Collapse and Conservation of Shark Populations - Essay Example

Critically review the current conservation status of one named group of marine vertebrates Abstract The conservation of fishes in the class chondrichtyes is gaining importance due to the life history features and the threat of extinction for various species that comprise the marine vertebrates. A detailed discussion is conducted based on various researches around the world with respect to the various factors that cause the vulnerability of the species towards extinction and the various measures taken by conservation institutions to protect the species and the flaws in implementing conservation measures in various parts of the world. Introduction Chondrichthyans are low productivity populations when compared to teleost fishes. This is due to the varying life history strategies. The consequence of fishing chondrichthyans worldwide is presently a topic of international concern. The poor number of sustainability of sharks in target fisheries reveals the same(Stevens, J.D., Bonfil, R., Dulvy, N.K. & Walker, P.A. 2000 pp.476-494). International organisations have instituted various plans to reduce the fishing of chondrichthyans in specific regions and thereby conserve the species but lacks progress and proper management Discussion Marine vertebrates are categorised under class vertebrata and phylum chordata. Main classes of vertebrata are classified into seven. They are Amphibia, agnatha, reptilia, osteichtheyes, chondrichthyes, mammalia and aves. Class chondrichtyes comprises 800 species of cartilaginous sharks and rays. Fishes in the chondrichtyes class do not have bones. The skeletal structure of these fish are made of cartilage. This class of fish is one of the primary marine species to form paired fins. Since fishes in this class does not have swim bladders they have 5-7 gill slits. The skin of fish in this class is covered in denticles instead of scales and has a coarse sand paper like texture. These fishes adopt internal fertilisation for reproduction. Large liver is another feature of fishes in class chondrichtyes. The large liver holds a large quanity of oil that enables the fish to stay buoyant in the water. The absence of swim bladder requires the sharks to swim continuously to prevent sinking. Continuous swimming enables the fish to maintain the flow of oxygen and water through the mouth and over the gills (Marine vertebrates 2009). A statistics of worldwide chondrichthyan fisheries indicate that 50% of the universal catch is considered as by catch and does not reflect in the official fishery records and are not managed. When chondrichthyans are considered as by-catch, they usually undergo high rates of mortality in target groups. Therefore, sawfish, deep water dogfish and some skates extinct in certain region and recolonise in a different region. Certain chondrichthyans are flexible to fishing while some species are vulnerable. This feature varies according to the life history of the fish and the quantum of its population. The population of rays and sharks increases when it feeds on discards from trawlers. The fishing of sharks results in long term reactions from the ecosystem and a considerable decline in stocks (Stevens, J.D., Bonfil, R., Dulvy, N.K. & Walker, P.A. 2000 pp.476-494). The Class Chondrichthyes is classified into an estimated 923 -1117 in 55 families and 171 genera of cartilaginous fishes. These fishes are recognized in 43 species in ten genera and 4 families of rays and sharks that move into freshwater marines beyond the tidal reach of river mouths. This movement of freshwater elasmobranchs takes place in tropics though some movement of these fishes is visible in warm temperate waters. The movement of huge number of Chondrichthyes is found in South America, specifically in Atlantic drainages where there are lesser pockets for endemism. This phenomenon is also common in southeastern and southern Asia, West Africa, Indonesia, southern China, Philippines, New Guinea and Australia. Fresh water elasmobranches are also found in the Tigris River in Iraq, a number of rivers in Africa, South Europe, North America and certain rivers flowing into the Mediterranean Sea. Elasmobranchs found in freshwater abide by all the factors that restrict the diversity and biomass of marine elasmobranches. Additionally they are also bound by a factor that normally does not concern marine elasmobranches. Elasmobranchs live in limited environments that strictly restrict their chance to avoid habitat modifications, pollutants, incidental or direct capture in regional fisheries. The presence of freshwater elasmobranches in tropical waters is of great concern due to the lands adjoining these waters with enormous human populations found mostly in developing nations. The increase in the rate of direct modification, exploitation or destruction of lacustrine and riverine ecosystem tends to drive several freshwater elasmobranchs stocks to extirpate and several other freshwater species to extinct. Scientists face a major concern to ascertain the level of this phenomenon since these species have not obtained much attention from fisheries conservation and management institutions (Compagno, LJV & Cook, SF 1995). Overexploitation of fishes pose a threat to the future of several large marine vertebrates. While concerns for conservation of sea turtles and tunas have increased for this reason, the conservaion status of shark species on the contrary remains uncertain. An analysis of the data from Northwest Atlantic indicates that there is a rapid decrease in the number of oceanic as well as coastal shark populations. Thresher sharks, scalloped hammerhead and white sharks are estimated to have declined by more than 75% over the last fifteen years. Closed area methods of assessment requires conservation of sharks based on priority in certain regions and the requirement to opt for reallocation and selection of sites for marine reserves that will benefit the conservation of multiple threatened species (Baum et al. 2003 pp.389 – 392). Further, comparatively less marine fishes are ascertained by the World Conservation Union policy as threatened species and it is observed that the rate of extinction of marine fishes has been underestimated. Traditional assessment system lacks the data required to assess the level of extinction. Therefore potential correlations between vulnerability and extinction are considered to prioritize species that require conservation. A research into 230 species of rays and sharks indicate the vulnerability of this class of marine fishes. A further research into documented cases of regional extinction gives information on the extinction of the species based on body size, depth difference and latitudinal difference of the species. It is also observed that certain species that have moved out of substantial region of their range constitute fishes with large body when compared to other skates however depth and latitudinal differences are same as that of other species. The correlation of the size of the body of fishes may be because of the high rate of mortality and the correlation of life history factors like late age of the fish during maturity. A research on regional extinct species with small size and ranges as the criteria reveals that body size formed a least species list by excluding the available local extinctions when compared to the list generated on the basis of depth range and latitudinal size. Therefore body size is the only factor that precisely points out known local extinction which suggests that body size is a useful tool rather than range size to find out potentially vulnerable species of skates. The research provides an objective and simple method to prioritize the continued evaluation of species and balances the process in direct evaluation methods that are time consuming and data intensive (Dulvy, N.K. & Reynolds 2002 pp. 440 - 450). A research on the correlation between latitude, temperature and depth and the comparative abundance and distribution of chondrichthyans in Norway’s northern coast was observed. The catches from scientific trawls during the period 1992 to 2005 north of 62°N indicate that species like Squalus acanthias, Chimaera monstrosa, Galeus melastomus and Etmopterus spinax are plenty in south while the species Amblyraja radiate was abundant in the north. The comparative abundance and distribution of the species does not indicate any significant change to the availability level though the temperature of water was high during 1992 to 2005. Current statistics reveal that there is no impact on the population of common species of chondrichthyans in the region though the status of rare species in the region is vague in the recent scientific trawls (Williams, T., Helle, K. & Aschan, M. 2008 pp: 1161-1174). A comparison of life history attributes and the extinction risk of rays, sharks and chimaeras (chondrichthyans that fall under the category of high conservation factor) was conducted on the basis of three prominent marine habitats (open ocean, continental shelves and deep sea) to determine its longevity based on evolution correlation. It is observed that deep water chondrichthyans mature at a higher age and have longevity with a lower rate for growth completion when compared to the same species found in shallow water. It is also observed that the average fishing mortality rate required to lead a deep water chondichthyan to extinction is 38-58% that of the rate of mortality of continental shelf and oceanic species. The size of the fish body has a poor rate for risk of extinction while mode of reproduction is an important factor that determines the risk of extinction. The risk of extinction is correlated to the phylogeny of the species. Therefore, it is necessary to conserve species of all sizes since the vulnerability of small species living in the deep ocean is also vulnerable to extinction. An essential step to conserve deep water species is to reduce exploitation of the species through fishing and prevent the risk of extinction (Garcia, V.B., Lucifora, L.O. & Myers, R.A.2007). A research off the Australian western coast using stochastic demographic model was developed to study the populations of the dusky shark Carcharhinus obscurus and the sandbar shark Carcharhinus plumbeus using inputs from empirical biological information collected from fishery independent investigation