Whilst the harbour porpoise {Phocoena phocoena) is the most j&equently observed cetacean in the UK, it. is at risk from habitat degradation, pollution, incidental capture in fishing nets and anthropogenic disturbance. UK, European and International initiatives have highlighted the conservation need to obtain a better understanding of this species. This research, conducted over 5 years (1999-2004) aimed to fiirther the scientific knowledge of the harbour porpoise in order to influence fiiture conservation initiatives. Studies into the relative abundance, density, distribution, habitat use and behaviour were conducted through combinations of shore- and boat-based research. Technical trials of a towed acoustic device (T-POD) were also conducted from onboard the research vessel. Additional analysis of previously collected acoustic data from bottom-set gillnets was carried out This research presents some of the first examination of the west coast for the harbour porpoise. Porpoise density was found to be exhibit significant inter-anniial differences, with increases noted off West Scotiand and a decrease observed for the South West In the case of Northern freland these results are some of the first quantitative analysis of the harbour porpoise within the region. A statistically significant relationship with depth and in particular the 100m depth contour was also observed. The area off north Devon (Morte Point) is considered to offer an important feeding site for the species, where porpoises were found to aggregate in areas of high tidal flow. Site-specific differences in behaviour, group size and distance from shore were observed depending on time of day and tidal cycle. A full description of the ethology of the harbour porpoise is also given and the potential affects of dolphin watching tourism assessed. Porpoises were observed to regularly engage in cooperative feeding and aerial activity, previously considered rare. The behaviours, as recorded from shore, differed considerably to those recorded from the boat. This highlights the need for precautionary management to increasing numbers of dolphin watching tour operators in UK waters. Acoustic detection of harbour porpoises around bottom-set gillnets revealed that porpoises are present around the nets on a 24 hour basis. This is highly significant in terms of bycatch mitigation as it indicates that porpoises are successfiilly avoiding entanglement for 99.75% of encounters. In terms of monitoring populations the T-POD was found to present a cheap and quick method which detected an additional 10% of all porpoise encounters which were not detected visually. The results presented here make a significant contribution to the scientific knowledge of the harbour porpoise, which will provide a basis for future research and conservation initiatives.

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