Concern is growing over the conservation status of elasmobranch populations. An estimated 25% of species are thought to be threatened with extinction worldwide, due to increasing threats from direct overfishing, being caught as accidental bycatch and habitat degradation. Despite this concern, managing and conserving elasmobranch populations has proven to be a difficult task- but why?
Elasmobranchs (sharks, skates and rays) are a highly diverse group of cartilaginous fish (chondrichthyans) and one of the oldest lineages of vertebrates; arising some 420 million years ago they rapidly diverged to occupy predatory roles in almost all aquatic ecosystems (Compagno, 1990; Kriwet, et al., 2008). Elasmobranchs can be found everywhere from intertidal continental shelf waters, to the pelagic ocean and the deep sea, with forms even being found up to depths of 4,000m! A variety of species can also be found in freshwater and estuarine ecosystems and some can even be classed as euryhaline (meaning they can survive in a range of salinities).
Despite their high diversity and wide range, elasmobranchs remain a group vulnerable to extinction. Typical of predatory vertebrates, they can be characterised by a large size, slow growth rates, late maturity, and low reproduction rates. Elasmobranchs also exhibit some of the highest levels of maternal investment and longest gestation periods of the vertebrates (Cortes, 2000; Dulvy et al., 2014).
These characteristics result in slow rates of population increases, meaning that they are vulnerable to overexploitation by fisheries. Numbers can decline very quickly, and recover slowly, hindering conservation efforts. Key examples of this issue are the Scalloped hammerhead, white, and thresher sharks which are all thought to have declined by over 75% in the past 15 years (Baum et al., 2003).
Above: Scalloped hammerhead shark (Sphyrna lewini).
Although the decline of Elasmobranch populations is well-documented, very little is actually known about the state of species worldwide. Many conservation issues for these Elasmobranchs are exacerbated by a lack of available scientific data (Dulvy et al., 2014) and public misunderstanding. For example, a large number of species are listed as ‘data deficient’ on the IUCN Red List of threatened species, meaning there isn’t enough data to assess their conservation status.
Even for species that are relatively well-studied, there is often a lack of historical fisheries information. This means that the true decline of populations cannot be fully assessed, as there are no accurate historical population data to compare current numbers to. This can result in an underestimation of the true decline of many Elasmobranch species and hence mismanagement of populations.
Surprising discoveries about Elasmobranchs are being made every day that have strong implications for their conservation. A key issue is that many Elasmobranch species are morphologically and ecologically very similar. These similarities cause high amounts of cryptic speciation (animals that look alike but are genetically distinct), which results in taxonomic confusion and unstable nomenclature (naming of species).
Above: An example of cryptic speciation. The Blonde ray (A) (Ellis et al., 2009) and the Spotted ray (B) (Ellis et al., 2007) look very similar, but are actually two genetically different species.
A prime example of cryptic speciation and its impact on conservation has been the separation of the common skate (Dipturus batis) into two distinct species in 2009: the smaller-growing D. cf. flossada and the larger D. cf intermedia (Iglesias et al., 2010; Griffiths et al., 2010). These two novel species look almost identical, but their different sizes mean they could historically have been impacted by fisheries differently and hence have experienced verying degrees of population declines.
This discovery means that before 2009 these species were likely managed ineffectively; given that the ‘common skate’ is now classified as critically endangered, this is quite significant. Now, however, with more research into the status of these two separate species, conservation efforts can be targeted accordingly.
The long-term impacts of elasmobranch declines are also poorly understood, but as they play important roles in regulating most marine ecosystems, the impacts are likely to be far-reaching. As top predators, Elasmobranchs regulate the population numbers of their prey, including fish, crabs, worms, shellfish and marine mammals, thus preventing their over-population. In this way. sharks, rays and skates are able to maintain ecosystem balance. It has even been suggested that changes in North-Atlantic fish communities are, in part, a consequence of the increased vulnerability of elasmobranchs relative to teleost (bony) fish (Rogers et al., 1999).
Hence, now more than ever, assessing Elasmobranch decline accurately and managing populations effectively is of vital importance. Thankfully, scientists are developing new genetic tools, sampling methods and data analysis techniques to increase our understanding.
Baum et al., 2003. Collapse and Conservation of Shark Populations in the Northwest Atlantic. Science. 299(5605); 389-392.
Compagno, 1990. Alternative life-history styles of cartilaginous fishes in time and space. Environmental Biology of Fish I0020es. In: Bruton M.N. (Eds.) Alternative life-history styles of fishes. Developments in environmental biology of fishes (pp 33–75) Springer, Dordrecht.
Cortés, E., 2000. Life history patterns and correlations in sharks. Reviews in Fisheries Science, 8(4), 299-344.
Dulvy, N. K., Fowler, S. L., Musick, J. A., Cavanagh, R. D., Kyne, P. M., Harrison, L. R. (2014). Extinction risk and conservation of the world’s sharks and rays. eLife. DOI: 10.7554/eLife.00590
Ellis et al., 2007. Raja montagui. The IUCN Red List of Threatened Species 2007: e.T63146A12623141. http://dx.doi.org/10.2305/IUCN.UK.2007.RLTS.T63146A12623141.en. Downloaded on 21 June 2018.
Ellis et al., 2009. Raja brachyura. The IUCN Red List of Threatened Species 2009: e.T161691A5481210. http://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T161691A5481210.en. Downloaded on 21 June 2018.
Griffiths et al., 2010. Molecular markers reveal spatially segregated cryptic species in a critically endangered fish, the common skate (Dipturus batis). Proceedings. Biological sciences / The Royal Society, 277(1687), 1497–1503.
Iglésias et al., 2010. Taxonomic confusion and market mislabelling of threatened skates: important consequences for their conservation status. Aquatic conservation: marine and freshwater ecosystems, 20(3), 319–333.
Kriwet et al., 2008. First direct evidence of a vertebrate three-level trophic chain in the fossil record. Proc Biol Sci., 275(1631), 181-6.
Rogers et al., 1999. The taxonomic distinctness of coastal bottom-dwelling fish communities of the North-east Atlantic. Journal of Animal . Ecology, 68: 769–782.