Fish Fights: Weighing Up the Opposition
When rival fish size each other up from afar, the most common outcome is that one of them will recognize the other's superiority and keep its distance, or be chased off without putting up any resistance. But when fish are fairly closely matched in terms of their size and motivation, then the aggression stakes are raised. This does not necessarily mean that a fight is inevitable -- far from it. Out-and-out fights are rare because of the very real danger of serious injury, even for the victor. In most cases, disputes are settled by a series of ritualized displays and signals that are calculated to communicate the prowess of each fish without actual physical contact.
What form do these displays and signals take?
The potentially deadly cost of injuries from fighting means that fish have evolved a wide range of different displays. The most obvious ones to a human observer are the visual displays that fish use at close quarters. For example, when rivals come face-to-face, they may flare their gill covers or, in the case of royal grammas and some gobies, open their mouths wide.
Damsels more usually display by spreading their dorsal and anal fins and swimming in a stiff, exaggerated manner. In doing so, they display their flanks to one another and often circle like tentative boxers. These visual displays are frequently backed up physically or by audible signals. Lateral displays are very often supported by each fish sharply contracting its muscles, causing its body to wave and sending a pulse of water towards the opponent This allows each fish to assess its rival's strength very directly, via the pressure-sensitive lateral line.
Most territorial reef fish, including butterflyfish and damsels, are also highly capable of making warning noises to rivals in exactly the same way as animals such as deer or lions.
What do the signals mean?
The whole purpose of all the signals is to try to impress a rival. Spread fins act, if not to exaggerate, then at least to emphasize a fish's size. This is important because, nine times out of ten, a large fish will overcome a smaller opponent. Wide open mouths and flared gill covers attempt to achieve the same thing by demonstrating size and power. Displaying by spreading the fins also maximizes the impact of a fish's color pattern, which is in turn related to its health and, again, its strength. The growls and grunts that different fish make also pass on information. Fish are highly attuned to small differences in these calls and can tell a great deal from them. Although it is possible for fish to make the most of themselves using displays, they cannot fake them outright A fish cannot pretend to be twice the size it is nor develop strong colors without being in the peak of hearth, so all these displays are honest signals of a fish's quality.
How else might fish assess their opponents?
Remarkably enough, fish can also pick up information about potential rivals by watching them in contests with others.These so-called bystander effects have been shown in male gobies, which adjust their level of aggression towards other males if they have been able to pick up information by watching them fight. When they are confronted with a fish that has just won an aggressive encounter -- that has proved itself in a direct fight -- they are less aggressive than when they face up to a loser!