Habitat modelling of the harbour porpoise (Phocoena phocoena) in southwest UK: effects of depth, slope and tidal state

Abstract

The harbour porpoise, Phocoena phocoena, is a widespread cetacean species in the Northeast Atlantic, with a year-round distribution around the UK. The south-west experiences the highest bycatch rates in the UK and is the main threat to P. phocoena populations. Yet here there is limited information on their abundance, distribution, and habitat preferences. This study aims to understand harbour porpoise species-habitat relationship in the UK south-west, which may be used to inform conservation management and sustain valuable populations. From boat-based visual surveys conducted over summers 2017-2019 from Plymouth to St Marys in the Scilly Isles, habitat modelling using generalised additive models (GAMs) were used to analyse harbour porpoise occurrence in relation to variables including seabed depth, slope, and tidal state. A total of 197 harbour porpoises were spotted over 111 hours and 35 minutes survey time. The best model explained 15.7% of the deviance with variables sea state, month, depth, slope, and time of day as significant predictors. A low sea state, in the month of August, and at 35-60m depth all resulted in increased sighting rate, as well as regions of higher slope and at 10-12am. These results suggest seasonal movement patterns, reliance on static bathymetric features, and diurnal changes in surfacing behaviour. This study has successfully identified important predictors of P. phocoena distribution within a previously understudied area, to aid towards the species conservation particularly in terms of management and reduction of bycatch.