Harbour porpoises (Phocoena phocoena) are the most common cetacean in the Belgian Part of the North Sea (BPNS). They are marine top predators and indicators for the health of an ecosystem. This makes them very important to study, yet there is no high resolution data available on the seasonal abundance and distribution patterns as well as the most important foraging zones for these animals in the Belgian Part of the North Sea (BPNS). The biggest reason for this is that they are hardly seen because they spend most of the time below the surface. Previous studies used aerial and ship-based surveys in order to learn more about their distribution. The problem with this is that no high resolution long-term data can be obtained due to the limitations of this type of study. It is not possible to obtain data during night, during wind speeds of more than 2 Beaufort, in foggy weather… Because porpoises have high energetic demands they echolocate almost continuously to navigate and forage. Therefore we can use Passive Acoustic Monitoring (PAM) devices to study the seasonal distribution and feeding patterns by recording the sound characteristics of the porpoises. The devices used are C-PODs, which are basically hydrophones that are placed in the sea and record the characteristics of underwater sound. With nine different stations in the BPNS, our main goals were to obtain more knowledge of the seasonal presence and distribution patterns of harbour porpoises, determine where their main feeding hotspots are and see if there is a diel pattern in feeding behaviour. We observed a seasonal pattern with the least detections made in May and June, while the highest peak in detections was in January-February and a second and lower peak around September-October. Midshore has significantly higher detections than coastal areas and offshore areas had more detections than coastal areas, although not significant. The feeding ratio (encounters indicating feeding behaviour subdivided by the total amount of encounters) also seemed to have a seasonal pattern with the lowest ratios in August. We were able to show that significantly more feeding takes place in offshore areas compared to midshore areas. Both the seasonal patterns and the feeding ratio showed a diel pattern with significantly more detections during night-time than during daytime. Our results showed that the peak in seasonal presence is earlier than found in the previous research based on aerial surveys and PAM in the BPNS, and contradicts results from the SCANS-surveys that say there were only porpoises present in offshore areas in the summer. Due to a lack of overlapping data we couldn’t see patterns over years, but in the close future the PAM network in the BPNS will be updated with a new mooring type so that both the data and the data-availability will be increased. There will also become more stations in the under-sampled offshore north-western part of the North Sea, an area where a lot of porpoises are found during aerial surveys. This will hopefully help us to find long-term trends besides seasonal trends and help us to link the presence and behaviour to environmental factors. |
