Breeding Tropical Fish to Save Their Schools

For every tropical fish that becomes a pet, four are killed. Joan Holt wants to take the violence out of their world.
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For every tropical fish that becomes a pet, four are killed. Joan Holt wants to take the violence out of their world.

Inside her humid Texas lab, Joan Holt weaves through 36 tall, cylindrical PVC tanks containing the nearly invisible larvae of one of the tropical fish species she studies. In each tank, the oxygen and salinity levels are slightly different. These tanks, with their unique filtering systems, are nothing like the home tanks familiar to ornamental-fish enthusiasts, but the research and innovations they represent just may change the $1 billion aquarium industry.

Holt, a 70-year-old aquaculture specialist, is renowned for her groundbreaking research at the University of Texas Marine Science Institute, in Port Aransas, where she studied the rearing of commercial marine species such as redfish. As a child, the Fort Worth native spent time on her grandparents’ farm, which nurtured her interest in biology. She saw the ocean for the first time shortly after arriving in Houston for college. “It made quite an impression,” she says. She earned a doctorate in fisheries science at Texas A&M University; then she joined the institute.

In those days, research could be challenging, says the slight, soft-spoken scientist. “I was often the only woman, and some men wouldn’t go out on boats with me.” Women were sometimes considered bad luck on boats, but Holt didn’t let that stop her from tackling the notoriously difficult problems of culturing marine fish — or later from extending her research to ornamental species, an interest that grew out of a dive trip to Central America 20 years ago. “In Belize, I saw all these juvenile fish hanging out around little hummocks of coral in the sea grass beds,” she says. “It got me interested in learning more about them in the lab.”

The March-April 2012

This article appears in our March-April 2012 issue under the title "The Age of Aquariums." To see a schedule of when more articles from this issue will appear on, please visit the
March-April magazine page.


But when she set out to obtain tropical specimens to study, Holt learned that nearly all the fish in home aquariums are taken from the wild. The practice removes large numbers of fish — an estimated 1.5 million to 3.75 million a year just from around Hawaii — which damages reef habitats. And to make the fish easier to catch, collectors often use cyanide as an anesthetic. Around 80 percent of the fish die either before or soon after reaching home tanks. Holt decided that if popular tropical fish could be produced commercially in captivity, they would be healthier and live longer, and their native reefs would be healthier, too.

First she needed to figure out how to breed them. She began by trying to find a method of collecting wild fish that wouldn’t kill them or harm the reef. Near the oil platforms off the Texas coast, she tested several methods before hitting on one that worked: “One diver sort of herds the fish, and another collects them in a large clear plastic bag, which the fish can’t see.” She collected more species in the waters off Belize and Mexico, carting the fish to her lab in small coolers.

Eventually Holt was able to get 18 ornamental species to spawn or reproduce. But feeding the larvae proved an even greater challenge. “All marine fish larvae need live prey,” she explains. “We had figured out how to grow live prey for redfish, trout, and flounder,” but many of the tropical fish hatched as tiny, translucent larvae — with no eyes, mouth, or stomach. She had to figure out what they were feeding on in the wild.

Using traps she and her team developed — clear Plexiglas cylinders with a light in the middle and fluted openings that allowed fish to swim into the cylinder but not out — the researchers collected baby fish from the ocean and soon discovered that, after they develop a bit more, the diet consisted of microscopic plankton, including copepods, diatoms, and dinoflagellates.

As any aquarium owner knows, just dumping food in a tank can make a mess of the water. So Holt and her team tinkered with containers and filters until they had a design that let them feed the larvae but keep the water clear.

Obtaining money for research on the ornamentals was a constant struggle. “Maybe raising tropical fish didn’t sound serious enough,” Holt says. She got some funds from the World Wildlife Fund’s coral reef conservation program and found that graduate students were often willing to help out just for fun. SeaWorld recently launched an initiative called Rising Tide to help fund research on the captive breeding of tropical fish, and interest from commercial aquariums is increasing.

Thanks to Holt’s work, seven of the species she’s worked with, along with eight more from other labs, could now be raised and sold commercially. Given that more than 700 tropical species are sold for home aquariums, there’s still a long way to go. But Holt is optimistic: “With some money for research, perhaps from Rising Tide, researchers can make more progress. We could produce enough variety, and cheaply enough, to satisfy the needs of the market.”

After 36 years at the institute, Holt is now semi-retired but is still the associate chair of marine sciences, and she’s editing a book on marine aquaculture. It’s a kind of how-to for raising tropical fish. “All the information is there,” she says. “You could do it on a small scale, in your garage.”

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