Quantifying harbour porpoise foraging behaviour in CPOD data: identification, automatic detection and potential application

Harbour porpoises (Phocoena phocoena) are regularly monitored to assess how they are impacted by the construction and operation of offshore wind farms. A suitable method to do this is passive acoustic monitoring (PAM), and in particular using specific stationary hydrophones called CPODs. These devices provide information on click activity, which can then be analysed to investigate habitat use over time, differences between areas and the impact of human activities. Due to their small size and high metabolism porpoises are thought to need a more or less constant supply of prey to survive. Prey occurrence is thus considered one of the main drivers in porpoise distribution. And successful feeding is vital to the fitness and survival of individual porpoises. Information on foraging behaviour, however, is difficult to obtain in the field, in particular as animals feed under water. Recently the tagging of animals has provided new insights into porpoise behaviour, but it has been done for a limited number of individuals and for short times only. CPOD data have been used in Dutch waters to monitor harbour porpoise habitat use and behaviour before, during and after the construction of wind farms. The analyses have focussed on using a number of parameters that can be derived from the data, such as porpoise positive minutes, hours or days, encounter and waiting times. From other studies, primarily in captivity, we know that during foraging porpoises produce a characteristic pattern of clicks, starting with an approach phase and ending with a so-called “terminal buzz”. Aim of our study was to investigate if we could quantify foraging behaviour from CPOD data, and we were able to use an existing data set of harbour porpoise click activity from the Gemini wind park (June 2015 to February 2016). The study consisted of three phases. First, the different existing methods were applied to a sample set of data to determine the most suitable approach to identify foraging behaviour. The r