Sieving a living: a review of the biology, ecology and conservation status of the plankton-feeding basking shark Cetorhinus maximus.

The basking shark Cetorhinus maximus is the world's second largest fish reaching lengths up to 12 m and weighing up to 4 tonnes. It inhabits warm-temperate to boreal waters circumglobally and has been the subject of fisheries exploitation for at least 200 years. There is current concern over its population levels as a consequence of directed harpoon and net fisheries that in the north-east Atlantic Ocean alone took over 100,000 mature individuals between 1946 and 1997. As a consequence, it is not known whether populations are recovering or are at a fraction of their historical, pre-fishing biomass. They are currently Red-listed as vulnerable globally, and endangered in the north-east Atlantic. The basking shark is one of only three shark species that filter seawater for planktonic prey and this strategy dominates key aspects of its life history. Until recently, very little was known about the biology, ecology and behaviour of this elusive species. The advent of satellite-linked electronic tags for tracking has resulted in considerable progress in furthering our understanding of basking shark behaviour, foraging, activity patterns, horizontal and vertical movements, migrations and broader scale distributions. Genetic studies are also beginning to reveal important insights into aspects of their global population structure, behaviour and evolutionary history. This chapter reviews the taxonomy, distribution and habitat, bionomics and life history, behaviour, population structure, exploitation, management and conservation status of the basking shark. In doing so, it reveals that whilst important behavioural and ecological information has been gained, there are still considerable gaps in knowledge. In particular, these relate to the need to resolve population sizes, spatial dynamics such as population sub-structuring and sexual segregation, the critical habitats occupied by pregnant females, and the distribution and scale of fishery by-catch rates. Although challenging, it is arguable that without achieving these goals the conservation status of the basking shark will be difficult to assess accurately.