Model assesses chrondrichthyan extinction risk

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A study by Canadian scientists published in the most recent issue of the journal Proceedings of the Royal Socety B: Biological Sciences has shown that there is a relationship between habitat, life history and extinction risk in chondrichthyans (sharks, rays and chimaeras).

The study by Vernica Garca, Luis Lucifora and Ransom Myers of Dalhousie University used data on habitat and life history from 127 populations of 105 species of chondrichthyans, calculated the fishing mortality necessary to drive a species to extinction, and analysed the data using different mathematical models and statistical methods.

The authors found that life-history traits and extinction risk in chondrichthyans are highly associated with habitat.

The turnover times for deep-water chondrichthyans are longer (ie. they grow more slowly, mature at a later age, and live longer) and, as a consequence, they face a higher extinction risk than oceanic and continental shelf chondrichthyans.

The authors also found chondrichthyans to have a higher extinction risk if they are matrotrophically viviparous (i.e. embryos are nourished by their mothers during development) and, less importantly, if they have a large body size.

The different reproductive modes in chondrichthyans significantly affect extinction risk, being lowest for oviparity, increasing with lecithotrophic viviparity and was highest in adelphophagic, oophagic, histotrophic and placental viviparity.

According to the authors' estimates, an average fishing mortality approximately 58 and 38% of that applied to continental shelf and oceanic species, respectively, is sufficient to drive deep-sea chondrichthyans to extinction.

The study identified the clades containing the order Lamniformes (mackerel sharks) and Squaliformes (dogfishes) as the most extinction-prone groups. Mackerel sharks comprise some of the largest chondrichthyans and the species most seriously threatened with extinction, such as the sand tiger shark (also known as the grey nurse shark, Carcharias taurus), the basking shark (Cetorhinus maximus), the porbeagle (Lamna nasus) and the white shark (Carcharodon carcharias).

Dogfishes are generally small species that inhabit almost exclusively deep-sea ecosystems, the habitat associated with the highest extinction risk.

The authors write: ...we found vulnerable species at both extremes of the size continuum, from the big mackerel sharks to the small dogfishes. Focusing conservation efforts only on large species will leave highly vulnerable species without any protection.

Given their high extinction risk, we recommend that all deep-water chondrichthyans should be given high conservation priority regardless of its size.

Deep-sea fisheries are expanding rapidly and the very low levels of fishing mortality needed to drive deep-sea chondrichthyans to extinction may have already been reached in some areas...

In addition, deep-sea fisheries appear to have already reached the maximum depths attainable by chondrichthyans, leaving them without any depth refuges...

Minimizing fishing mortality in deep-water habitats already exploited and preventing new deep-water ecosystems to be exploited are necessary to avoid the extinction of these species.

For more information, see the paper: Garca VB, LO Lucifora and RA Myers (2008) The importance of habitat and life history to extinction risk in sharks, skates, rays and chimaeras. Proceedings of the Royal Society B: Biological Sciences 275, pp. 83"89.