Fish Territories on the Reef

The coral reef is home to huge numbers of animals, each seeking to gather food, to shelter, and to find suitable space to breed. But these resources are scarce; there is rarely sufficient to go around so the competition is intense. Under these conditions, the only way that reef fish can guarantee their supplies is to exclude their rivals and defend a territory that allows them private access to the resources they need. The exact requirements vary from species to species; some fish defend only their foraging patches, others include a sheltered nest site within their territory.

How large are fish territories?

Fish territories vary considerably in size. Fish such as grunts, that rest throughout the day before dispersing to feed with the onset of night defend territories containing hiding places that may be as small as an 8-inch (20-cm) square. Moray eels are equally aggressive in defense of refuge holes. At the other end of the spectrum, some triggerfishes defend enormous territories of up to 2,150 square feet (200 square meters).

Territory size also varies between members of the same species — the larger the territory, the more food it will provide for its owner. However, this comes at the cost of increased work in defending it, so only the strongest individuals can defend the largest and best territories. The cost of defending a territory increases when there are more competitors to fight off. The territories of dusky gregorys (Stegostes nigricans) tend to be larger in winter, when there are fewer competitors, than in high-density summer conditions.

Where's the best place to have a territory?

For a fish to go to the extent of defending a territory — a behavior that uses up considerable time and energy
— there has to be considerable benefit. The territory must provide the fish with plenty of food and that food usually has to be renewable. For herbivorous fish, such as damsels, the best site for a territory is one where the algae can gain the most light possible, which usually means shallow surface waters. Under these conditions, the algae grows slowly but continuously, providing a steady harvest for the fish that is prepared to defend it. Research has shown that many damselfishes actively culture an algal garden, weeding out some of the faster-growing but less nourishing species of algae in favor of other types, which are richer in nutrients. Their effectiveness in achieving this is shown when fish are experimentally excluded from patches of reef — the fishe's carefully cultivated salad gardens are rapidly overgrown by the algal equivalent of weeds.

What else should a territory have?

It is essential that territories provide some cover — a bolt-hole for the fish to shelter in if a hunter comes along. For some fish, including clownfish, the protection provided by a territory — in their case an anemone — is the number one requirement Sheltered areas are often also used for the safe deposit of eggs, as far as possible out of sight of sneaking egg predators. Damselfish defend territories that include grazing areas, shelters and, for some of the males, a nest site. In many cases this nest site is a small patch of red algae that may be cultured by the male. Once he finds a satisfactory site, the male can remain in place for several years.The importance to a male of having the “full package” in terms of a territory was suggested by a study on the threadfin wrasse (Cirrhilabrus temminckii) where the female chooses her mate on the basis of the quality of his territory.