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Behavioural state modelling of long-term harbour porpoise movement data and the influence of environmental variability

Dominique Stalder, Floris M. van Beest, Signe Sveegaard, Rune Dietz, Jonas Teilmann, Jacob Nabe-Nielsen*

*Corresponding author:

ABSTRACT: The harbour porpoise (Phocoena phocoena) is a small marine predator with a high conservation status in Europe and the United States. To protect the species effectively it is crucial to understand its movement patterns and how the distribution of intensively used foraging areas can be predicted from environmental conditions. Here, we investigated the influence of both static and dynamic environmental conditions on large-scale harbour porpoise movements in the North Sea. We used long-term movement data from 57 individuals tracked during 1999–2017 in a state-space model (SSM) to estimate the underlying behavioural states, i.e. whether animals used area-restricted or directed movements. Subsequently, we assessed whether the probability of using area-restricted movements was related to environmental conditions using a generalized linear mixed model. Harbour porpoises were more likely to use area-restricted movements in areas with low salinity levels, with relatively high chlorophyll a concentrations, low current velocity and in areas with steep bottom slopes, suggesting that such areas are important foraging grounds for porpoises. Our study identifies environmental parameters of relevance for predicting harbour porpoise foraging hot spots over space and time in a dynamic system. The study illustrates how movement patterns and data on environmental conditions can be combined, which is valuable to the conservation of marine mammals.