Aarhus University Seal / Aarhus Universitets segl

No. 68: Porpoise monitoring in pinger-net fishery. Final baseline report

Teilmann, J., Sveegaard, S., Balle, J.D., Kyhn, L. & Carstensen, J. 2015. Porpoise monitoring in pinger-net fishery. Final baseline report. Aarhus University, DCE – Danish Centre for Environment and Energy, 38 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 68. dce2.au.dk/pub/TR68.pdf

Summary

Harbour porpoises are part of the assignment of 16 Natura 2000 areas in Danish waters and Denmark are obliged to monitor and protect the species. The harbour porpoise faces the threat of entanglement in gill nets but by using acoustic alarms, so-called “pingers” placed on the nets, bycatch can be reduced. Pingers may, however, scare the porpoises out of important areas such as the Natura 2000 areas. This project aim to examine if porpoise density will change in the Great Belt, if mandatory use of pingers in all set net fisheries are to be enforced during a limited time period following the baseline period. By comparing the presence of porpoises before, during and after pingers are  introduced in the Great Belt, with a control area in Kalundborg Fjord, we will be able to estimate the effect of pingers on porpoises in relation to density and acoustic behaviour. Porpoise presence is examined by deployment of acoustic data loggers (C-PODs) that can detect echolocation sounds emitted almost continuously by porpoises during foraging, communication and orientation.  This report covers the baseline period from July 2011 to September 2015 for the 14 C-POD stations that have listened for porpoises more or less continuously during this period. The porpoise detections were analysed as PPM (Porpoise Positive Minutes) and CPPM (no. of clicks during each PPM. This may be interpreted as a measure of acoustic behaviour). Both are aggregated as average values per 24 hours. The results show that the seasonal patterns in the study area are significant and common to the control and impact areas. Over the four years with baseline data, echolocation activity (CPPM) has remained relatively constant over the entire study area, but there have been seasonal shifts in porpoise presence (PPM) between stations, most pronounced in the winter period, where porpoises apparently move from shallower stations to the deeper stations. Power analysis show that the current baseline data and a continuation of the monitoring program during the employment of pingers for one year, would allow for detecting relative changes of density (PPM) around 22% and echolocation behaviour (CPPM) around 42%. If monitoring continue for up to four years the relative changes that can be detected is reduced gradually to 14% and 25%, respectively.