Fish Fights: The Lead-Up

The aggressive nature of many tropical marine fish, particularly their intolerance for their own or similar species, can present a real problem for fishkeepers. Like all animals, fish must compete to acquire resources such as food, territories, or mates. On the coral reef, food, hiding places, territories and mates are almost always in short supply To acquire these precious commodities, or even just to hold on to what they have, fish must usually compete and this competition can often escalate into direct aggression. But actual fighting is extremely costly — even the victor can sustain serious injuries — so fish go to great lengths to size up their opposition, to try to scare their rival, to impress it into backing down, while all the time trying to work out their own chances of winning if it should come to a fight.

How do fish decide whether or not to fight an opponent?

The likelihood of one fish attacking another individual is affected by two mam factors. First of all, the fish will assess several physical characteristics of its rival in order to decide its chances of coming out on top. This usually means its opponents size — fish are much more likely to attack small fry than a big bully. However, if fish is defending precious eggs it is likely to throw caution to the wind and fearlessly attack members of species that are several times larger than itself. Some fish, such as triggerfish, will even attack divers!

Secondly, the potential aggressor will determine the extent to which its rival presents a threat to its precious possessions: its food, territory, or mates.

What other characteristics do fish assess in their rivals?

Coral reef fishes tend to be most aggressive towards individuals that use the same resources — food and territories — as themselves, and the biggest competitive overlap is with members of their own species. Color is extremely important, especially on the coral reef and fish tend to attack when confronted by an individual that is that same color as themselves.

Matching colors can indicate fish of the same or similar species that would compete for exactly the same resources.This matched color aggression, seen in butterflyfishes, among others, is one reason why juvenile reef fish tend to be so differently colored to adults of the same species — to discourage aggression. Similarly, some fish, including damsels, are known to attack fish according to their shape.This is because a fish's shape, like its color, gives a clue to its diet and therefore the threat it presents as a competitor.

However, fish do not just rely on hard-and-fast rules for this. It has been shown that damselfish are able to learn what is, and what is not, a competitor, getting clues by watching the behavior of other fish to work out what they are eating. As well as all this, male fish are sometimes more aggressive to other males than to females that stray into their territories. Some male wrasses and pygmy angels will ferociously exclude other males, while welcoming potential mates.