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Basking Shark Population Assessment Basking shark population assessment Final Project Report for Global Wildlife Division of the Department for Environment, Food and Rural Affairs Basking shark population assessment Final report for Global Wildlife Division of Defra Tender CR 0247 D.W. Sims1, E.J. Southall1, J.D. Metcalfe2, M.G. Pawson2 1 Marine Biological Association of the UK, The Laboratory, Citadel Hill, Plymouth, Devon, UK PL1 2PB. Contact: [email protected] 2 Centre for Environment, Fisheries and Aquaculture Science, The Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk UK NR33 0HT. Contact: [email protected] Acknowledgements: The authors wish to thank the following for their help and assistance at various stages during the fieldwork, data analysis and report writing: David Uren, Peter Harris, Gareth Fraser, Steve Moszolics, Victoria Wearmouth, Alec and Donald MacKenzie, John and Nick Boyle, the late Alan Russell (MBA tagging team), Peter Miller (NERC RSDAS), Anthony Richardson (SAHFOS), Matt Witt (University of Exeter), Vin Fleming (JNCC), Sarah Fowler (Nature Bureau), Phil Lewis and Sally Davis (Defra, GWD), Jeremy Stafford-Deitsch and the Shark Trust for the use of the basking shark photograph (front cover), and the Marine Conservation Society for the use of the map of historic UK basking shark fishing grounds (p. 24). Copies of this report can be downloaded as an Adobe Acrobat file from the Defra web site at: http://defraweb/wildlifecountryside/resprog/findings/index.htm Department for Environment, Food and Rural Affairs Nobel House 17 Smith Square London SW1P 3JR Telephone 020 7238 6000 Website: www.defra.gov.uk © Crown copyright 2005 Copyright in the typographical arrangement and design rests with the Crown. This publication (excluding the logo) may be reproduced free of charge in any format or medium provided that it is reproduced accurately and not used in a misleading context. The material must be acknowledged as Crown copyright with the title and source of the publication specified. EXECUTIVE SUMMARY Introduction Populations of many large marine vertebrates are threatened by high levels of fisheries exploitation (both targeted and as bycatch). This applies particularly to sharks, skates and rays (elasmobranch fishes) that have life-history traits that make them especially vulnerable to levels of harvest mortality that are above that of natural mortality. In particular, many elasmobranchs have a late age at maturity and low fecundity leading to low rates of reproduction. This results in little scope for the compensatory mechanisms that enable many “bony” fish species like cod or mackerel to withstand unnaturally high levels of mortality. As a consequence, elasmobranch fisheries not only exhibit rapid declines in catch rates as exploitation increases, but there is a greater potential for the fishery to collapse. The basking shark is the world’s second largest fish and is widely distributed in coastal waters on the continental shelves of temperate zones in both northern and southern hemispheres. Individuals take 12-20 years to reach maturity, females have long gestation periods (1-3 years) and give birth to a few, large young. This inherent vulnerability to exploitation, together with concern over the strong possibility that populations are depleted as a result of exploitation by fisheries and the lack of scientific knowledge of the species, has led to the basking shark being listed as Vulnerable worldwide, and Endangered in the north-east Atlantic, in the 2004 IUCN Red List (IUCN, 2004). In 2002, the species was listed on Appendix II of the Convention on International Trade in Endangered Species. They are also protected in British territorial waters under Schedule 5 of the Wildlife and Countryside Act (1981). The current approach to conservation of basking sharks relies heavily on the precautionary principle, which states that insufficient scientific knowledge about biology and stock status is no defense for a lack of action. In particular, very little is known about migration routes, whether there are discrete local populations of basking sharks, or the relationship between regional population abundance and global trends. There is also a need to distinguish between the effects on population status of climate change on the sharks’ environments and the legacy of the impact of fisheries, and to ascertain whether stocks are depleted or recovering. The latter concern is recognised in the UK basking shark Biodiversity Action Plan, which states that improved long-term (many decades) monitoring of the UK population is necessary to enable population trends to be identified. Though some monitoring data for this species are available, most are based on sightings of sharks feeding on plankton near the sea surface during spring and summer and no analyses done to date has provided information on population trends. Aims of the study This study set out to use modern satellite telemetry to determine movements and behaviour of basking sharks in the north-east Atlantic, especially that part of the population that occurs at some time within UK territorial waters. This