The Basking Shark Cetorhinus maximus (Gunnerus) in Irish Waters: Patterns of Distribution and Abundance

A sighting scheme was established during 1993 to determine the distribution and abundance of the basking shark in Irish waters. One hundred and forty-two sighting records were made of 425 individuals during 1993. Sightings were received from around the entire Irish coast, but concentrations were evident off the east, south-west and northern coasts. Sightings were made between April and November, but peaked in June and September. Although group size varied from one to twenty animals, most records were of single animals. Most sharks were between 4m and 6m in length, with only one under 2m. Twenty-eight records were of basking sharks entangled in fishing nets, usually surface gill-nets, and specimens were reported washed up on all coasts. A database, including historic records made before 1993, was established. When compared with historical accounts of basking sharks in Irish waters, the present study suggests that sharks are scarce in areas in which they were once fished and are now more abundant at other locations. They are appearing later in the year than previously reported. Surface sightings were correlated with sea surface temperatures, with a temperature of 11.5°C being critical. Basking sharks appeared later along the north coast than along the south, and this was attributed to the timing of the spring plankton bloom in coastal waters.