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Ecology of cartilaginous fish

The ecology of cartilaginous fishes is not well known since most cartilaginous fish researchers have concentrated on other aspects of their biology. Cartilaginous fishes are often seen as peripheral and unimportant to marine ecosystems, which reflects a bias towards bony fishes, sea birds, cetaceans and pinnipeds by most marine ecologists that work on vertebrates. Most marine community studies are focused on bony fishes or invertebrates while overlooking the importance of cartilaginous fishes to these very same communities. Ecologists have expended great efforts in defining the various niches, habitats, and roles occupied by bony fishes, but failed to recognize the importance of cartilaginous fishes in the ecosystem. More often than not the cartilaginous fishes are merely lumped into a catch-all group termed "sharks" or "rays" with the ecologists glossing over their presence and their importance to the community. This failure on the part of many ecologists stems from a lack of training and from preconceived notions about cartilaginous fishes.

What marine ecologists need to recognize is that each species of cartilaginous fish occupies a distinct niche within a particular community. Critical studies are necessary to properly elucidate the role each cartilaginous fish species plays in a given community. Just as with bony fishes there exist cartilaginous fish species that are generalists and specialists. Some species like the Whitetip reef shark (Triaenodon obesus) are exceptionally well adapted to extracting small fish and crustaceans from coral reefs, and is able to penetrate cracks and crevices where their preferred prey hides. The Silvertip shark (Carcharhinus albimarginatus) is poorly adapted to crevice feeding, but is efficient at picking off active bony fish species which tend to stay out in the open or just above the reef. A whitetip reef shark, in contrast, is inept at capturing free-ranging bony fishes.

Cartilaginous fishes have to be viewed as an integral part of any ecosystem since in most instances they are among the top predators. Even among cartilaginous fishes there exists a hierarchy with some species being prey to others. In a typical coastal temperate bay environment in southern Africa or California, for example, the cartilaginous fish fauna will differ seasonally and spatially. The bay with its deep central portion fans out north and south into large areas of mud flats which are exposed during spring tides. These mud flats are usually bisected by deeper channels formed by runoff from small creeks and rivers. In the bay's ecosystem several species of cartilaginous fishes reside with each occuping a distinct niche within this habitat. Common in these bays are several species of houndsharks (family Triakidae) of the genus Mustelus and Triakis, along with several species of rays, particularly eagle rays (Myliobatoidei) and skates (family Rajidae). Among the houndsharks one species which readily roots out mud-dwelling organisms may be seen foraging in the shallows of the mud flats, feeding on clam siphons, echiurid worms and mud shrimp. Another species of houndshark which is inept at mud-rooting is actively swimming just off the bottom in the deeper channels hunting for crustaceans and small bottom fishes. Also hunting along the bottom of these channels is an eagle ray which locates and cracks such hard-shelled prey as oysters, which cannot be handled by the weaker jaws of the houndsharks, and also roots in the mud flats for echiurid worms and mud shrimp. Cruising up off the bottom and foraging through the channels and occasionally onto the mud flats are powerful predators like the Great white shark and Spotted sevengill shark, both of which feed on other cartilaginous fishes and on marine mammals. In tropical areas these species might be replaced by the Zambezi and Tiger sharks. Over the course of the year the abundance and composition of the species residing in this bay will change with changes in prey composition, temperature, and salinity.

Along the open coastal areas different species occupy a variety of habitats ranging from rocky reefs with kelp beds, sandy or mud bottom with little vertical relief, and coral reefs with inner lagoons which will have a different cartilaginous fish fauna than the outer deeper reef. Some species like the Tiger shark may stay out in the deeper recesses of a reef during the day, but at night move into the shallower lagoons to hunt. The outer edge of the continental shelf will contain an entirely different fauna than will be found on the upper slopes, though some slope species may move up onto the shelf at night or seasonally to feed or give birth. Although intensive studies are lacking the numerous seamounts and troughs scattered throughout the world's oceans may each comprise a distinct fauna with cartilaginous fishes specially attuned to surviving on the endemic prey species present.

Cartilaginous fishes have many species at or near the apex of the food pyramid and care should be taken to examine them in this context. Ignoring them is as fallacious as ignoring lions, leopards, and hyaenas if one was studying the ecology of Kruger Park. The removal of one or several species of cartilaginous fishes may have a domino effect on the whole ecosystem. This is evidenced by the reduction of large sharks such as the Zambezi, Tiger, Great white, and Ragged-tooth sharks, which include as a part of their diet juveniles of such large species as the Dusky shark (Carcharhinus obscurus) and adults of small species of sharks, which in turn feed on bony fish including popular recreational species. Increases in small sharks after the onset of anti-shark programs to reduce the number of potentially dangerous, large sharks have been noted in Hawaii and off KwaZulu-Natal. In the case of the well-known KwaZulu-Natal `small sharks controversy' a question that most people have overlooked is where are the droves of juvenile Dusky sharks coming from? The answer is obvious: from adult Dusky sharks! Though juvenile Dusky sharks tend to stay nearshore in nursery grounds, the large adults migrate offshore to the edge of the shelf and to offshore banks. Incremental reduction of shark predation on juvenile Dusky sharks can produce a higher survival of individuals to adulthood, which in turn can produce a greater amount of little Dusky sharks when pregnant females come inshore to pup. Besides the effect on the nearshore environment no one has yet considered the effect the adult dusky sharks are having on the offshore environment which harbors several important fisheries for bony fishes.

Researchers and anyone else who has a genuine interest in the sea will have to consider the impact of cartilaginous fishes on the whole environment. These fishes cannot be lumped in a catch-all group, but must be examined as individual species which are integrated biological units with a long evolutionary history in the marine environment. One must throw away preconceived notions on the ecology of cartilaginous fishes and begin to look at these organisms as versatile, precision biological machines, each occuping a specific niche, habitat, and a role in the marine environment. These are more advanced and complex animals than indicated in most literature accounts, and have been immensely successful in exploiting virtually every niche and habitat available to large predators in the marine environment. Studies on the ecology of cartilaginous fishes are not easy due to the inherent difficulties one can encounter, but are well worth the effort in allowing us to attain a better understanding of the whole marine environment, the BIG PICTURE so to speak, rather than one-sided aspects of it that appeal to human biases and emotions.

Text by Leonard J.V. Compagno, David A. Ebert and Malcolm J. Smale

  Iziko Museums of Cape Town, 2008

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