Shark Ecology 111
How can you tell if a shark is sick?
A healthy shark swims normally. A sick shark would appear disoriented,
swimming erratically or lying on the bottom on its side or back. Such an
animal would be eaten quickly by other sharks or by a variety of scavengers
in the wild. Public aquariums have specially trained veterinarians to deal
with sick animals, but there is little the average home aquarium keeper can
do. This is one of the reasons we advise against keeping sharks in fish tanks
at home.
Are sharks good for the environment?
In some areas, rays serve as “ecosystem engineers” by digging up clams,
crushing their shells and turning them into sand, and moving sediment
around in the process. Their digging often pushes soft sediment aside, ex-
posing hard bottom and objects that become settling habitat for a variety of
invertebrates. A study in western Australia estimated that three ray species
reworked 42% of the sediment in a bay each year.
Rays are an example of the different roles played by different species in
natural ecosystems. When species are eliminated, relationships and pro-
cesses are affected. Predators play important ecological and evolutionary
roles, preventing overpopulation by reducing prey numbers and remov-
ing weak or sick individuals. Predators therefore maintain healthier prey
populations with less disease and fewer genetic defects. When predators
are removed, prey populations become overpopulated. Deer populations
throughout the United States have reached unnaturally high numbers and
have become pests because their natural predators (wolves, mountain lions)
have been “controlled.” Deer then outstrip their food resources and starve,
contract fatal epidemic diseases, and cross highways too often. Rabbit pop-
ulations skyrocket and strip vegetation bare in the absence of predators.
The same rules hold for marine systems. Coral reefs that are overfished
become covered with coral-killing algae because the fishes that ate the al-
gae have been removed. When sea otters were hunted almost to extinc-
tion in the North Pacific, the sea urchins that they used to eat increased.
Urchins eat algae such as kelp and consequently create “urchin barrens,”
which are rocky areas devoid of plants. Kelp beds provide habitat for a vast
array of nearshore fishes and invertebrates. Without kelp, these animals are
homeless, and their populations diminish.
Sharks are at the top of the food chain in many systems (ignoring their
parasites for the moment). When they are overfished, their absence affects
the different levels of the food chain below them.