Foods of nestling double-crested cormorants in Penobscot Bay, Maine, USA: Temporal and spatial comparisons

Early research on the ecology of Double-crested Cormorants (Phalacrocorax auritus) in Penobscot Bay, Maine, USA indicated that cormorants are primarily benthic foragers (Mendall 1934, 1936). However, in the six decades since Mendall's work, the benthic community in the Gulf of Maine has changed, with a decline of groundfish species, such as flounders (Bothidae and Pleuronectidae) and Atlantic cod (Gadus morhua). Further recovery of tags of Atlantic salmon (Salmo salar) smelts from cormorant breeding colonies has prompted concerns about predation by cormorants on smelts after the fish enter marine waters. In June-July 1993, we examined foods of nestling Double-crested Cormorants on 10 island colonies located in Penobscot Bay, Maine, USA, a 35 x 60 km estuarine/marine ecosystem. We ranked prey taxa according to frequency of occurrence, average percent volume, and numerical abundance. Five benthic taxa, sculpins (Cottidae), sand shrimp (Crangon septemspinosa), wrymouth (Cryptachanthodes maculatus), rock gunnel (Pholis gunnellus), and cunner (Tautogolabrus adspertus) were consistently among the highest ranking prey taxa in temporal and spatial comparisons. Atlantic salmon (Salmo salar) smolts were represented only by a single individual, indicating that predation by breeding cormorants occurs prior to the nestling phase of the cormorant breeding cycle. Further, in contrast to historic data, groundfishes were negligible components of the nestling diet.