Background

What's special about basking sharks?

Basking sharks belong to the elasmobranch (sharks and rays) family of fish and, like most of their relatives, have a number of life-history traits that make them particularly vulnerable to fishing, whether it is either deliberate or accidental. In particular, their late age at maturity and low fecundity (number of young) lead to low rates of reproduction, and therefore low potential rates of population growth compared to many marine teleost (bony) fish like cod or mackerel. Basking sharks also have relatively little scope for the compensatory mechanisms that enable fish like cod and mackerel to withstand fishing mortality well above the level of natural mortality. As a consequence, elasmobranch fisheries not only exhibit rapid declines in catch rates as fishing increases, but there is a greater potential for the fishery to collapse.

A bit of basking sharks biology.

Basking sharks take 12-20 years to reach maturity, have long gestation periods (1-3 years) and produce a few, large, young (the only recorded litter is of six very large pups). It is possible that populations of basking sharks may have localised distributions (most sharks are characterised by large scale migrations), at least in the feeding areas where they are exploited. Population replacement may therefore depend more on recruitment of juveniles rather than immigration from other parts of the population. Indeed the Achill Island basking shark fishery off the west coast of Ireland appeared to collapse in the early 1960's after only ten years of peak catches. However, this apparent decline in numbers also coincided with a long-term decline in their zooplankton food supply in that region, so a shift in distribution rather than over-exploitation could also account for the trend.

What current protection is there for basking sharks?

Because of its inherent vulnerability, together with concern over depleted populations as a result of over-exploitation by fisheries, and the lack of scientific knowledge of the species, the basking shark was listed as vulnerable in the 1996 IUCN Red List. Basking sharks are also protected in UK waters under Schedule 5 of the Wildlife and Countryside Act (1981), and have now been listed on Appendix II of CITES by the UK.

The basking shark is also a priority species under the UK Biodiversity Action Plan, with a published species action plan which recognises that improved long-term (many decades) monitoring of the UK population is necessary to enable population trends to be identified. The basking shark is protected in UK territorial waters and, though fishing for basking sharks in European waters can be regulated by the European Commission (EC) under the Common Fisheries Policy (CFP), the only catch control is on Norwegian vessels fishing for the basking sharks in Community waters. This Total Allowable Catch (TAC) is not based on any stock assessment, or on any knowledge of the geographical range of the exploited "stock", which may include the population around the UK. Norway has reportedly ceased to fish for basking sharks in EC waters, presumably because of poor catches.

What is the current state of our knowledge?

Given the acknowledged gaps in scientific knowledge about basking shark biology, the current conservation measures rely heavily on the precautionary principle. In particular, very little is known about stock structure, annual migration circuits, the relationship between regional population declines and global trends, and whether populations of basking sharks are discrete or not.

Basking sharks are widely distributed in coastal waters on the continental shelves of temperate zones in both the northern and southern hemispheres. They appear near the surface (feeding on plankton) in UK waters during spring and summer (April to September). Though some monitoring of this species takes place, nothing done to date has provided information that enables population trends to be determined with any reliability. Catches of basking sharks are recorded by some fisheries departments, including Norway, New Zealand (incidental catches) and, formerly, Scotland. However, the fishery for basking sharks in the North Atlantic is currently at such a low level that fishery-derived data are inadequate as a means of monitoring population changes. Even where catches have been reported, it is unlikely that catch per unit effort series are available that enable the status of the stock to be assessed.

Three public sightings recording schemes for the species are presently underway in the UK. All sightings are heavily dependent on weather conditions (sharks a short distance below the surface will usually not be recorded), and observer effort is neither standardised nor stratified with respect to population sampling requirements. Variation between years in numbers recorded cannot, therefore, reliably be attributed to changes in population size.

As a consequence, we need to obtain much more reliable data for the species on population structure (sex ratios, relative numbers of juveniles and adults, etc.), reproductive biology, and migrations between wintering and summer grounds and pupping areas.