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Passive acoustic monitoring (PAM) was used to study the occurrence and distribution of feeding behaviour of harbour porpoises (Phocoena phocoena) in the Ems estuary, on the border between the Netherlands and Germany. Occurrence was expressed as detection positive hours (DPH) per month or station, and feeding behaviour was described as feeding buzz ratio (FBR). Three types of analyses were undertaken: 1. A year-round analysis of FBR and DPH for one PAM station close to the Ems harbour; 2. An analysis of FBR and DPH for 10 PAM stations in the Ems estuary in March and September 2010; and 3. A comparison of porpoise clicks and fish density in the area for September/October of 2010. The year-round analysis results showed a variable seasonal pattern of porpoise occurrence, with in general lower values in April–July, and higher values in August–December. FBR and DPH per station differed between March and September 2010. The March data shows an increase of DPH when moving from the Wadden Sea into the estuary, with at the same time an increase in FBR. In September 2010, DPH decreased from outside to inside the Ems estuary, coinciding with an increase in feeding behaviour. Fish density was analysed for 5 potential prey taxa (smelt, whiting, goby, flounder and herring) at sampling stations in 4 areas along the estuary. Flounder and smelt increased in occurrence towards the inner estuarine waters. Smelt is an anadromous fish that is a known prey species for porpoise. The results of this study suggest that while feeding activity and occurrence of porpoises is observed all along the estuary and throughout the whole year, the presence of a preferred prey might be the reason for porpoises to move far into the Ems estuary at specific times. The Ems is highly used by humans and some activities, such as construction work and intense shipping, could have potential harmful consequences to the locally occurring porpoises. As this study has only covered a short time frame, the results should be considered preliminary. Future studies on the investigation of fish and porpoise occurrence in this area would allow a more in-depth understanding of this relationship and would be of high relevance for conservation and management actions.
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