and commercial target fisheries. Factors like age specific survival obtained from mortality rates from fishing from a tagging research and indirect assessment of natural mortality was used by the stochastic demographic model. It was observed that when fishing was absent, the median calculation of the quantity of intrinsic population was high for both the species of sharks. However, while including the age specific parameter the rate of mortality from fishing was low for Carcharhinus obscurus born during the year 1994 and 1995. The research using this model also suggests that these shark populations are vulnerable to exploitation when they fall into an older age group than was previously conceived. The research on Carcharhinus plumbeus indicate that the fishing mortality rate of these fishes were high during the period 2001 and 2004 and therefore fishing on these species may not be continued. Carcharhinus plumbeus do not have somatic growth, fecundity and longevity to adapt to the environment and hence requires the intervention of conservationist to avoid the depletion of this particular stock in Australia (McAuley, R. B., Simpfendorfer, C. A., and Hall, N. G. 2007 pp.1710-1722). A World Conservation Union (IUCN) report reveals that 40 percent ray and shark species seen in the Mediterranean face the threat of extinction. The first Red List assessment has reported this status of rays and sharks due to overfishing and extensive by catch (incidental non target species catch). Other reasons that contribute to the decline of various species of rays and sharks are recreational fisheries, degradation of habitat and other disturbances caused by human beings. The Red List was published after an expert workshop that assessed the populations of 71 Mediterranean species of rays, sharks and chimaeras. The workshop concluded that 30 species faced the threat of extinction comprising 13 species that were categorised under high levels of threat called Critically Endangered, while 8 species were classified under Endangered and 9 species were classified under vulnerable. The workshop also classified another 13 species as near threatened and the inadequacy of data categorised 18 species as data insufficient. The Least concern status was recorded only for 10 species of chondrichthyans in the Mediterranean. Around 80 percent decline in population of Leucoraja melitensis (the Maltese skate) found exclusively in the Mediterranean is due to bottom trawl fishing. Similarly three species of Squatina spp or the angel sharks and Oxynotus centrina or the angular rough sharks are also declining and are assessed as Critically Endangered. The Mediterranean poses a threat to the Mobula mobular or giant devil ray and has been considered in the Endangered category. The vulnerability of this species is due to the huge size of the females to get trapped in net fisheries (Mediterranean Sea 2007). The IUCN Red List reports that 26 percent rays, sharks and chimaeras in the northeast Atlantic face threats of extinction while 20 percent fall in the Near Threatened classification. The report specifically observes that 7 percent species fall under critically endangered category, 7 percents as endangered and 12 percents are vulnerable due to overfishing. The European Union has laid specification for fishing restriction only for four species of rays, chimaeras and sharks out of the 116 species found in the region. Great white and basking sharks have legal protection. Though catch limits are applicable for porbeagle sharks and spiny dogfish the fishing rate for these species are usually high. It has been recommended that the assessment of vulnerability to extinction offers an important guideline to conserve the species at risk in the ocean by reducing fishing in crucial areas and by implementing strategies to avoid by catch. It is also recommended to protect the endagered species of rays and sharks (Quarter of northeast Atlantic sharks 2008). A growth and age based comparative research of Carcharias taurus , sand tiger sharks in western North Atlantic with previously derived data on the same factors revealed that these sharks have a slow growth rate than recorded previously. This data is important for precisely evaluating the ability of the species to recover its population level and also suggests the need to completely protect the species (Goldman, K.J., Branstetter, S. & Musick, J.A. 2006 pp.241-252). Euryhaline and freshwater elasmobranchs feed at high trophic level like the species in marine waters. But these elasmobranchs have habitat limitations specifially their limited number based on the physicochemical variability and makes them more vulnerable to the outcomes of human activites than their marine species. To conserve these species in freshwater special emphasis should be given to conserve sharks of a particular genus pristids, Glyphis and potamotrygonids. The geographic limitation of elasmobranchs is serious in the case of euryhaline elasmobranchs since it requires the species to move into the sea for breeding. Therefore it is important to conserve euryhaline and freshwater elasmobranchs alongwith the conservation of fresh water habitats (Martin, R.A. 2005 pp: 1049 – 