Active acoustic examination of the diving behavior of murres foraging on patchy prey

Abstract

Van Holliday used sound to study nearly every type of organism living in the ocean, contributing to our understanding of acoustics as well as marine ecological processes. To honor Van, I will talk about the one animal group that I could not find in his long publication list—seabirds. We combined visual observations of murres with active acoustics, fish trawls, zooplankton net tows, and hydrographic measurements in the area surrounding breeding colonies in the southeastern Bering Sea. We detected thousands of unique acoustic targets that were strongly correlated with the number of visually detected murres, providing a new tool for quantitatively studying the foraging ecology of diving birds. Diving murre abundance was related to the combined availability and vertical accessibility of squid, krill, and pollock. Individual krill patches targeted by murres had higher krill density and were shallower than mean values but were similar in total krill abundance and overall size. Murres found outside of krill showed a depth distribution similar to that of juvenile pollock. The high proportion of diving murres in aggregations and their consistent inter-individual spacing support the hypothesis that intra-specific local enhancement may facilitate foraging in these predators.