# Spatial Dynamics of Harbour Porpoise Phocoena phocoena Relative to Local Hydrodynamics and Environmental Conditions

## TL;DR

This study models where harbour porpoises are most active near shorelines, identifying key environmental factors influencing their habitat use.

## Contribution

A multidisciplinary workflow for predicting porpoise distribution using hydrodynamic and environmental data is presented.

## Key findings

- Porpoises are most concentrated within 500 m of shorelines, near headlands with coarse sediments.
- Environmental factors like slope, aspect, and sediment grain size significantly predict porpoise distribution.
- Porpoise occurrence peaks during specific tidal phases, highlighting dynamic habitat use.

## Abstract

Understanding the spatial dynamics of harbour porpoise (
Phocoena phocoena
) is crucial for effective conservation and management. The study presents a multidisciplinary approach to modelling and analysing the site occurrence and habitat use of
Phocoena phocoena
within the Skerries and Causeway Special Area of Conservation (SAC), identifying areas where they were seen surfacing and/or spending the most time. Using data derived from multibeam echosounders (MBES), particle size analysis of sediments, hydrodynamic modelling, and theodolite tracking observations, the study examines the influence of local hydrodynamics and environmental conditions on the spatial distribution of harbour porpoises. Kernel density analysis of 451 porpoise sightings over an 11‐day survey demonstrated that dense clusters and higher aggregations occurred within ~500 m of the shoreline. Generalised Additive Models (GAMs) identified slope, aspect, backscatter intensity and sediment grain size as the most significant environmental predictors, accounting for 47.6% of the deviance in harbour porpoise distribution. Porpoises' occurrence was particularly spatially coincident with coarser sediments (4.25–5 mm), and their distribution was highly concentrated around headlands, shoreline and within a 3‐h window before and after high water. Overall, these findings highlight the dynamic nature of harbour porpoises' use of habitat in space and time, with models predicting a high probability of porpoise encounters (> 0.6) nearshore, particularly in headland areas characterised by local flow acceleration and coarser seabeds. The study presents a robust workflow for developing a porpoise‐specific monitoring program. By leveraging multidisciplinary methodological approaches, the study provides a scientific basis for refining marine conservation measures, delivering long‐term protection for harbour porpoise habitats under existing legal and management frameworks both within and beyond the SAC boundaries.

This study uses a multidisciplinary approach to model the habitat use and site occurrence of harbour porpoises, identifying areas where they were seen spending most time in the Skerries and Causeway Special Area of Conservation. The results show that porpoises are most concentrated within 500 m of the shoreline, particularly near headlands with coarse sediments and during specific tidal phases. The study highlights key environmental factors influencing porpoise distribution and emphasizes the need for targeted and strategic management of critical areas to support the protection of harbour porpoises and wider biodiversity.