Differential use of zooplankton prey by Ancient murrelets and Cassin's auklets in the Queen Charlotte Islands

Abstract

The distribution at sea and the food of two similar sized plankton-feeding alcids were examined during the 1981 breeding seasons in the northwestern Queen Charlotte Islands, British Columbia. The two alcids, the Ancient murrelet (Synthliboramphus antiquus) and the Cassin's auklet (Prychoramphus aleuticus) have different chick-rearing strategies. Both species fed predominantly at the shelf break, although the Cassin's auklet also foraged over seamounts. The feeding distributions of the species appear to be related to those of their prey. Zooplankton sampling indicated that each alcid selects a small and different portion of the zooplankton available in surface waters. The Ancient murrelet's main foods were euphausiids (Thysanoessa spinifera and Euphausia pacifica) and larval and juvenile fishes. The Cassin's auklet chicks fed chiefly on calanoid copepods (Neocalanus cristatus). euphausiids (mostly Thysanoessa longipes in 1981, but in other years also Thysanoessa spinifera), and larval and juvenile fishes. The Cassin's auklets took smaller prey than the Ancient murrelet. Differences in the diets of the two alcid species were associated with differences in morphology and chick-rearing strategies.