Short-finned pilot whales (Globicephala macrorhynchus) depredate pelagic longlines along the shelf break ecosystem of the Mid-Atlantic Bight (MAB). The mortality and serious injury of short-finned pilot whales in the U.S. pelagic longline fishery exceeds Potential Biological Removal levels, and mitigation techniques developed to date have been unsuccessful. We quantified overlap between pilot whales and longlines relative to observed interactions by combining data from: satellite telemetry of pilot whales (n=33); logbook records of fishing effort; and the Pelagic Observer Program in 2014 and 2015. We examined the distribution of pilot whales and longlines relative to environmental variables, and assessed spatial overlap in two ways: using Williamson’s Spatial Overlap Index (SOI) with Monte Carlo simulations; and by quantifying the area of overlap under kernel density plots of pilot whale and longline distribution relative to environmental variables. Pilot whale distribution was centered inshore of the shelf break, represented by the 1000 m isobath, and longline distribution was centered at the shelf break itself. Spatial overlap between whales and longlines was significantly higher within 10 km of the shelf break and 88% of observed interactions occurred in this region. A temporal analysis of overlap from kernel density curves showed that overlap relative to the shelf break was strongly and significantly correlated with observed interactions per unit effort (Pearson’s correlation= 0.83, p=0.010 for 2014, Pearson’s correlation= 0.83, p=0.022 for 2015). Our results suggest that interactions between pilot whales and pelagic longlines are driven by proximity to the shelf break, and that shifting longline effort away from the shelf break could provide a means of decreasing the frequency of these interactions. Ongoing work is exploring the use of dynamic oceanography to further delineate smaller “high risk” regions that could be avoided by fishermen to decrease the frequency of pilot whale-longline interactions.
Stepanuk, J., A. Read, R. Baird, D. Webster, L. Thorne. 2017. Spatial overlap of short-finned pilot whales and pelagic longlines in the Mid-Atlantic Bight of the United States: Towards a spatiotemporal approach to reducing bycatch. Abstract (Proceedings) 22nd Biennial on the Biology of Marine Mammals, Halifax, Nova Scotia, October 22-27, 2017.