Aarhus University Seal / Aarhus Universitets segl

No. 97: Monthly variation in fine-scale distribution of harbour porpoises at St. Middelgrund reef

Sveegaard, S., Balle, J.D., Kyhn, L., Larsen, J., Mohn, C., Teilmann, J. & Nabe-Nielsen, J. 2017. Monthly variation in fine-scale distribution of harbour porpoises at St. Middelgrund reef. Aarhus University, DCE – Danish Centre for Environment and Energy, 34 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 97. http://dce2.au.dk/pub/TR97.pdf

Summary 

The study was commissioned by the AgriFish Agency, Ministry of Environment and Food of Denmark to explain the drivers for bycatch of harbor porpoises in gillnet fisheries by Store Middelgrund. Previous studies by DTU Aqua have shown St. Middelgrund to be a fishing area where a large number of porpoises drown in the gillnets. DCE was therefore asked to examine when and where harbor porpoises utilize the St. Middelgrund reef. 

This report presents the monthly distribution of harbour porpoises (Phocoena phocoena) at and around St. Middelgrund reef in Kattegat. Presence data was obtained by means of passive acoustic monitoring using CPODs. 11 CPODs were deployed in a systematic grid at and around the reef for a year (February 2016 – January 2017), continuously collecting the presence of harbour porpoises based on the clicks they use for foraging and navigating. This yielded data on minutes and hours with porpoise clicks, so called porpoise positive minutes or hours. Data were analyzed using a generalized linear model (GLM) to relate hourly porpoise presence to six environmental predictor variables including water current strength, temperature, and salinity in order to assess their potential role as drivers for porpoise presence at the St. Middelgrund Reef. The GLM was used to produce monthly maps of porpoise presence in the area. 

The study shows that porpoises are present around the reef all year, but that the densities are higher in May to August with a peak in June on most stations while the period from September to December had the fewest detections. Insufficient data in January prevented the creation of robust model, and thus all January data were excluded from further analysis and only 11 monthly harbour porpoise distribution maps were created (February – December 2016). The model that best explained presence/absence of porpoises per station and month included all the predictor variables suggesting that all variables in the full model were important for predicting the presence of porpoises at Store Middelgrund. Relative importance of each variable was calculated, which showed that month and bathymetry were the most important variables, followed by chlorophyll A. Porpoises occur mainly in the deeper part of the study area and not as frequent within the shallowest part of the reef that is designated as Natura 2000 for harbour porpoises.