Influence of environmental variability on harbour porpoise movement

D Stalder,S Sveegaard,Fm Van Beest,J Teilmann,R Dietz,J Nabe-Nielsen

doi:10.3354/meps13412

Abstract

The harbour porpoisePhocoena phocoenais a small marine predator with a high conservation status in Europe and the USA. 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 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, relatively high chlorophyllaconcentrations and 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.

Highlights

Movement ecology research provides valuable knowledge to inform conservation management (Allen & Singh 2016) and can help to better understand the mechanisms influencing population dynamics (Morales et al 2010)
Since we suspected the influence of environmental conditions on behavioural states to be different in the Kattegat than in the North Sea due to the prevalence of frontal systems associated with inflow of brackish Baltic Sea water into the Kattegat, we excluded the Kattegat from the study area
Our study demonstrates a great deal of variability in porpoise movements, with some animals staying in the same region using AR movements for many months whereas others roamed over large distances and shifted between AR and directed movements

Summary

Introduction

Movement ecology research provides valuable knowledge to inform conservation management (Allen & Singh 2016) and can help to better understand the mechanisms influencing population dynamics (Morales et al 2010). Tidal current strength is the most important driver of ecosystem dynamics due to its association with high primary productivity (Sharples et al 2007, Embling et al 2012), and in estuaries with strong tides, prey densities appear to be related to salinity (IJsseldijk et al 2015) Both current strength and the location of highly productive upwelling zones are in turn influenced by seabed slope (Genin 2004, Embling et al 2012, Amorim et al 2017). These environmental variables are not independent, their relative importance can yield information about the underlying drivers of changes in animal movement behaviour

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Discussion

Conclusion