Territorial Reef Fish
Perhaps the majority of reef fishes, including many damselfish, butterflyfish, surgeons, tangs, parrotfish, triggers, and groupers, stake out and defend territories to feed or to lay their eggs. For any strategy to be so routinely adopted it must provide considerable benefits, yet territoriality is often referred to as a "density-dependent" strategy, meaning it only occurs when large numbers of fish are concentrated in a small area.When densities are low, fish usually do not bother to spend energy on defending a territory, but this is rarely the case on coral reefs.
How do fish gain possession of a territory?
Finding and occupying a territory is an activity usually associated with the onset of adulthood. With age comes size, and with size comes power and increased competitive ability.This means that juvenile fish of most species must bide their time; for example, young parrotfish tend to live in groups. As they mature they go it alone, becoming solitary and starting to seek out territories. In many species, such as the dusky gregory younger fish are forced to gather in areas away from territorial adults and where food is comparatively scarce.
For coral-dwelling gobies, such as the broad-barred goby (Gobiodon histno) the size of the fish is directly related to the size of the territory; smaller individuals have to make do with equally small corals, while larger fish occupy the largest coral territories. When given a free choice, fish of all sizes prefer to set up their territories in large corals, so the fact that smaller fish do not is due to competition. As fish grow they can trade up, fighting for better territories. However, one factor does count against them: to win a territory they must drive off the current owner who is, of course, on home ground. Studies have shown that where two competitors are equal, home advantage counts for a great deal.
Are territorial fish equally aggressive to all other fish?
Fish with feeding territories save most of their aggression for fish with the same diet as themselves. Butterflyfish tolerate the presence of fish with different diets in their territories, but are quick to drive out direct competitors. It has been shown that a damselfish presented with an unknown fish is extremely quick to learn what the stranger eats and to tailor its aggression accordingly. Damselfish also show more aggression towards larger invaders, such as parrotfish and surgeonfish, that could have the greatest impact on their carefully tended algal gardens. Species such as Ward's damsel (Pomocentrus wordi) will happily tolerate the presence of fish such as the jewelled blenny (Salarias fasciatus) within their territories, possibly because this aggressive fish contributes to the damsel's territory defense by helping to keep out other blennies.
What happens to fish that can't get territories?
When all available territories are taken, certain individuals, especially juveniles, are sometimes forced to follow other options. Failing to acquire a territory is bad news for fish and means that their food supply is likely to be heavily restricted.
An experiment on fairy basslets showed that smaller fish were able to almost double their feeding rates when larger, more dominant individuals were absent from their reef. This is one reason why many fish, such as blue tangs, are differently colored as juveniles; territorial adult reef fish tend to attack fish of the same color as themselves. However, there are options available to fish that fail to secure a territory. Where there are high densities of fiercely territorial damselfish, blue tangs often abandon attempts to defend their own territories and gang together to raid the damselfishes' territories en masse. The aggressive territorial defense of the jewel damsel (Plectroglyphidbdon lacrymatus) is thought to have driven juvenile chocolate surgeonfish (Acanthurus pyroferus) to mimic different species of pygmy angelfish. By doing this, the juvenile mimics are thought to deceive the damsels into thinking that they are, like pygmy angels, not direct competitors. Certainly the mimics suffer less aggression from the damsels and are able to spend more time feeding.