The IWDG sucessfully tagged two basking sharks off Slea Head, Co Kerry. This pilot study will provide knowledge and experience of basking shark tagging and hopefully provide some indication of movements and site fidelity in Irish waters.
In 1993, Dr Simon Berrow received funding from the EU Fisheries Department to learn more about the elusive basking shark in Irish waters. Although historically very important to coastal communities, especially off Conemara, very little was known about their basic ecology. This funding included establishing a sighting scheme and attempting to tag basking sharks. The results of the sighting scheme were published in the literature (Berrow and Heardman 1996. Biology and Environment: Proceedings of the Royal Irish Academy) but no tags were deployed. This was due to the scarcity of basking shark sightings and their unpredictability.
Last year (2007) was a fantastic year for basking shark sightings reported to the IWDG and this year to date we’ve received 96 records to date, and increase of 54.8% on the same period last year. These sightings allow us to predict where the sharks will be and support attempts to tag them. This weekend was the first time the IWDG have attempted to tag these sharks and are delighted with their success. We hope to tag more over the next few weeks and months to increase our chances of tag returns.
Basking shark fishing in Ireland
The best documented basking shark fishery in the world was off Achill Island, Co Mayo. Basking sharks were typically netted off Keem Bay on the west of the island. The nets were spread at right angles to the shore to trap the sharks and a lookout alerted the fishers who rowed around in their currachs and killed the shark with a jab of a lance behind the head. Between 1950 and 1964, 9000 sharks were killed with a record 1,808 killed in 1952 alone. From 1955 the catch declined and the fishery closed in 1975 after 12,342 sharks had been killed. The collapse of this fishery suggested a local stock had been over-fished. Basking sharks were continued to be fished commercially by Norwegian vessels off Co Waterford up to 1986 when 2,465 sharks were killed, with boats often seen in the port of Dunmore East.
Basking shark ecology
It had long been thought that basking sharks hibernated during the winter. This was thought to be a strategy to survive periods when plankton was very scarce and the squalene in the liver was a food deposit to sustain them during this period. This strategy was thought to lead to very slow growth and reproductive rates with a gestation period of 3.5 years suggested. As basking sharks are ovoviporous, bearing live young, and producing only around 6 young at a time. These life-history characteristics suggested a very low reproductive rate, making them vulnerable to over-exploitation. Recent research has suggested these estimates are too extreme but it highlights how little we know about this impressive species.
Basking shark tagging and tracking
The tags were deployed on 31 May from the IWDG 6m RIB Muc Mhara and are individually numbered red tags which are anchored in the shark using a modified Casey tag. The tag is pushed through the skin via a pole, when the tag swivels and is firmly lodged. The red tag is clearly visable below the dorsal fin. Simon Berrow and his team were joined by Nick Massett and they searched the waters around the Blasket Islands for surface feeding sharks.
The first tagging attempt was unsuccessful as the tag was too firmly held by the tag applicator and was not release