|Author: || Dale Speirs|
|Publisher: || The Calquarium|
|Volume: || Volume 42, Number 8|
|Date Published: || April 2000|
Full-contact fighting between individuals of the same species is relatively rare in nature. Even during the mating season most disputes are decided without actual contact. This may not be believable to the average aquarist who has had pet fish kill each other in the tank, but that is not a violation of this law of nature. In habitat, the loser of a bluffing contest flees the site and leaves the victor in possession of the territory or mate. In an aquarium, however, the loser has no place to escape. The winner of such a fight is hard-wired to expect the loser to go away without any actual physical combat. If this does not happen because it cannot happen, the winner instinctively escalates to physical violence, even though it is not the loserís fault he failed to depart the scene.
The non-contact phase of aggression between fish is pretty much universal among all species. These fights are normally won by the biggest and healthiest fish, but if size is not an obvious difference then the fish resort to fin flaring and rushing each other. The purpose of fin flaring is to make the fish look bigger to its opponent. Rushing at an opponent or waving oneís body side-to-side in an S-shape is designed to produce a pressure wave in the water to be felt by the opponent. The bigger the wave, the heavier or bigger the opponent must be.
Aquarists tend to think of their pets interacting only visually, but fish commonly use sound as well. Sound, not surprisingly, is another method used to decide aggressive contests without resort to blows. A study done at the University of Vienna  showed that the winners of croaking gourami (Trichopsis vittata) fights produced a higher sound level and at a lower dominant frequency. In other words, the contestants, in addition to the usual visual cues such as fin flaring, were shouting at each other. The winners could shout the loudest and deepest. In the aquatic world, baritones beat tenors.
Aquarists may also think that if two fish are involved in aggression, then that is a matter between the two of them and not the rest of the fish in the tank. One study on Betta splendens shows this is not necessarily so. Male bettas will watch other bettas fighting and observe who is the winner and who was not. They use that information in any subsequent fights they might get into with the males they were observing . For the home aquarium, usually there is only one male betta per tank, but it seems logical to deduce that this behaviour could be found in other anabantoids as well. Following from there, a fight between two fish could then have consequences that spread elsewhere, as tank mates move in.
The aquarist should feed a tank of many fish by sprinkling the food along the length of the tank, but in actual practice most people just open the lid, dump the food in, and leave. Spreading the food is important because it helps the weaker individuals get their fair share. This is more important where there are only a few fish in a tank than many. The reason why this is so was illustrated in a study on blue gouramis (Trichogaster trichopterus) where they were fed at a concentrated location. If there were only a few fish in the tank, aggression between individuals was quite high. As the group size increased, aggression declined . This is because an aggressor would be too busy trying, unsuccessfully, to chase away all its competitors, and would get little or no food. An aggressive gourami can hold off a few fish in a small group, but in a large group while it was chasing away a competitor several more would zip in behind and grab off some food. This is a scientific demonstration of a principle that aquarists have long been aware of, namely that larger groups are safer for individual fish because aggression is spread out over many individuals, instead of one or two individuals being pecked to death.
1] Ladich, F. (1998) Sound characteristics and outcome of contests in male croaking gouramis (Teleostei). ETHOLOGY 104:517-529
2] Syariuddin, S., and D.L. Kramer (1996) The effect of group size on space use and aggression at a concentrated food source in blue gouramis, Trichogaster trichopterus (Pisces: Belontiidae). ENVIRONMENTAL BIOLOGY OF FISHES 46:289-296
3] Oliveira, R.F., P.K. McGregor, and C. Latruffe (1998) Know thine enemy: fighting fish gather information from observing conspecific interactions. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON 265B:1045-1049