Despite being the second largest fish, basking sharks (
Cetorhinus maximus
) have been assumed to remain in discrete populations. Their known distribution encompasses temperate continental shelf areas, yet until now there has been no evidence for migration across oceans or between hemispheres. Here we present results on the tracks and behaviour of two basking sharks tagged off the British Isles, one of which released its tag off Newfoundland, Canada. During the shark's transit of the North Atlantic, she travelled a horizontal distance of 9589 km and reached a record depth of 1264 m. This result provides the first evidence for a link between European and American populations and indicates that basking sharks make use of deep-water habitats beyond the shelf edge.
Animal migration is ubiquitous in nature with individuals within a population often exhibiting varying movement strategies. The basking shark (Cetorhinus maximus) is the world’s second largest fish species, however, a comprehensive understanding of their long-term wider-ranging movements in the north-east Atlantic is currently lacking. Seventy satellite tags were deployed on basking sharks over four years (2012–2015) off the west coast of Scotland and the Isle of Man. Data from 28 satellite tags with attachment durations of over 165 days reveal post-summer ranging behaviours. Tagged sharks moved a median minimum straight-line distance of 3,633 km; achieving median displacement of 1,057 km from tagging locations. Tagged individuals exhibited one of three migration behaviours: remaining in waters of UK, Ireland and the Faroe Islands; migrating south to the Bay of Biscay or moving further south to waters off the Iberian Peninsula, and North Africa. Sharks used both continental shelf areas and oceanic habitats, primarily in the upper 50–200 m of the water column, spanning nine geo-political zones and the High Seas, demonstrating the need for multi-national cooperation in the management of this species across its range.
Migratory movements in response to seasonal resources often influence population structure and dynamics. Yet in mobile marine predators, population genetic consequences of such repetitious behaviour remain inaccessible without comprehensive sampling strategies. Temporal genetic sampling of seasonally recurring aggregations of planktivorous basking sharks, Cetorhinus maximus, in the Northeast Atlantic (NEA) affords an opportunity to resolve individual re-encounters at key sites with population connectivity and patterns of relatedness. Genetic tagging (19 microsatellites) revealed 18% of re-sampled individuals in the NEA demonstrated inter/multi-annual site-specific re-encounters. High genetic connectivity and migration between aggregation sites indicate the Irish Sea as an important movement corridor, with a contemporary effective population estimate (N e) of 382 (CI = 241-830). We contrast the prevailing view of high gene flow across oceanic regions with evidence of population structure within the NEA, with early-season sharks off southwest Ireland possibly representing genetically distinct migrants. Finally, we found basking sharks surfacing together in the NEA are on
Large-scale genetic sampling by non-invasive methods is of vital importance for the conservation of vulnerable or elusive species. In the marine environment, non-invasive genetic sampling can provide a powerful alternative to conventional biopsies. We designed and implemented mucus swabbing for a free-ranging elasmobranch, thereby demonstrating the utility of this method in the field. We report the first attempt at mucus collection from 30 plankton-feeding basking sharks Cetorhinus maximus from 3 spatially distinct 'hotspots' in Irish waters. C. maximus DNA was successfully extracted and verified using DNA barcoding of the mitochondrial DNA cytochrome c oxidase 1 gene (99% sequence similarity) and basking shark species-specific multiplex PCRs derived from the nuclear ribosomal internal transcribed spacer 2 locus. Mitochondrial control region sequencing (1086 bp) showed that Irish samples were dominated by 2 haplotypes previously found to be globally distributed. Additionally, 1 novel haplotype was defined from western County Kerry. On-going genetic tagging will eventually provide more accurate estimates of global basking shark population structuring, abundance and behavioural ecology.
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