| Summary: | Harbour porpoise (Phocoena phocoena) is one of the most abundant cetaceans in Scotland, where it forages for prey such as whiting, sandeel and sepiolids. Currently, whiting is among primary prey, while herring, an important prey historically for porpoises, seemed not to be common in their diet anymore. The present work aimed to study spatial and temporal variability in porpoise diet, and the effects of prey availability and porpoises physical characteristics on the consumption of herring and whiting. Besides presenting new information, this study helps extend the data on porpoise diet beyond a published study from 2004 (by Begoña Santos), for Scotland. The stranded monitoring scheme (SMASS) provided the dietary samples from 1992 to 2014. Regression equations were used to back-calculate the amount of prey eaten based on measurement of fish hard parts (e.g. otoliths) recovered from stomach contents. GAMs and X2 test analysed the variables that significantly influence the variability of porpoise diet. Porpoise sex didn’t affect the consumption of whiting and herring. On the other hand, geographical location, year and seasonality were significant factors in the presence and size of these prey species in the diet. Porpoises seem to prefer small sizes of herring and whiting than the ones fishery harvested. Over the years, whiting was more important and constant than herring in porpoise diet. Whiting consumption was consistent with trends in whiting abundance (based on ICES stock assessments for the North Sea). Additionally, bigger whiting was eaten by bigger porpoises. The consumption of herring by porpoises did not appear to be affected by the same or equivalent (as in the case of herring abundance) variables. Although, even with an incomplete knowledge of the variables that affect porpoise diet, the present study provides new insights. Porpoise diet seems to be highly linked to the relationship predator-prey, and changes in the biology or ecology on one or the other, could have consequences for porpoise population ...
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