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Basking Shark Survey: tagging and tracking
Simon Berrow 1 and Emmett Johnston 2
Shannon Dolphin and Wildlife Foundation, Merchants Quay, Kilrush, Co Clare
Ballynarry, Buncrana, Co Donegal
Final Report to the Heritage Council
This project was funded by the Heritage Council under the Wildlife Research Grant Scheme 2009
The aims of the Basking Shark Survey 2009 were to extend the basking shark tagging program started in
2008 including the deployment of satellite tags. Secondary aims included collecting images useful for photo-identification and tissue samples for genetic analysis.
A total of 101 coloured numbered tags were deployed at three sites. Most tags were deployed off Inishowen in North Donegal and around the Blasket Islands, west Kerry. In addition two archival satellite tags were deployed off Slea Head, west Kerry. A total of eight re-sightings of tagged sharks were recorded. Most of these were within a site between one and 10 days later but one re-sighting was between North Donegal and Tiree in the Inner Hebrides, a distance of 140km and duration of 22 days. In time it is expected more and longer distance/duration recoveries will be recorded.
The tagging project provided an unexpected opportunity to use mark-recapture modelling to estimate the abundance of basking sharks in Trawbreaga Bay, North Donegal. During three days fieldwork from 1-3 June we tagged 50 individual sharks. Twenty-three were tagged on 1 June, 17 on the 2 June and 12 on the 3 June. Of those tagged on 1 June four re-sighted on the 2 June which provided a crude estimate of around around 135 basking sharks in Trawbreaga Bay during this period.
Images of 71 photo-identification images were obtained, which form the basis of an Irish Basking Shark Photo-ID Catalogue. Over one-half (56%) were considered good quality images but only 21% had marks useful for photo-identification. Most sharks are not well marked but the occasional well-marked individual can be used to check the longevity of tags as there is a chance this shark will be identified again from these wounds and thus it can be checked for the presence of tags.
Attempts were made to biopsy sample three sharks using the biopsy pole and two sharks with the crossbow in order to provide samples for genetic analysis. Only one sample was obtained but a sample of slime from one shark, which was scrapped off the bow of the research vessel after being hit by the tail after tagging was also recovered. DNA was extracted from this basking shark slime which demonstrated the value of shark slime for genetic work. A further five samples were obtained from basking sharks in the Blasket Islands using a modified mop handle and scourer pad. This technique was also used successfully in the Isle of Man and a paper is being prepared on this technique as it offers a non-invasive efficient method of obtaining samples for genetic analysis.
As part of this project we facilitated a film crew who are making a documentary on animal migration for RTE. In addition to this media opportunity a large number of interviews were carried out on radio and articles appeared in a number of Irish and UK papers. The level of interest in basking sharks in Ireland has been increased considerably.
The project team gained huge experience of basking sharks, tested and developed new techniques and enjoyed considerable media interest. There is no doubt that we have established a very important basking shark research project and a lot of people in Ireland are now aware and interested in basking sharks in Ireland. A number of recommendations were made on how to take this project forwarding future years.
Introduction Basking sharks The basking shark (Cetorhinus maximus) is the second largest fish in the world. It is one of only three species of shark that feed on plankton, the others being the whale shark (Rhincodon typus) and the megamouth (Megachasma pelagios). The morphology and anatomy of basking sharks was described by Matthews (1950) and Matthews and Parker (1950). Kunzlik (1988) carried out a review of basking sharks in the northeast Atlantic and more recently Simms (2008) updated this review.
Basking sharks are reported to have grown up to 12.2 -13.72m but most sharks do not exceed 10m in length. A maximum weight of 5-6 tonnes has also been reported but around 2 tonnes is more typical. The liver makes up between 15-25% of the body weight and was a target for many fisheries both historic and modern due to the presence of large quantities of squalene oil. The basking shark is a harmless planktoneater whose appearance on the surface in Irish coastal waters coincides with the occurrence of high densities of zooplankton, especially the copepod Calanus, whose dense swarms often turn the water red.
Sharks filter this plankton by cruising slowly with their mouth open. Inside the mouth are gill-rakers made from keratin (like baleen from the great whales). About every minute or so the shark closes its mouth and ‘gulps’, contracting the gill-arches, where the mass of plankton and mucus passes into the stomach. It has been estimated that a 7m shark, with an open mouth of 0.5m2 and travelling at 2 knots could filter 1484 cubic metres of seawater every hour.
Until recently it was believed that basking sharks hibernated during the winter. This was thought to be a strategy to survive periods when plankton is 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 are thought to produce only around 6 young at a time, though this is based on the one account of a shark giving birth, soon after being harpooned in Norwegian waters. Sexual maturity is thought to be reached at around 6.8-8.1m in males and 7.7-8.2m in females. This equates to around 6-7 years for males and 10 years for females. 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 elusive species.
Up until 2006, there had been no published studies on the worldwide genetic status of basking sharks to explore global population structure. Hoelzel et al. (2006) found genetic diversity in basking sharks to be comparatively low worldwide and suggested a lack of significant genetic differentiation among ocean basins based on mitochondrial markers. The recent transatlantic movement of a basking shark tagged in the Isle of Man (Gore et al., 2008) supports the idea of low population differentiation across the Atlantic.