information has improved our understanding of the status of basking shark stocks that will inform decisions in relation to conservation measures and help ensure recovery and sustainability of basking shark populations. Methods The movements and behaviour of 20 basking sharks was monitored by tagging them with pop-up archival transmitting (PAT) tags. The tags record swimming depth, water temperature and light level while being towed by a shark. At a pre-specified time, the tag detaches from the shark, floats to the sea surface and transmits summarised i information via the Argos system aboard NOAA polar-orbiting satellites. In addition to using PAT tags, 3 sharks were tagged with pop-up archival tags that could transmit data via the terrestrial cellular-telephone network once they had drifted near to shore. Geographical movements of the sharks were calculated using retrieved data on daily light intensity to estimate the local times of midnight or midday for longitude calculations. Latitude was then determined using night time Advanced Very High Resolution Radiometer (AVHRR) remote-sensing images of sea surface temperature along the longitude and temperature data recorded by the tag. Retrieved data also provided summary information about time-at-depth and the time-at-temperature. A number of tags were also returned by members of the public who found them in the sea or on beaches. These tags provided detailed minute-by-minute data on the vertical movements of the sharks. Results and conclusions Data sufficient for reconstructing geographical movements were retrieved from 8 of the 23 basking sharks tagged. The results show that the sharks spent most of their time on the European continental shelf, moving between centres of high zooplankton productivity characterised by tidal fronts and fronts associated with the shelf break. Individuals foraging along fronts off the south-west peninsula of England moved to three main areas: the Celtic Sea front, the Goban Spur and the north Biscay regions of the shelf edge. Two individuals moved northward from these areas along the shelf edge into rich feeding areas in the Hebridean Sea that are also characterised by strong tidal fronts that aggregate zooplankton. None of the sharks tagged off south west England in spring moved northwards through the Irish Sea during summer, whereas those sharks tagged in the Clyde Sea off western Scotland in summer travelled relatively rapidly south through the Irish Sea in late summer and early autumn to areas off south-west England. In winter, some individuals remained or moved into shallow coastal waters, generally in the southern region of the shelf. These results suggest that there are no separate populations of basking sharks inhabiting northern or southern UK waters. Individuals move freely between these areas and probably represent a single population. The depth data show that there is generally a preference for deeper depths in the water column (or deeper water) on the shelf during winter compared with summer, a shift that at least in part explains why basking sharks are rarely observed at this time. Patterns of movement show that, contrary to earlier belief, basking sharks do not hibernate on the sea bed in the winter. The existence of differing patterns of diel (daily) vertical movement in individual sharks in different ocean habitats in UK waters resulted in very different surfacing frequencies. The daytime-surfacing frequency of a tracked individual feeding in an inner-shelf area near a front was over 100 times higher than another shark feeding in well-stratified water. This large difference in ‘basking’ behaviour between regions was reflected in our shark survey data with 11.5 times more shark sightings per unit effort in frontal areas than in stratified water. This suggests sightings per unit effort may not necessarily reflect real differences in geographic (horizontal) abundance between areas, because the probability of sighting a basking shark may be about 60 times higher in frontal sea areas than in sea areas that are well-stratified. This has profound implications for the use of sighting data both in defining population distribution and estimating abundance trends. These results suggest that bias-reduction according to habitat type (and zooplankton behaviour) should be incorporated into analyses of survey data when attempting to estimate abundance. ii The distribution of tag geolocations for individual sharks has been compared with sightings of basking sharks compiled as part of the “Conserving endangered basking sharks” project and sightings made during scientific surveys by the MBA, Hebridean Whale and Dolphin Trust, The Shark Trust, UK Wildlife Trusts and the International Fund for Animal Welfare. Whilst the broad distribution patterns revealed by these different methods are similar, there are considerable differences in density distributions, with a strong emphasis in the sightings data in the Hebridean Sea, Clyde Sea, Irish Sea and close inshore around Devon and Cornwall, areas that clearly represent important habitat for basking sharks, most probably in relation to feeding opportunities.
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