Utilising land watch data to determine long-term trends in abundance.

摘要: Data on diet and age of harbour porpoise, Phocoena phocoena, have been assembled over 23 years based on the Scottish cetacean strandings monitoring programme, part of the UK CSIP. Here we integrate previously unpublished data from the mid-2000s onwards. While the same prey species continue to predominate in the diet, the contribution of sandeels (Ammodytidae) has declined over the last decade while Gadidae are more dominant. The way in which sandeel abundance has been assessed by ICES has changed since 2009, making it hard to directly compare porpoise diet selection and fish abundance trends; however diet composition has likely followed trends in prey availability. Age data were used to construct life tables and to estimate mortality rate; observed changes in mortality are mainly driven by the varying proportion of young animals, among which starvation/loss of condition is an important component of mortality, suggesting a link to food availability. Focusing on the period 1994-2005, we examine the changes which coincided with the apparent southwards shift in distribution recorded by the SCANS surveys. The importance of sandeels in the diet generally declined, and sandeel abundance in the North Sea declined after 1998. The importance of whiting in the diet fell during the first part of the 2000s, corresponding to low levels of stock abundance in the North Sea, and there was an apparent increase in porpoise mortality at the end of the 1990s. Finally we test the sensitivity of the provisional conclusions to likely biases in the data, including under-representation of age zero animals and changes in data collection protocols.