Diet composition and food consumption rate of harbor porpoises (Phocoena phocoena) in the western Baltic Sea

Abstract

Stomach content composition and prey‐specific consumption rates of juvenile and adult harbor porpoises (Phocoena phocoena) were estimated from a data set including 339 stomachs collected over a 32 yr period (1980–2011) in the western Baltic Sea. The stomach contents were mainly hard parts of fish prey and in particular otoliths. The bias originating from differential residence time of otoliths in the stomachs was addressed by use of a recently developed approach. Atlantic cod and herring were the main prey of adults, constituting on average 70% of the diet mass. Juvenile porpoises also frequently consumed gobies. Here, the mass contribution by gobies was on average 25%, which was as much as cod. Other species such as whiting, sprat, eelpout, and sandeels were of minor importance for both juveniles and adults. The diet composition differed between years, quarters, and porpoise acquisition method. Yearly consumption rates for porpoises in the western Baltic Sea were obtained in three scenarios on the daily energy requirements of a porpoise in combination with an estimate including the 95% CLs of the porpoise population size. Cod of age groups 1 and 2 and intermediate‐sized herring suffered the highest predation from porpoises.

Resilience of harbor porpoises to anthropogenic disturbance: Must they really feed continuously?

“…, Leopold , Andreasen et al . ). Notably in the North Sea, juveniles feed primarily on small lean fish (<10 cm) such as gobies (consistent with Wisniewska et al .…”

“…, Leopold , Andreasen et al . ). Search effort, rates of encounter, and prey capture almost certainly differ between adults and juveniles, as well as with space and time (Read , Santos and Pierce , Spitz et al .…”

“…, Leopold , Andreasen et al . ). Furthermore, previous published studies on harbor porpoise diet are based on robust samples collected over large spatial and temporal scales; the five animals observed by Wisniewska et al .…”

“…Hence, a juvenile porpoise of ~20 kg would need to take 2,000 such small prey per day, and this is shown to be the case by stomach content analyses (Leopold , Andreasen et al . ). Still, this would only need an intake rate of 80 prey per hour, with continuous feeding.…”

Using the otolith sulcus to aid in prey identification and improve estimates of prey size in diet studies of a piscivorous predator

“…Similar experimental studies have not been performed on cetaceans (Bowen & Iverson, ). In vitro experiments mimicking digestion in marine mammals have been performed on otoliths to estimate erosion rates (Christiansen, Gamst Moen, Hansen, & Nilssen, ; Wijnsma, Pierce, & Santos, ), but their application is not straightforward (Andreasen et al, ).…”