Interestingly, a genetic bottleneck was suggested as an explanation for the low variability in mtDNA haplotypes whilst a low effective population size was estimated for this globally distributed species (Hoelzel et al., 2006). More samples for genetic analysis are required throughout the world to explore this important issue in more detail.
Results from recent satellite telemetry have shown long distance movements between Cornwall and Scotland with one shark travelling 1,878 km in 77 days across the Celtic Sea and up the western seaboard of Ireland before the tag stopped transmitting in the Minches in the outer Hebrides (Sims et al. 2003).
These tags also recorded sharks at up to 850m off the edge of the continental shelf and were active throughout the winter when they tend to occur in deeper water but in similar habitats as the summer. More recently Gore et al. (2008) tracked a female basking shark nearly 10,000km over 90 days from the Isle of Man to Newfoundland producing the first evidence of a trans-Atlantic movement.
The appearance of basking sharks on the sea surface provides an opportunity to carry out visual sighting surveys. Sighting schemes involving casual sightings often submitted by the public have identified important concentrations of basking sharks off southwest England and Scotland but satellite telemetry has shown other important concentrations may not be identified through surface sightings (e.g. Celtic Deep). A combination of both methods is best for identifying important habitats for this species (Southall et al., 2005). Surfacing behaviour also facilitates behavioural studies and provides opportunities for tagging and tracking. Sims and Quayle (1998) suggested sharks feeding on fronts tended to be at the surface over 60 times more frequently than sharks feeding in stratified waters and these sharks were selective filter-feeders that chose the richest plankton patches within which to forage.
Basking sharks in Ireland
The Basking shark may seem an exotic and mysterious creature to many people in Ireland today but it was a very familiar sight to coastal communities along the western seaboard of Ireland, from Counties Donegal to Cork. The basking shark was traditionally called the sunfish, which should not be confused with the ocean sunfish Mola mola, which has become increasingly abundant in Irish waters in recent years due to climate change. The basking shark was known in County Kerry as ainmhide na seolta ‘monster with the sails’ and liop an dá lapa‘ unwieldy beast with two fins’. More generally in the west of Ireland it was called liabhán mór (signifying a great leviathan) or the most evocative liabhán chor gréine ‘great fish of the sun’ (Went and Suilleabhain, 1967).
There is a bank, marked on old charts of the western seaboard about 30 miles west of North Conamara and west Mayo at 11º12’W, called ‘The Sunfish Bank’ (McNally, 1976). This was where shark fishermen ventured, especially from the Ballyconnelly region of Conamara, in six-man open boat or Galway hookers in the spring and early summer with hand-held harpoons to locate sharks. Once captured they would row back towing the shark behind them. Although the meat was used for food or fertiliser and its tough hide for moccasin type shoes, it was the shark’s liver that was the most valuable prize. Basking shark liver may weigh up to one-third of the weight of the animal and is rich in squalene. This oil was used for dressing wounds, preserving wood, in manufacturing processes and most commonly for lighting. In 1742 the street lights of Galway and Waterford were lit with sunfish or basking shark oil.
The best documented basking shark fishery in the world was off Achill Island, Co Mayo (McNally 1976).
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, however Simms and Reid (2002) suggested this decline may have be due to long-term zooplankton decline in the north-east Atlantic. Basking sharks continued to be fished commercially by Norwegian vessels based in Co. Waterford up until 1986 when 2,465 sharks were killed, with boats often seen in the port of Dunmore East.
Basking shark sighting schemes
Records of sightings of basking sharks from Britain and Ireland have been collected since 1987 by the UK Marine Conservation Society (Doyle et al., 2005). In 1993 a survey of basking shark sightings (Berrow and Heardman, 1994) obtained 142 records of at least 425 sharks. Most sightings were from fishing boats especially off the southwest coast, with 20% of records coming from entanglement in fishing nets. A Irish basking shark database was established in 1993 containing 279 records of 868 animals. Since 1992 the Irish Whale and Dolphin Group have been receiving sighting records through their cetacean sighting schemes and to date they have 282 records of 955 sharks. Nearly one-half (43%) of these records were from 2007 which was a phenomenal year for basking shark sightings. The increase in basking shark sightings has continued which is likely to be a product of better recording but also possibly an increase in occurrence.
Most records are in June (36%) and May (20%).
Number of basking shark sightings and b. Number of individual sharks reported 2007-2009.
If we compare the number of sightings reported to the IWDG during 2009 compared to the previous two years we can see that there was a large decline in sightings in May but an increase in July. This trend was even stronger in May if individuals are considered but numbers in June weregreater than reported in 2008 but less than that reported in 2007 (Fig. 1).
Basking shark conservation
There are strong indications that the population of basking sharks in Irish waters was over-exploited and that populations are still depleted (McNally, 1976). However Sims and Quayle suggested this decline was due to a decline in zooplankton. Basking sharks are frequently caught in surface and bottom-set gillnets in Irish waters (Berrow, 1994). The recent ban on salmon drift-nets in Ireland should reduce this impact but the extent of other fisheries interactions is poorly known.