Sperm whale (Physeter macrocephalus) acoustic ecology at Ocean Station PAPA in the Gulf of Alaska – Part 1: Detectability and seasonality

Sperm whales Physeter macrocephalus produce loud, stereotypical click sequences and are an ideal species to be studied with passive acoustic techniques. To increase our limited knowledge of sperm whale occurrence patterns in remote and inaccessible locations of the North Pacific, we analyzed a five-year-long (June 2007–April 2012) acoustic data set recorded at Ocean Station PAPA (OSP; 50°N, 145°W) in the Gulf of Alaska (GOA). Firstly, we assessed the sperm whale detection performance of the Passive Aquatic Listener (PAL), and secondly, we investigated temporal patterns of sperm whale presence at OSP. The PAL proved highly efficient, with above 50% probability of detecting more than two sperm whales, a condition met for over 50% of the recordings. Results indicated that sperm whale clicks were recorded year-round, with a clear seasonal pattern. The number of detections during the summer months was approximately 70% higher compared to winter. An ambient noise analysis showed that differences in detection rates were likely not driven by seasonal changes in ambient noise levels. The average propagation range of sperm whale clicks ranged between 7 and 8 km between summer and winter, with slightly decreased detection distances observed in winter. Seasonal shifts in the intensity of the Alaska Current and the latitudinal oscillations of the North Pacific Transition Zone results in changes in water mixing, transport of nutrients and the concentration of prey such as squid, which likely drives sperm whale distribution.