Abstract

Since 2005, harbour porpoise (Phocoena phocoena) sightings in the lower course of the river Weser have been reported after decades of apparent absence. The aim of this study was to document the habitat use and distribution using an opportunistic sightings scheme (2007–2010) and two acoustic click detectors (March–July 2010). Seventy-nine sightings with 125 animals have been reported. Most sightings occurred during April (44%) and May (35.7%). Acoustic data at Weser river km (RKM) 51 revealed 413 harbour porpoise click trains. Sightings indicate a strong seasonality. C-POD data at RKM 51 indicated a presence 7 km upstream (south) of the southernmost sighting on 18 days. Acoustic data includes typical click trains associated with feeding (n=34) and communication (n=29). Ramming of sheet piles in 2010 at Brake (RKM 42) may have deterred porpoises from migrating further upstream. The reasons why the small cetaceans nowadays enter the Weser are not clear. They may follow anadromous fish shoals such as herring, smelt and twaite shad representing the most dominant species by biomass in April/May. However, due to size and life-history considerations of these fish, it appears unlikely that a single fish species attracts porpoises into the river.