John A. Musick
Sharks and their relatives, the batoids (including skates, rays, guitarfishes and sawfishes) and chimaeras are all chondrichthyans, or cartilaginous fishes. Chondrichthyans are a rather small (about 1,200 species), evolutionarily conservative group that has functioned successfully in diverse ecosystems for over 400 million years. Despite their evolutionary success some may now be threatened with extinction as a result of human activity and the very conservative life-history traits. Many, if not most chondrichthyans grow rather slowly, mature at relatively late ages and have a small number of young. These characteristics result in very low rates of potential population increase so that populations have little capacity to offset excess losses from fishing (either direct or indirect) and other sources of mortality wrought by humans. Therefore, chondrichthyan populations are vulnerable to overfishing, local extirpation and population collapse from which they are slow to recover.
Most chondrichthyans are marine creatures, although many utilise estuaries, particularly as nurseries and some enter or are endemic to fresh water. Chondrichthyans range from the immediate subtidal zone offshore to coastal, bathyal (200–2,000m) and even abyssal habitats (>2,000m). Some species are strictly benthic, like the skates (Rajoidei) and angel sharks (Squatinidae), whereas others like the mako shark Isurus oxyrinchus (Lamnidae) are pelagic, restricting most of their activities to the upper layers of the ocean. Threats posed to chondrichthyan habitats by humans are directly proportional to the habitat's proximity to land. Freshwater species have been affected by the construction of dams, deforestation (and siltation), eutrophication and chemical pollution. Estuarine species have been affected by the destruction of marsh and mangrove nursery habitats. Likewise coastal species have been impacted by habitat change brought about by human activities such as trawling and dynamite fishing. Offshore species are buffered the most from human-induced habitat degradation. Vast oceanic habitats remain relatively clean and unaltered, although the spectre of ozone depletion and global warming will probably affect the geographic ranges of some oceanic species of chondrichthyans.
A school of silky sharks Carcharhinus falciformis.
Yves Lefevre, Fondation Malpelo
Most chondrichthyans are predators, and the variety of their prey is great. Some species of skates may specialise on small benthic infaunal animals, such as polychaetes or amphipods. Some rays, particularly the myliobatids, may consume hard-shelled bivalve molluscs. Most sharks eat a wide variety of fishes and crustaceans, although white sharks Carcharodon carcharias prefer marine mammals, and basking sharks Cetorhinus maximus and whale sharks Rhincodon typus filter zooplankton from the sea. Despite an extensive literature on the food habits of chondrichthyans, very little is known of the dynamic function they serve in their ecosystems.
The centre of greatest chondrichthyan biodiversity lies in the Indo-West Pacific Region (as with many other fishes). Some of the galeomorph sharks (requiem sharks and their relatives, order Carcharhiniformes, carpet sharks, order Orectilobiformes and bullhead sharks, order Heterodontiformes) have radiated and reach their highest diversity there. The ancient Orectolobiformes (carpet sharks) have low diversity outside the area and are represented elsewhere only by the cosmopolitan whale shark R. typus and one relict species (the nurse shark Ginglymostoma cirratum) in the tropical Atlantic. In contrast, the squaloid sharks, certain carcharhinoid sharks (catsharks, family Scyliorhinidae), skates and the chimaeras have reached their greatest diversity in the cool, dark reaches of the bathyal zone, with the smaller species, such as the lantern sharks (Etmopterus spp.) and many skates, showing regional endemism. In fresh water, the Amazon basin has the highest incidence of chondrichthyan endemism and species diversity because of the radiation within the river stingrays (family Potamotrygonidae).
Despite the infamous yet erroneous public image of sharks as threatening man-eaters, the probability of shark attacks is minute (there are fewer than 15 deaths per year worldwide). Humans have a much higher probability of being struck by lightning than being attacked by a shark. Rather, it is the sharks that are increasingly threatened by humans. Shark fisheries have proliferated around the world in response to lucrative markets for shark fins used for soup in Asia. In addition, the burgeoning human population and its demand for food, along with the collapse of many traditional fisheries, have created market demand for the meat of sharks, skates, rays and even chimaeras. Fishers, who once discarded chondrichthyans (often alive) because of their low value, now land them or cut their fins off and discard the dead or dying animals. The most insidious source of mortality is bycatch, where species are not the target of specific fisheries but are killed incidentally in fisheries aimed at other species. For example, the barndoor skate Dipturus laevis, a large and obvious species, was severely reduced in abundance in the western North Atlantic before scientists noticed, because it is taken as bycatch and often discarded dead in the bottom-trawl fishery for cod, haddock or other teleosts.
