The following paper is now available online: Sveegaard S, Andreasen H, Mouritsen KN, Jeppesen JP, Teilmann J, Kinze CC. 2012. Correlation between the seasonal distribution of harbour porpoises and their prey in the Sound, Baltic Sea. Marine Biology, available online from February 2, 2012. DOI: 10.1007/s00227-012-1883-z The study examines the relationship between distribution of harbour porpoises and their prey in a small strait between Denmark and Sweden, the Sound (Øresund). Abstract: Abstract Low densities of harbour porpoises in winter (November-March) and high densities in summer (April-October) were found in the Sound, connecting the Baltic Sea and Kattegat. Due to their high energy requirements, it is hypothesized that the density of harbour porpoises is related to local prey abundance. This was tested by examining the stomach content of 53 harbour porpoises collected between 1987 and 2010 in the Sound (high season, 34 porpoises; low season, 19 porpoises). A total of 1,442 individual fish specimens from thirteen species were identified. Twelve of these were present in the high-porpoise density season and seven in the low-density season. The distribution of occurrence and the distribution of number of fish species were different between seasons, indicating a shift in prey intake between seasons. Furthermore, during the high-density season, the mean and total prey weight per stomach as well as the prey species diversity was higher. However, no difference was found in the number of prey species between the two seasons, indicating a higher quality of prey in the high-density season. Atlantic cod was found to be the main prey species in terms of weight in the high-density season while Atlantic herring and Atlantic cod were equally important during the low-density season. Prey availability and predictability are suggested as the main drivers for harbour porpoise distribution, and this could be caused by the formation of frontal zones in spring in the northern part of the Sound, leading to prey concentrations in predictable areas. The study builds on the results of recently published papers on harbour porpoise distribution, which are also available: Sveegaard S, Teilmann J, Tougaard J, Dietz R, Mouritsen KN, Desportes G, Siebert U (2011) High density areas for harbour porpoises (Phocoena phocoena) identified by satellite tracking. Mar Mamm Sci 27:230-246 Sveegaard S, Teilmann J, Berggren P, Mouritsen KN, Gillespie D, Tougaard J (2011) Acoustic surveys confirm areas of high harbour porpoise density found by satellite tracking. ICES J Mar Sci 68:929-936 Please send any enquiries or questions to sign@dmu.dk Best regards Signe Sveegaard PhD, researcher Department of Bioscience Aarhus University Frederiksborgvej 399 4000 Roskilde Denmark