Sharks belong to the class Chondrichthyes, together with rajiformes, torpedos, sawfish, chimaeras and elephant fish. These fish differ from the Osteichthyes or bony fish as they possess a cartilaginous skeleton instead of a bony skeleton. This class is divided by main taxonomists into two subclasses: Holocephalii (chimaeras or ratfish and elephant fish) and the Elasmobranchii, which include sharks (classified into 35 families and roughly 465 species[1], as can be seen in figure 1) and a group known as the batoids (composed of rajiformes, torpedoes and sawfishes). The great majority of the commercially important species of chondrichtyans are elasmobranchs[2].

Figure 1 Classification of sharks

Sharks and the Chondrichthyes in general are not well known in terms of their population dynamics, in particular concerning their biology and stock assessment, except for certain species. The migratory habits of most species have made it difficult for researchers to keep track of individual populations. Moreover, sharks were traditionally considered as low-value commercial fish so no priority was given to collecting data about them and only limited research has been done. The situation has been changing over the last twenty years as the increase in demand for and value of fins and cartilage and the expansion of the market for shark meat have caused a consistent growth of recreational and commercial shark fisheries. At the same time, public interest in ecology and concern about the state of shark resources, considered to be under pressure, has risen.

Shark populations are extremely heterogeneous and are represented by a great variety of species, which differ markedly in habits and biology, in particular in growth and reproduction. They inhabit wide-ranging environments from the bottom of marine oceans to freshwater rivers, lakes, inshore estuaries and lagoons, from polar waters to warm tropical regions. They live in coastal waters, open sea, at different depths, even as low as 2 000 meters. Only 5% are really oceanic. There are species, such as the megamouth shark (Megachasma pelagios), which inhabit the ocean depths, while hammerheads (Sphyrna spp.) and sandbar sharks (Carcharhinus plumbeus) prefer shallow coastal waters. Sharks vary greatly in size from the colossal whale shark (Rhincodon typus), which can exceed 12 meters, to the pygmy sharks (Euprotomicrus bispinatus), which are full grown at 25cm. Most species are highly migratory and travel great distances, such as mako sharks (Isurus spp.), blue shark (Prionace glauca) and whitetip shark (Carcharhinus longimanus), which are pelagic, but other species are localized. There are species with a limited distribution and those which are widespread. There are also wide differences in their social behaviour: some species are solitary, like the thresher sharks (Alopias spp.), but they can assemble when attracted by shoals of prey. There are species that tend to segregate geographically on the basis of age and sex, such as sandbar, blue and dogfish (Squalidae). They can live as long as 60/70 years like the tope sharks (Galeorhinus galeus), or they can have a shorter lifespan like the roughly 20 years of the blue shark. They can take up to 20-25 years to mature like the dusky shark (Carcharhinus obscurus), or mature within one year like the Australian sharpnose shark (Rhizoprionodon taylori). Not all sharks are totally carnivorous. There are species such as the whale, basking (Cetorhinus maximus) and megamouth sharks which eat predominantly plankton. Shark reproductive strategies also vary widely from one species to another. They can be oviparous, viviparous or ovoviviparous as many species lay eggs, others have placentas, and some produce embryos. The gestation time can be long (two years) and the number of offspring can reach 300 (e.g. the whale shark) or be limited to one (e.g. the sandtiger shark (Carcharias taurus)).

In general the life history of sharks is characterized by slow growth rates, low fecundity potential, relatively late sexual maturation, long life spans and so they are classified by ecologists as strong K strategists. Furthermore, they are usually the top predators in their communities and are therefore comparatively sparsely distributed. So, they possess biological peculiarities and an ecological role that indicate that they could be particularly sensitive to an increase in fishing exploitation[3].

There are enormous differences between sharks and other fish. In particular, unlike other vertebrates, sharks have a skeletal structure of cartilage instead of bone. Urea and trimethylamine in their blood and tissues help to maintain their osmotic balance. They are without the usual urinary tract, so they concentrate urea in their blood and excrete it through their skin. Urea is a nitrogenous, non-toxic by-product of protein metabolism, which must be removed by immediate bleeding, dressing and icing the shark after it is caught to prevent urea from contaminating the meat. Urea is converted by bacteria to ammonia and lodges in the tissues. Improper handling causes a strong ammonia odour and taste. Due to urea and ammonia the shelf life of fresh products is limited to a few days.