There are close to 200 million fish living in aquarium tanks across the United States, and untold numbers of them are angry. So angry, in fact, that when they are kept with other fish, many are driven to pester, injure, and even kill their fellow tank-dwellers.

What's driving them to such displays of violence? According to the first scientific study of its kind, the answer is simple: fish need their space โ€” and an engaging one, at that โ€” and many of them just aren't getting it.

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In their natural ecological environments, fish have plenty of reasons to contend with one another; in the wild, for example, the need to compete for mates, for food, or for shelter can often lead to aggressive interactions. But biologist Ronald Oldfield wanted to look at what causes aggressive behavior in fish when they have no need to compete for any of the aforementioned resources.

Instead, Oldfield wanted to examine patterns of aggression in groups of fish competing for two rather unnatural ecological factors, namely: the amount of available space (corresponding to the size of the tank in which they are kept) and the amount of habitat complexity (corresponding to the presence of obstacles and hiding places like rocks and plants) within the tank.

To accomplish this, Oldfield observed the behavior of Midas Cichlids โ€” one of the most popular tank fish in North America โ€” across a variety of tank conditions. What he found was somewhat disturbing.

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Fish that were grouped in what Oldfield describes as "aquaria of sizes typically used by pet hobbyists" were found to be irascible, and prone to hostility towards their fellow fish. Manifestations of aggression usually started at a low simmer โ€” beginning with visual displays of vexation like the flaring of fins โ€” before boiling over into more violent and even deadly interactions between fish.

But when fish were grouped together in larger aquariums filled with complex habitats, aggression between them was observed to be significantly lower.

"The public should be aware," writes Oldfield in a recently published paper that documents his findings, "that this and similar species require larger aquaria with complex habitats, which elicit more natural behavior."

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And according to Oldfield, these "similar species" could even include humans.

"This study might help us to better understand how human behavior changes when people are placed in different social environments," he said.

Oldfield's findings are published in the latest issue of the Journal of Applied Animal Welfare Science
Via Discovery News

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Top image via Davor Pukljak/Shutterstock