Study Finds Aquarium Fish More Aggressive In Cramped Environments

November 2011

An angry glare from the family goldfish might not be the result of a missed meal, but rather a too-humble abode. Fish in a cramped, barren space turn mean, a study from Case Western Reserve University has found. Ornamental fish across the United States might be at risk, all 182.9 million of them.

“The welfare of aquarium fishes may not seem important, but with that many of them in captivity, they become a big deal,” said Ronald Oldfield, an instructor of biology at Case Western Reserve. Why, then, has the welfare of pet fish been overlooked within the scientific community?

Oldfield is the first to scientifically study how the environment of home aquariums affects the aggressive behavior of ornamental fish. The results are published in the online edition of Journal of Applied Animal Welfare Science, Volume 14, Issue 4.

Oldfield compared the behavior of a popular aquarium fish, Midas cichlids (Amphilophus citrinellus), in a variety of environments: within their native range in a crater lake in Nicaragua, in a large artificial stream in a zoo, and in small tanks of sizes typically used by pet owners.

The study focused on juvenile fish to remove aggressive behavior related to mating. Also, resources like food and shelter were removed prior to observation to eliminate direct competition.

Along with the size of the environment, Oldfield tested the complexity of an environment and the effects of the number of fish within tanks. The addition of obstacles and hiding places using rocks, plants or other similar objects can increase the complexity of the aquarium environment. He found that an increase in tank size and complexity could reduce harmful aggressive behaviors and make for healthier fish.

Oldfield quantified aggressive behavior as a series of displays and attacks separated by at least a second. Displays are body signals such as flaring fins. An attack could be a nip, chase or charge at another fish. In aquariums, these behaviors can lead to injury and in extreme cases to death.

Aggressive behavior was not correlated with small-scale changes in either group size or habitat size alone. However, a significant difference was observed in environments sufficiently large and complex: fish spent less time exhibiting aggressive behavior. A more natural environment elicits more natural behavior, which is more interesting to observers, Oldfield said.

And for the fish themselves, their lives can be vastly improved with these simple changes to their environments.

But why should anyone beyond fish enthusiasts care about fish behavior?

“If we are going to try to create a society as just as possible, we need to do everything we can to minimize negative effects,” Oldfield said.

Minimizing negative effects extends beyond the treatment of ornamental fishes. Interactions between humans and other species could apply. Humans have intimate relationships with a variety of fish: they provide food and sport for many people, some are used in decorative settings, and others are well-loved pets. Additionally, conditions for animals in the rapidly growing field of aquaculture and on factory farms are issues of contention.

Oldfield is not trying to support any extreme agendas. “I’m not trying to ban human use of animals – I just think that if we are going to interact with them, then we should be as informed as possible.”

Relatively simple fish behavior can also serve as a basic model for more complex behaviors. In the future, Oldfield said, “this study might help us to better understand how human behavior changes when people are placed in different social environments.” For instance, violence in prisons might be linked in part to the smaller quarters and reduced stimuli provided to inmates.

For now, at least the 182.9 million ornamental fish in the United States may benefit from this study. Maybe the family goldfish can swim in greater peace, enjoying a newly remodeled space.

Jeremy Rayl is a Case Western Reserve University student.