As a group, elasmobranchs present an array of problems for fisheries management and conservation. Their life–history characteristics make them a fragile resource, more susceptible to overfishing than most teleost fishes. Assumptions of traditional fisheries models do not always fit the biological traits of elasmobranchs, making their assessment and management difficult. The high mobility of many species, sometimes involving trans–boundary migrations, incorporates another level of complexity to their assessment and highlights the need for proper knowledge about stock delimitation and dynamics if adequate management to be implemented. Elasmobranch fisheries assessment is complicated further because of a general lack of baseline information about their fisheries throughout the world. Furthermore, the economics driving elasmobranch exploitation involve a paradox: sharks and rays have a relatively low economic value making them low priority resources when it comes to research or conservation, while the demand for some of their products, such as shark fins, is very high and stimulates increased exploitation. The demand for shark fins sometimes results in substantial waste when only the fins are kept and the rest of the fish is discarded.
Considering these circumstances, it is not surprising that there is a history of nonsustainability in the exploitation of elasmobranchs. In recent years however, there has been growing international concern over the conservation of some elasmobranch stocks and it seems that now, more than ever, there is a need for a more systematic approach to the problem of elasmobranch assessment and management.
Fisheries for elasmobranchs have not increased in the same way as because of other fisheries worldwide. The low market value of these fishes, and their relatively low abundance. Compagno (1990) indicates that in terms of the commercial catches and according to FAO statistics, cartilaginous fishes are a minor group which contributed an average of 0.8% of the total world fishery landings during 1947–1985, while bony fishes such as clupeoids, gadoids and scombroids, accounted for 24.6%, 13.9% and 6.5%, respectively. Furthermore, elasmobranch catches increased only threefold over this time whilst the other three groups showed fivefold to sixfold increases and total world catches increased fourfold. Recorded world chondrichthyan commercial catches totalled 704 000t in 1991 (present study) making 0.7% of the total world fisheries catches; even considering an unreported catch of 50% to those recorded, this is still only about 1% of the world fisheries catch. Despite these facts, elasmobranch resources are of prime importance in some regions of the world and have sustained very important fisheries in some countries. Also, they have been, and remain, a cheap source of protein for millions of humans from coastal communities dependent on subsistence fisheries.
Traditionally, elasmobranchs have not been a highly priced fishery product. Their economic value ranks low among marine commercial fisheries (e.g. in the Taiwanese gillnet fisheries of the Central Western Pacific, shark (trunks) prices attain only 20% and 60% of those of tunas and mackerels (both whole) respectively (Millington 1981)). Exceptions are: sport fisheries, which can be of considerable economic value; certain species for whom a gastronomic demand has recently developed in some parts of the world (e.g., mako and thresher sharks in USA), or those species which, unfortunately, are highly-sought only for their teeth and jaws, such as the great white shark. The only highly-prized elasmobranch product is shark fin for oriental soup, a commodity for which there has recently been a considerable increase in demand (Cook 1990). On the other hand, anthropocentric points of view have substantially biassed public opinion against some elasmobranchs labelling them either as malevolent or as trash fish and thus undesirable species. A further issue is that of shark attacks on humans and the damage that some sharks cause to fishing gear and catches. These problems are real, but now are probably insignificant compared to the threats that humans represent to some populations of elasmobranchs.
Sharks and rays have biological characteristics and an ecological role which suggests they could be particularly vulnerable to fishing pressure. All elasmobranchs are predators and most exist at the top of the food chain. Their abundance is therefore relatively small compared to groups situated in lower trophic levels. They are typically slow growing and long-lived and mature at a late age. This, together with their low fecundity, results in a low reproductive potential for most of the species. Recoveries of population numbers from severe depletions (caused either by natural phenomena or human action) should take many years for the majority of elasmobranchs. Additionally, the removal of top predators from marine ecosystems might trigger undesirable consequences for the environment and other fishery resources (van del Elst 1979).
The vulnerability of the group, together with the past history of collapses in elasmobranch fisheries (see Anderson (1990) for a review), are causes for concern. The continuing increase in their catches and the continuing increase in demand for shark fins may be endangering the sustainability of elasmobranch fisheries. However, adoption of any widespread conservation measure is likely to affect the fisheries of many countries for whom the resource is of considerable importance. These impacts are difficult to assess without good basic information about their fisheries on a global scale.
Given the relative low value of elasmobranchs it is not surprising that information on their fisheries, or even their basic biology, is scarce, patchy and scattered, especially when compared to the amount of literature on other fishery resources or even that focusing on the problem of incidental catches of marine mammals in fisheries. An example is the amount of scientific literature generated during the past 16 years. The results of a fisheries database (ASFA) query for papers published between 1978 and 1993 including the name of six different fishery resources in the article's title are shown in Figure 1.1. Sharks and rays rank last after the salmons, shrimps and prawns, clupeoids, tunas and even lobsters. Although these figures may be biassed in some cases (e.g. inclusion of numerous environmental studies for salmonids and aquaculture studies for shrimps and prawns), they still are a measure of the importance of each resource.
To properly assess the current situation of elasmobranch resources, address the various problems associated with their exploitation and contribute new ideas about their study and management, it is essential to increase our knowledge about the characteristics and diversity of their fisheries, the species exploited, the size of the catches, discards at sea and past or current management measures adopted for the fisheries. While recent workshops and symposia have expanded our knowledge, specially in relation to their biology, much of the existing information about their fisheries is not only dispersed but is also not usually published by those concerned with the studies or management. This review is a contribution towards providing this information by compiling in a single volume the most important information available about world elasmobranch fisheries and providing an analysis of the global situation.
Elasmobranchs are part of the Chondrichthyes. The Class Chondrichthyes comprises a diverse group of fishes whose most obvious common feature is the possession of a cartilaginous skeleton as opposed to the bony skeleton of the Osteichthyes or bony fishes. The cartilaginous fishes form an ancient and successful group dating back to the Devonian era. The basic models remain largely unchanged since their last large speciation during the Cretaceous era. Despite their ancient origin they possess some of the most acute and remarkable senses found in the animal kingdom which allows them to coexist successfully with the more modern teleost species.
Figure 1.1. Number of articles published* during 1978–1993 for each of six groups of fishery resources, and appearing in the ASFA database. (* those with the name of the group in the article's title)
The chondrichtyans are grouped into two main subclasses by many taxonomists: Holocephalii (Chimaeras or ratfishes and elephant fishes) with three families and approximately 37 species inhabiting deep cool waters; and the Elasmobranchii, which is a large, diverse group (including sharks and rays) with representatives in all types of environments, from fresh waters to the bottom of marine trenches and from polar regions to warm tropical waters. The great majority of the commercially important species of chondrichtyans are elasmobranchs. The latter are named for their plated gills which communicate to the exterior by 5–7 openings. The classification of elasmobranchs is a subject of continuous debate but they are generally divided into three groups, i.e., squalomorphs, galeomorphs and squatinomorphs, which include 30 families and approximately 368 species and a group known as the batoids composed of rays, skates, torpedoes and sawfishes, embracing 14–21 families and about 470 species (Compagno 1977, 1984; Springer and Gold 1989).
For this review, all the Chondrichthyes, (sharks, skates, rays and chimaeras) are often treated together as “elasmobranchs” or “sharks and rays”. Although this is strictly inaccurate, it simplifies writing and reading by avoiding uncommon or lengthy terminology such as “chondrichthians” or “sharks, skates, rays and chimaeras” every time referance is made to the group.