Elasmobranchs are part of the class Chondrichthyes. The Class Chondrichthyes comprises a diverse group of fishes whose most
obvious common feature is the possession of a cartilaginous skeleton. This is in contrast to the bony skeleton of the Osteichthyes or bony fishes. Of all the fishes in this class, the great majority of the commercially important species of chondrichtyans are
elasmobranchs.
Elasmobranch is the scientific name for a very ancient group of fishes which includes the sharks, skates as well as rays. Generally, elasmobranchs are more often taken as bycatch in fisheries. According to research done by the Joint Nature Conservation Committee of the United Kingdom, trawlers and seines working in mixed demersal fisheries on the continental shelf take a bycatch of skates and rays, spurdog, spotted dogfish and occasionally porbeagle shark and tope. Gill netters and longliners targeting flatfish, cod and other species take dogfish and rays and in some cases these may actually become the target species. Anyhow, nowadays there is a great concern on the catches of sharks and rays which is on the rise. These fishes are generally caught using a longline. Longline fisheries are the most important source of shark kills in the high seas, mainly because of the magnitude of their effort. They contribute about 80% of the estimated total elasmobranch by catch in weight and about 70% in numbers of fish. Mixed species deepwater fisheries take a number of species of sharks including the Portuguese dogfish and the leafscale gulper shark and a very large bycatch of rabbitfish while offshore fisheries for tuna take a large bycatch of blue and thresher sharks. As a result of pressure from fisheries, many species are now severely depleted. Elasmobranchs are particularly vulnerable to over-fishing because of a number of unusual characteristics of their biology. Most species are slow growing, mature at a late age and produce very small numbers of young, sometimes as few as one or two every two years. Species such as the common skate, long-nosed skate and angel shark, which once supported profitable fisheries, have now been exterminated from large parts of their former range and may be close to extinction in our waters. Others, such as the spurdog and porbeagle shark have also suffered severe population declines and are in urgent need of effective fisheries management.
Several large-scale fisheries operating in the high seas around the world are known to take a substantial bycatch of elasmobranchs, particularly sharks. Although sharks are retained and utilized in some of these fisheries, they usually are dumped, sometimes alive after their fins have been chopped off. This activity of finning is well known in several Asian countries. The survival of released sharks varies depending on the type of gear used. Trawl and gill nets and perhaps purse seines, almost certainly cause 100% mortality. While longlines permit prolonged survival of sharks by allowing limited movement and thus some respiration, survival rates depend on the metabolism and endurance of individual species. As a whole, it is believed that most of the bycatches of sharks in large-scale fisheries have high mortality. The amount of elasmobranches of various species, being killed in large-scale high seas fisheries is poorly known and has not been systematically assessed and an unknown part of the bycatch is discarded at sea. This diversity contributes to the difficulty in studying the fisheries and to the problems of collecting accurate data on yields and fishing effort.
The management of elasmobranch fisheries is rather difficult to implement due to economical and social factors mainly. First, research and management of sharks and rays are hampered by their low economic value as research funds are usually given to resources economically more important than elasmobranchs. Second, the high price attained by shark fins in the international market. This stimulates fisheries totarget sharks and explains why incidental catches are usually finned. The low economic value of elasmobranchs results in fishery statistics which are not accurately maintained together with problems of species identification, especially for tropical species. Most records aggregate skates in a single group and sharks in two categories, large and small. Without accurate statistics by species or species groups it is difficult to assess the population of elasmobranch species specifically.
Therefore, as a solution to management difficulties, installation of shark deterrent devices similar to the Turtle Excluding Device in passive fishing gears should be promoted. This can be a way to exclude sharks which are not being targeted species. Another method of effective management and protection of elasmobranchs should begin with education and awareness. Through intensive and widespread educational programmes is it possible to motivate fishermen, scientists, the public and governments to achieve effective protection and management of sharks and rays. Such measures are vital for the ecological balance of the marine environment between predator and prey.