Aggression and Stress in Freshwater Fish

It is not only humans that can feel “stressed out.” As serene as an aquarium may look, its occupants can suffer from the pressures of day-to-day life. In fish, stress is usually a response to aggressive behavior and can occur whether the fish is the target or the instigator. Fish exposed to extended periods of stress can suffer all kinds of effects, from reductions in growth rate to greater susceptibility to disease. Minimizing the stress that fish suffer is essential to maintaining a healthy aquarium.

How can you tell if a fish is stressed?

In some circumstances, it is obvious that fish are stressed; they lose color, they lose their appetite, and behave in a generally listless fashion. However, high stress levels can also occur in fish that look and behave perfectly normally This was discovered by taking blood samples from fish and measuring levels of particular hormones, such as Cortisol, which are produced by the body in response to stress.

What do these stress hormones do?

The purpose of hormones such as Cortisol is to prepare the body for action, to keep the metabolism ready to go into battle. For example, one of the functions of Cortisol is to prevent the body from storing its energy reserves, thereby keeping them available for instant use. Extended periods of high stress hormone levels can affect the fish's osmoregulation and reduce the amount of energy that the body puts towards fighting disease.

Which fish get stressed and why?

In the home aquarium, any fishes that are involved in aggression — either doling it out or taking it — become stressed. This includes fish that are breeding, fish that are behaving territorially, and fish that are in dominance hierarchies — quite a large group, when you consider it. Levels of stress vary from fish to fish, but Cortisol can be especially high in subordinate fish and in fish that spend long periods of time guarding their young.

Do hormones affect the way a fish behaves?

Hormones do affect the way a fish behaves and this relates directly to its social status and also to the way it looks and even smells. Male fish with high levels of blood Cortisol and low levels of androgens (male sex hormones) are less likely to display or fight. Catfish can distinguish between dominant and subordinate conspecifics on the basis of their smell; the difference in their odors relate to hormone levels in the body

Do fish hormones affect the way a fish fights?

One fascinating aspect of the aggression that can occur between fishes closely mirrors a phenomenon sometimes seen in human sporting contests. When a male swordtail wins a fight with another male, he gets (in simple terms) a testosterone rush.This rush can last for a few days and makes him much more likely to solicit another fight and, remarkably, also makes him more likely to win. His vanquished opponent suffers by comparison and the defeat makes him more likely to lose subsequent battles. These occurrences, sometimes termed “winner and loser effects,' can help lead a fish on to a winning run or a losing streak