Chondrichthyes (cartilaginous fish,
including sharks, rays and chaemeras) >
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
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