Shark Behavior 71
that are frequently filmed in groups include Blacktip, Blacktip Reef, Silky,
Porbeagle, Whitetip Reef, Gray Reef, Sand Tiger, and Basking sharks.
Spiny Dogfish—probably the most abundant shark in the sea—form
schools numbering in the hundreds and even thousands. These schools
create problems for fishing operations because migrating schools move
through areas and destroy fishing gear, get tangled in nets set for other
fishes, eat large numbers of fish and invertebrates, and may slow the recov-
ery of depleted stocks of fishes such as Atlantic Cod. Spiny Dogfish schools
are usually made up of animals of similar size and, after the fish mature, of
one sex. Immature dogfish tend to school offshore whereas mature females
school farther inshore.
Among batoids, the myliobatid rays are well known for forming groups.
Manta Rays and Spotted Eagle Rays frequently occur in small groups of a
half dozen or more animals, swimming in synchrony. Spotted Eagle Rays
fly through the water in small groups of 5 to 50 individuals. The groups
take on predictable shapes, including one ray following another or the
group swimming in a diamond formation, much like planes at an air show.
Manta Rays in Hanifaru Bay, the Maldives, can form trains of dozens of
animals that perform what has been called cyclone feeding. The mantas
form a giant linked chain of more than 50 animals, the ray at the end of
the chain followed by the ray at the head of the chain, forming a spiraling
vortex of Manta Rays.
The record holders for large schools of sharks are smaller mylioba-
tids, such as the Bat Ray (Myliobatis californica) and especially the Cownose
Ray. Migrating hordes of Cownose Rays, called “fevers” when migrating
in large groups, have been counted from airplanes. One fever in Sarasota
Bay, Florida, was estimated to contain between 600,000 and 1 million in-
dividuals; another group, in the Chesapeake Bay of Virginia, was so large
that numbers had to be calculated from multiple aerial photographs. This
migration contained more than 5 million individuals, covering an area of
over 450 ha (1,110 acres, or more than 850 football fields), at a density of
more than 1.1 ray per square meter (or yard).
Can sharks tell one another apart?
Most sharks and rays clearly can distinguish between their own and
other species. This is shown in the many species that form aggregations
and schools of only one species, often of only one sex, and mostly made up
of individuals of similar sizes. In laboratory experiments, juvenile Lemon
Sharks preferred to associate with other Lemon Sharks rather than with
Nurse Sharks, and Nurse Sharks preferred to be with their own species
rather than with Lemon Sharks.