Abundance of marine mammals in the cardigan bay cSAC

Project Last Updated: 12/10/2004 20:37:00 Source : Fernando Ugarte

Project Start - 2003 / Project End - 2004

Collaborator: Sea Watch Foundation

In 1996, the UK submitted an area of 976 km2 in Cardigan Bay, Wales, to the EC as a candidate Special Area for Conservation (cSAC) for its bottlenose dolphins.

Our goal was to estimate the densities of marine mammals in this cSAC. Our budget allowed for 16 dedicated distance-sampling/photo-identification surveys per year, from a 10m vessel (observation height, 3m). In 2003, a pilot study indicated that a substantial increase in effort was needed to improve confidence intervals in abundance estimations based on the distance-sampling methodology. In order to increase the survey effort during 2004, we incorporated this methodology into dolphin-watching trips. Using Distance 4.0 software, we estimated densities of 0.2 grey seals/km2 (%CV 23.32) and 0.5 harbour porpoises/km2 (%CV 23.08).

These estimates were consistent with the ones obtained during the pilot study and confirm the importance of the area for the conservation of these species. The density estimated for bottlenose dolphins was unrealistically low (0.12 dolphins/km2, %CV 39.72), probably due to a small sample size combined with the detection function being skewed by atypical sightings. In addition to distance-sampling estimations, preliminary estimates of the numbers of dolphins were carried out using identification photographs. The flattening of a discovery curve showed that a large proportion of well-marked animals had been identified. Based on the size of our catalogue, the minimum number of dolphins utilizing the cSAC is 119. Based on the proportion of well-marked animals present in encountered groups (72%, SD=0.41, N=75 group counts), it was estimated that approximately 138 dolphins inhabit the study area.

This number is consistent with the high dolphin density estimated during the pilot distance-sampling study (0.3 dolphins/km2, %CV 36.58).

This study highlights both potential and limitation of monitoring marine mammals by combining distance-sampling and photo ID surveys, funded by a variety of sources, including the government, an NGO, the dolphin-watching industry and volunteers.