachs increased from 15 in late April to at least 31 through May. Cormorants collecte d above the head of tide consumed 12 fish species (including freshwate r, anadromous, and catadromous types), whereas birds collected below t he head of tide consumed 28 freshwater and seasonally-available estuar ine, marine benthic, and pelagic species. Salmon smelts were not recov ered from stomachs collected in April, rare in stomach samples during the first week of June, and absent from the diet thereafter In contras t, smelts were among the 5 most frequently occurring (1986-88) and hig hest ranking (1992-1993) prey taxa across the 3 river sections through May.