Little Tank, Big Attitude

Photo Credit: Douglas Quenqua/Getty Images
(ANIMAL BEHAVIOR) A study on common freshwater fish observed that fish kept in small tanks were significantly more aggressive than those in larger tanks. These fish are much more likely to flare their gills and quarrel with other fish as well as guard territory. Read on to learn more about the university’s findings. — Global Animal
Small tanks cause aggression amongst common freshwater fish. Photo Credit: Getty Images

New York Times, Douglas Quenqua

Tropical fish hobbyists will tell you their tanks are a source of relaxation, but recent research suggests the fish might disagree.

Nearly 13 million American households contain a fish tank, and the average tank size is less than 10 gallons. Yet a study comparing the behavior of common freshwater fish in a variety of habitats found that those kept in such small tanks were considerably more aggressive than those in larger ones — more likely to fight, flare their gills and guard whatever tiny alcoves they could find.

“In larger tanks, the fish were not in continuous eyesight of each other, and were swimming around checking everything out rather than beating the heck out of each other,” said the study’s author, Ronald G. Oldfield, a professor of biology at Case Western Reserve University.

The fish in question were Midas, or “red devil” cichlids, a species popular among hobbyists for their brilliant colors and active swimming habits. Dr. Oldfield used only very young fish to eliminate aggressive behaviors associated with mating.

Dr. Oldfield concedes that the emotional well-being of fish may not tug many heartstrings. “It’s probably not the end of the world,” he said in a telephone interview. Even the Humane Society, which routinely has commercials featuring slow-motion video of abused pets, does not offer guidelines for the treatment of pet fish.

“We work on almost every animal issue under the sun,” a spokesman said by e-mail, “but I don’t think this is one of them.”Still, Dr. Oldfield noted that the average household tank was only one-tenth the size of the smallest tank in the study to yield docile fish. “If people kept dogs in these conditions, they’d be put in prison,” he said. “It’s something we should think about.”

The study consisted of two experiments conducted side by side. In one, Dr. Oldfield tested the effects of overcrowding by keeping tank size constant while increasing the number of fish. In the other, he tested environment by placing three fish in consistently larger and more complex tanks. He then recorded their behavior at least two hours after feeding, to eliminate competitive behaviors related to food.

While aggression seemed to remain constant regardless of the number of fish in a tank, Dr. Oldfield observed that it dropped off considerably once the fish were placed in a 100-gallon tank with several plants and rocks to form alcoves.

The findings confirmed what he found when observing Midas cichlids in the wild. “If you go out and observe these fish swimming in a river,” he said, “they’re not aggressive at all, really.”

This is not the first study to suggest that water-dwelling creatures can become aggressive in small tanks. Biologists at the University of Alabama, Birmingham, recently found that sea urchins turned to cannibalism when kept in small, overcrowded tanks. The researchers were trying to recreate typical farming conditions for sea urchin, a delicacy in Japanese cuisine.

Nor is the idea news to Justin Muir, the owner of City Aquarium, a luxury fish tank business in Brooklyn.

“It just goes back to behavioral exercises,” said Mr. Muir, who has designed tanks for the Yankees pitcher C. C. Sabathia and the Dream Hotel in Manhattan. “More volume of water is always the better bet. It basically keeps the fish healthier, and the tank is more stable.” In that way, he said, fish are like any other pet kept in a small enclosure. But a major difference between fish and, say, Rottweilers is that aggressive behavior in small swimmy things can be entertaining, at least to humans.

“That’s why these fish sell,” Mr. Muir said, “because people like the way they act.”

Indeed, hobbyists who probably don’t view themselves as diabolical gleefully exchange online tales of clashing cichlids. Describing a case of “road rage” between two of her fish, a visitor to wrote:

“The two would stand in front of the other twitching their lower fins as in sign language, yelling at the other with ‘You almost hit me you blind fool. Didn’t you see me coming? I had the right of way!’ ”

Overcrowding has also become an issue on fish farms, where salmon or trout are sometimes packed into high-density pens, just as chickens or pigs are on industrial farms. The danger there is less about the happiness of the fish than about their health, said Alan Duckworth, a research scientist with the Blue Ocean Institute in Cold Spring Harbor on Long Island.

“Some species do better at high densities,” he said, “but the majority of species could be affected by overcrowding. It likely would stress them out, which could increase the amount of disease.”

One frequently cited problem with farmed fish, particularly salmon, is sea lice, whose spread has become such a problem in Scotland that the government is considering a ban on coastal fish farms. For Dr. Oldfield, the welfare of fish is a concern that dates to when he was 6 years old and won a goldfish in a small bowl at a county fair. He says he understands that a 100-gallon tank is beyond the means of the typical tropical fish hobbyist, but people who love their fish should be aware of the damage they may be doing by keeping them in small, bland environments.

“I’m not saying people need to stop keeping fish as pets,” he said, “but they do need to look at the ecology of animal aggression.”

As for goldfish won at carnivals, he said, “they should never, ever be kept in those bowls.”

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