Porpoise pasts: A journey through skull size variation in European waters

Commercial whaling peaked between the 1700s and 1800s, leading to declines in whale populations worldwide. While it is well constrained that baleen whales have shrunk in body size over the past centuries - likely due to whaling pressure and climate change – smaller toothed whale species, which were not the primary targets during the whaling era, have been largely understudied in this regard.

However, one of the major challenges in the face of global change and increasing anthropogenic influence is predicting population declines in order to establish suitable conservation strategies before a collapse can take place. Tracking declines in body sizes over large temporal scales has proven to be a reliable indicator preceding such population declines and collapses.

In this study we investigate body sizes of the harbour porpoise (Phocoena phocoena), which is one of the smallest toothed cetacean species and a common inhabitant of European waters. We use skull sizes as proxies for estimating body sizes of individuals collected from Scotland, England and the Netherlands, with collection dates spanning from the 17^th century to the present. The aim of this study is to assess whether a decline in harbour porpoise body sizes over time and additionally, if possible morphospecies from different regions of the North Sea, can be identified.