Temporal acoustic occurrence of sperm whales Physeter macrocephalus and long-finned pilot whales Globicephala melas off western Ireland

Sperm whales Physeter macrocephalus and long-finned pilot whales Globicephala melas are the most abundant species among the community of deep-diving cetaceans occurring off the west coast of Ireland, northeast Atlantic. To address a knowledge gap on these elusive species in an area subject to increasing levels of anthropogenic noise, fixed bottom-mounted autonomous acoustic recorders were deployed from 2014 to 2016 at 13 locations. Acoustic data were collected over 2410 cumulative days, for a total of 9179 h of recordings, with sperm whale clicks and pilot whale whistles detected on 79 and 53% of the days monitored, respectively. Diel, lunar and seasonal effects on the acoustic occurrence of sperm whales and long-finned pilot whales were investigated for individual recording sites and for each recording year using generalised estimating equations. Large differences in acoustic occurrence across stations for both species highlighted the existence of more critical locations throughout the year, especially to the north of the shelf edge. Temporally, significant modulations were found for both species at all scales investigated, but the lack of consistency across the study area emphasises the need to exercise great caution when inferring general tendencies based on local patterns. The variability of spatio-temporal patterns indicates a flexibility in the distribution of sperm whales and long-finned pilot whales off the west coast of Ireland, highlighting the challenge in establishing management and mitigation measures and stressing the need of long-term, year-round monitoring.