Defending Reef Territories: Fish Need to Protect Their Food Source

Defending a territory is an expensive business. A territorial fish has access to all the food its patch can supply, but must spend a large amount of time and effort in driving away possible intruders.When the benefits outweigh the costs, as can happen at certain times of the year, or even certain times of the day, fish may abandon their territories and adopt different strategies. Even when a territory is essential, say to maintain a breeding ground, fish have developed ways of cutting the costs of territory defense.

Do fish defend their territories constantly?

Territory defense uses up a great deal of a fish's energy, so it only happens when the payback to the fish is high. If the amount of food that a territory produces starts to decline, then the benefit to the fish in defending that territory will also decline and the fish is likely to give it up and move on after a while.

Some damselfish live in loose schools during the early hours of the day, only to stake claims for territories and become aggressive in late morning and throughout the afternoon, before abandoning them and reverting to non-aggressive schooling later in the day. It is thought that the reason for this lies with the nutritional value of the algae on which they feed. When the sun is at its peak, the algae respond by photosynthesizing and producing large amount of sugars, making them especially delectable to the damselfish during these hours and rewarding their efforts in defending a territory.

How are territories spread across the reef?

The largest and strongest fish are best suited to the hard work of winning and defending a good territory. Territories on the reef often build up into a mosaic across the surface. The best territories are usually found in the center of the group, where food supplies such as algae are greatest and where fish can find cover.

Other good territories may be arrayed around this, with poorer-quality territories on the outside of the reef, where they may be subject to attack by predators and have lower food supplies. A study on three-spot damselfish showed that males with central territories grew faster than their counterparts with edge territories.The central territories were richer in algae and also less at risk of a raid party of intruders
eating the precious food. More importantly, the central males attracted more females to lay eggs in their territories; female fish are good judges of territory quality and reward the efforts of a male who has staked his claim in a good territory by mating with him.

How do fish deal with their boundary disputes?

The crowded nature of reef life means that territorial fish always have neighbors. While the territory is being established and boundaries are being worked out, neighbors can frequently become confrontational. This usually involves a great deal of displaying and mock charging without the fish actually coming to blows, as each fish tries to impress upon the other that it is not to be messed with. But once the boundaries are determined and the relationship between neighbors is worked out, things tend to settle down. Boundaries are extremely important to fish and usually demarcated by landmarks, such as a piece of rubble or branched coral. When a territorial red-back butterflyfish (Chaetodon paucifasciatus) selects a new partner, the incoming fish rapidly learns where the boundaries are and, in doing so, prevents upheaval and an escalation in aggression.

Damselfish quickly learn to recognize their neighbors and declare a truce. A fragile non-aggression pact seems to ensue, sometimes called the “dear enemy” effect, that enables fish to avoid petty squabbling with the same few neighboring individuals while allowing them to defend vigorously against outsiders.The damsels are even able to tell the difference between the warning or aggressive sounds made by neighbors and those made by unfamiliar fish, responding more strongly to the newcomers in line with the greater threat that these may present to the territory.