Watching a fish or two swim around a tank can be relaxing for you — but surprisingly stressful for the fish.
A new study found that common aquarium fish fight more and act less like themselves when they're lonely. Just as people choose to squeeze into a crowded nightclub rather than roam around an empty bar, it seems, certain fish prefer to have lots of companions.
It was the first study to look at the well-being of fish in home aquariums, and the results suggest that we may owe more to our fish than just keeping them from going belly-up.
"I think we need to make sure they are not only alive," said Katherine Sloman, a fish biologist at the University of Plymouth in the United Kingdom. "I think we need to get them to display behaviors they might show in the wild."
Advocates for the welfare of animals talk about five freedoms: freedom from hunger and thirst; freedom from discomfort; freedom from pain, injury and disease; freedom to express normal behaviors; and freedom from fear and distress.
How many of those things fish can actually experience is still up for debate. Some still controversial evidence suggests that fish might feel pain.
Other studies have quantified ideal stocking densities that produce the largest number of big, healthy animals in fish farms. Whether fish can be happy or not is something we may never know, but people who spend a lot of time with fish can tell normal behavior from the abnormal.
To begin to quantify the conditions that are best
in home aquariums, Sloman considered two species of fish that are commonly kept as pets: neon tetras and white cloud mountain minnows.
She and colleagues put each fish in a 10-liter tank either alone, with one other fish, in a group of five, or in a group of 10. There were six replicas of each set-up. For two weeks, the researchers observed and videotaped the tanks, looking for key behaviors, including chasing other fish and darting around the tank.
In groups of 10, the team reported last month at the Society for Experimental Biology Meeting in Glasgow, fish were least aggressive, least likely to dart, and most likely to form schools and behave as they do in the wild. In home aquariums, the researchers concluded, people should keep neon tetras and white cloud mountain minnows in groups of at least 10.
"We're beginning to understand that fish are social animals," said Tony Farrell, a fish physiologist at the University of British Columbia in Vancouver.
To people, crowding might seem stressful and likely to spark fights, but in the real world, fish often stick more closely to each other than pet-owners might imagine. If your fish are fighting, Farrell said, adding more fish to the tank might work better than separating them.
"A lot of people would say it's nice to have as much space as you want or maybe have a partner," he said. "This type of work is telling you that somewhere around 10 fish is better than between one and 10."
Next, Sloman plans to study other species of fish and mixtures of species to learn more about the preferences of a wide array of common aquarium-dwellers.
"This work," Farrell said, "is the tip of the iceberg in asking questions about how many fish you should put in an aquarium to create the best environment for them."