1073). Unreported and unregulated trade of several shark species contributes to the decline in the number of stocks. Though the FAO has formulated International Plan of Action to conserve and manage sharks, there is insufficient development to achieve remarkable standards of conservation. Illegal fishing of sharks continues in various parts of the world irrespective of initiative taken by international organizations to conserve the species through various methods (Fowler, S., Raymakers, C. & Grimm, U. 2004 p.6). The conservation of chondrichthyans especially sharks are important due to their long life, slow growth rate and low rate of producing offspring. The fishing of sharks to reduce their number in bathing beaches should be avoided by considering the decline in shark stocks due to poor reproduction rates (Walker, T.I. 1998 pp:553-572). Conclusion A detailed study of IUCN Red List indicates the conservation status of chondrichthyans. Current conservation trends indicate that there is commercial exploitation of deep sea sharks and the vulnerability to extinction is due to the life history features and restricted distribution. Close monitoring of these species of chondrichthyans is needed with better research to understand the threat, biology and conservation needs and therefore result in the implementation of recovery and management plans for the sustainability of all the species under the class chondrichthyans (Cavanagh, R & Kyne, P.M. 2006 pp. 366-380). Bibliography Baum et al. (January 17, 2003). Collapse and Conservation of Shark Populations in the Northwest Atlantic. Science Magazine Vol. 299 (5605) pp. 389 – 392 Available: http://www.sciencemag.org/cgi/content/abstract/299/5605/389. Accessed on February 24, 2009 Cavanagh, R & Kyne, P.M. (2006) The conservation status of deepsea chondrichthyan fishes : Conference poster papers and workshop papers. Queenstown, New Zealand, 1-5 December 2003 and Dunedin, New Zealand, 27-29 November 2003. no. 3/2, pp. 366-380. FAO fisheries proceedings Available: http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=7448143&q=conservation+status+%2B++chondrichthyans&uid=1031188&setcookie=yes. Accessed on February 24, 2009 Compagno, LJV & Cook, SF (1995) The exploitation and conservation of freshwater elasmobranchs: Status of taxa and prospects for the future Journal of Aquariculture and Aquatic Sciences Available: http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=3877101&q=conservation+status+of+rays&uid=1012298&setcookie=yes. Accessed on February 24, 2009 Dulvy, N.K. & Reynolds, J.D. (November 22, 2002) Predicting Extinction Vulnerability in Skates. Conservation Biology Vol.16 Iss.2, pp. 440 – 450 Available: http://www3.interscience.wiley.com/journal/118954137/abstract. Accessed on February 24, 2009 Fowler, S., Raymakers, C. & Grimm, U. (2004) Trade in and Conservation of two Shark Species, Porbeagle (Lamna nasus) and Spiny Dogfish (Squalus acanthias) CoP13 Inf. 16. p. 6 Available: http://www.cites.org/common/cop/13/inf/E13i-16.pdf. Accessed on February 24, 2009 Garcia, V.B., Lucifora, L.O. & Myers, R.A. (October 5, 2007) The importance of habitat and life history to extinction risk in sharks, skates, rays and chimaeras. The Royal Society. Available: http://rspb.royalsocietypublishing.org/content/275/1630/83.abstract. Accessed on February 24, 2009 Goldman, K.J., Branstetter, S. & Musick, J.A. (September 6, 2006) A re-examination of the age and growth of sand tiger sharks, Carcharias taurus , in the western North Atlantic: the importance of ageing protocols and use of multiple back-calculation techniques. Environmental Biology of Fishes Vol 77 (3-4) pp.241-252 Available: http://www.springerlink.com/content/1641354885778460/. Accessed on February 24, 2009 Marine vertebrates. 2009 Available: http://marinebio.org/Oceans/MarineVertebrates.asp. Accessed on February 24, 2009 Martin, R.A. (2005) Conservation of freshwater and euryhaline elasmobranchs: a review. 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Accessed on February 24, 2009 Quarter of northeast Atlantic sharks and rays threatened with extinction (November 10 2008) Available: http://www.iucn.org/fr/ou/oceans/?2213/Quarter-of-northeast-Atlantic-sharks-and-rays-threatened-with-extinction. Accessed on February 24, 2009 Stevens, J.D., Bonfil, R., Dulvy, N.K. & Walker, P.A. (2000) The effects of fishing on sharks, rays, and chimaeras (chondrichthyans), and the implications for marine ecosystems Journal of Marine Science: Journal du Conseil 2000 57(3):476-494 Available: http://icesjms.oxfordjournals.org/cgi/content/abstract/57/3/476. Accessed on February 24, 2009 Walker, T.I. (1998). Can shark resources be harvested sustainably? A question revisited with a review of shark fisheries. Marine and Freshwater Research Vol.49(7) pp.553 – 572. Available: http://www.publish.csiro.au/paper/MF98017.htm. Accessed on February 24, 2009 Williams, T., Helle, K. & Aschan, M. (June 27, 2008). The distribution of chondrichthyans along the northern coast of Norway Journal of Marine Science: Journal du Conseil Vol.65(7) pp:1161-1174 Available: http://icesjms.oxfordjournals.org/cgi/content/abstract/65/7/1161. Accessed on February 24, 2009 Read More