Spatio-temporal patterns in harbour porpoise density: citizen science and conservation in UK seas

Abstract

Fisheries-induced mortality poses a threat to the conservation of small cetaceans, particularly those found in productive coastal waters. Harbour porpoisesPhocoena phocoenaare ubiquitous in UK seas, but despite their prevalence, knowledge gaps remain on fine-scale space use. A deeper understanding of how distributions vary in space and time is required for appropriate management. Citizen science programs which collect standardized data aboard platforms of opportunity can help uncover patterns and identify higher density areas in the time between surveys of absolute abundance. Here, we analyze citizen science data collected from ferries to investigate spatio-temporal patterns in porpoise densities along routes in UK waters. Region-specific detection functions and generalized additive models were used to estimate abundance and elucidate relationships between distributions and environmental variables. The highest densities during the study period (2006-2017) were found southwest of Cornwall, followed by the North Sea and the English Channel. Average density in the North Sea increased substantially during this time, suggesting new high-use areas along these routes. Density surface models showed strong relationships with coastal waters, sea surface temperature, chlorophyll, and water column dynamics. Contrasting preferences for stratified waters in the North Sea and well-mixed areas in the channel suggest distinct foraging behaviours. Higher density areas were identified in the southwestern and northeastern UK, indicating priority areas for conservation efforts, especially in Cornish waters where porpoises are highly vulnerable to bycatch. Findings highlight how citizen science, together with robust density estimation, can contribute to the conservation management of a common, although threatened, species.