Investigating the influence of the tidal regime on harbour porpoise Phocoena phocoena distribution in Mount’s Bay, Cornwall

Abstract: Investigating the influence of the tidal regime on harbour porpoise Phocoena phocoena distribution in Mount’s Bay, Cornwall Abstract Unintentional by-catch in fishing gear is a significant cause of mortality of harbour porpoises in UK waters. Understanding the spatial distribution of harbour porpoises at fine scales and how this changes over time is essential when trying to understand where these lethal interactions might occur. Mount’s Bay in Cornwall, UK is an important area for harbour porpoises. However knowledge of how they use the Bay is limited. Effort and sightings data were collected over eight years between 2011 and 2018 from a wildlife watching vessel. The data shows a high sightings rate for the Bay and confirms the relative high density of porpoises when compared to areas of the UK traditionally considered as strongholds. Harbour porpoises are present in high numbers year round. However, a seasonal pattern in their occurrence was identified, with numbers peaking in September. Areas where porpoises consistently occur are considered to be important foraging habitat due to their high foraging nature. To better understand the distribution of porpoises in the bay and how it changes with the tide cycle, Maximum Entropy Modelling (MaxEnt) with files correcting for survey biases was used. By analysing various environmental variables, the models identified the most important factors that influence the likelihood of porpoises being present. These include the longitudinal position within the bay, the depth of the water, the type of substrate on the seafloor, the distance from the nearest significant seafloor slope, and the distance from the nearest tide front. In addition, a General Additive Model for Location Scale and Shape (GAMLSS) was also used to model the likelihood of porpoise occurrence in relation to thermal front features remotely sensed by satellites. Identifying Important Areas for Harbour Porpoises The variables longitude, depth, distance from the nearest significant seabed slope, and distance from the nearest tide front can be combined to describe the location of turbulent oceanographic processes. The models identify specific spatially focused areas that are important for porpoises and demonstrate that the position of these areas changes throughout the tide cycle. The results indicate that these changes are associated with tidal processes. Tidal processes have been found to concentrate plankton and nekton, which would enhance the foraging success of harbour porpoises and likely explain this relationship. Key Words: Geography, Geographical Information Systems, GIS, Habitat Modelling, Harbour Porpoise, MaxEnt, Tidal Processes, Conservation Advisor: Valentijn Venus Master degree project 30 credits in Geographical Information Sciences, 2023 Department of Physical Geography and Ecosystem Science, Lund University Thesis nr 168