FOODS OF NESTLING DOUBLE-CRESTED CORMORANTS IN PENOBSCOT-BAY, MAINE, USA - TEMPORAL AND SPATIAL COMPARISONS

Early research on the ecology of Double-crested Cormorants (Phalacroco rax auritus) in Penobscot Bay, Maine, USA indicated that cormorants ar e primarily benthic foragers (Mendall 1934, 1936). However, in the six decades since Mendall's work, the benthic community in the Gulf of Ma ine has changed, with a decline of groundfish species, such as flounde rs (Bothidae and Pleuronectidae) and Atlantic cod (Gadus morhua). Furt her recovery of tags of Atlantic salmon (Salmo salar) smelts from corm orant breeding colonies has prompted concerns about predation by cormo rants 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 occurr ence, average percent volume, and numerical abundance. Five benthic ta xa, sculpins (Cottidae), sand shrimp (Crangon septemspinosa), wrymouth (Cryptachanthodes maculatus), rock gunnel (Pholis gunnellus), and cun ner (Tautogolabrus adspertus) were consistently among the highest rank ing prey taxa in temporal and spatial comparisons. Atlantic salmon (Sa lmo salar) smolts were represented only by a single individual, indica ting that predation by breeding cormorants occurs prior to the nestlin g phase of the cormorant breeding cycle. Further, in contrast to histo ric data, groundfishes were negligible components of the nestling diet .