The barndoor skate was allowed to decline because little information is recorded on bycatch or landings of chondrichthyans on a species-by-species basis, even in such well-monitored fisheries as the Canadian and New England ground fisheries. In other regions of the world where the fisheries management infrastructure may be rudimentary or non-existent, the situation is much worse. In South East Asia, where most fisheries land everything that is captured, 60–70 species of chondrichthyans may be landed in a single area. Some of the most common species of batoids are still undescribed. In such areas, taxonomic study and production of regional fish identification guides are needed if fisheries data are to be provided on a species level to enable effective management.
In other areas where species in the catch are known, the lack of data on fishing effort and size and age composition of the catch is problematic, even for fisheries like the US Atlantic shark fishery, where the management infrastructure is well established. In addition to a paucity of fishery-dependent data, basic biological information on age, growth and reproduction (all necessary for management) is only known for about 4% of shark species and less than 1% of batoids. Very little is known about the biology of chimaeras.
Responsible management of chondrichthyan populations has been hampered not only by the lack of biological and fisheries data, but also by the historically low priority given these fishes by fishery managers. Because chondrichthyans have traditionally been of low market value compared to most other fishes, very low (or no) priority has been placed on their management (Shotton 1999). Even today when some sets of sharks fins may bring in excess of US$700 per kg (Clarke et al. this volume), shark fisheries are virtually unmanaged with the exception of a few countries including Australia, Canada, New Zealand and the USA. Some causes for lack of management are the relatively recent and rapid development of chondrichthyan fisheries and lack of knowledge on the part of most fisheries managers about the extreme life-history limitations and high vulnerability of most species to rapid overfishing. Furthermore, the long-term economic impact of overfishing chondrichthyan stocks has been overlooked. Whereas most teleost stocks can recover from overfishing in less than a decade, chondrichthyan stocks take several decades to recover, during which time little, if any, economic gain can be realised from the fishery. Consequently, even if chondrichthyan landings may be of lower immediate value, the total economic loss to fishers over the extended period of recovery may be greater than that for more valuable species that recover from overfishing more rapidly.
The ultimate objective of the present document is to provide scientific information and advice that will lead to responsible fisheries management and effective conservation of chondrichthyan species worldwide.
Fishing harbour, Hodeidah, Yemen, the main shark landing point in the entire Red Sea/Gulf of Aden region.
The document begins with a brief description of the taxonomic diversity and interrelationships of chondrichthyans, general biology, ecology and life history of chondrichthyans, their socio-economic importance, threats to their populations and a discussion of global conservation initiatives. The next section includes regional overviews of the status of chondrichthyan populations in nine geographic regions prepared by IUCN/SSC Shark Specialist Group (SSG) teams from each region.
A large part of this document comprises status reports for individual species. The general format for these accounts includes a discussion on taxonomy, physical description, geographic distribution, ecology and life history, exploitation and threats and the IUCN Red List assessment as of 2000. Species chosen for review were generally prioritised by perceived degree of threat, ecological or economic importance and availability of expertise within SSG. Many additional chondrichthyan species have since been assessed, submitted to the Red List (www.redlist.org) and summarised on the SSG website (www.flmnh.ufl.edu/fish/organizations/ssg/ssg.htm). In addition, the species status reports in this volume will be updated on the SSG website as new information becomes available, as will the regional overviews.
Clarke, S., Burgess, G.H, Cavanagh, R.D., Crow, G., Fordham, S.V., McDavitt, M.T., Rose, D.A., Smith, M. and Simpfendorfer, C.A. This volume, Chapter 4. The Socio-economic Significance of Elasmobranchs.
Shotton, R. 1999. Species identification practices of countries reported landings of Chondrichthyan fishes in the FAO nominal catches and landings database. Pp. 904–920 in R. Shotton, ed. Case studies of the management of elasmobranch fisheries. FAO Fisheries Technical Paper, No. 378, Part 2. FAO, Rome.[link]
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