Effects of vessels and noise on the subsurface behavior of endangered killer whales (Orcinus orca)

Abstract

Prey availability and disturbance from vessels and noise are identified threats to endangered Southern Resident killer whales. Vessel noise can mask echolocation signals used for hunting and/or disrupt foraging with implications for energy acquisition in a likely prey-limited population. We utilized suction cup-attached digital acoustic recording tags (DTAGs), to measure received noise levels, understand killer whales’ use of sound, and determine effects of vessels/noise on subsurface behavior. During the 28 tag deployments, we collected vessel data concurrently along with opportunistic predation observations to validate feeding. Broadband received levels (dB re 1μPa) were significantly different across years. Of the vessel factors considered, both vessel count and speed were significant explanatory variables of received levels. Vessels emitting echosounder signals were commonly received by the DTAGs and overlapped with the echolocation frequencies use to hunt fish. Additionally, different phases of foraging were differentiated from the acoustic record, including the detection of crunching sounds after fish kills. Together with movement data analysis, these results allow the identification of different whale activities, including prey capture dives, to test hypotheses of vessel/noise effects on behavior. This work, along with a comparative investigation involving Northern Resident DTAG data, inform killer whale conservation and management measures.