Tiny Mandarin Fish Reveal Surprisingly Complex Spawning Ritual

Hillary Mayell
for National Geographic News
April 27, 2001
Each year, more than 40 million fish are removed from the wild to fill
the tanks of marine aquarium hobbyists. With the increasing popularity
of reef aquariums, that number is likely to grow.

What this popularity means to ocean fish populations worries scientists and conservationists. There is a real danger that excessive removals of wild fish for the aquarium trade could have a devastating impact on local populations, said Yvonne Sadovy, a biologist at the University of Hong Kong.

Very little is known about the life cycles and biology of many reef species. This makes it difficult to determine whether the fish need to be protected, and if so, how.

To help provide answers, Sadovy, along with Mariella Rasotto from the University of Padova and George Mitcheson from the University of Hong Kong, have studied populations of exotically colored mandarin fish in Palau, Micronesia. The project was funded with assistance from the National Geographic Society's Committee for Research and Exploration.

Insight Into Other Species

Mandarin fish are highly popular among aquarium owners, divers, and marine photographers because of their striking beauty. Their body designs of swirls, squiggles, dots, and stripes in improbable neon colors of orange, blue, yellow, and green make them look like something drawn by a five-year-old.

For the researchers, the fishes' exotic markings make it easier to detect individual specimens and observe their behavior.

Mandarin fish are one of the tiniest species among ocean spawners, which reproduce by depositing thousands of eggs into the water. Wide-ranging species such as tuna, whitefish, and sardines all belong to the same group, but are much harder to track and study than the tiny mandarin fish, which are only one to two inches (20 to 55 millimeters) long and live close to shore in the shallow waters of coral reefs.

What the scientists have learned so far about the mandarin fish has surprised them. "We never anticipated the complexity of the social structure and mating system that we encountered," said Mitcheson. "We also didn't expect that individuals could be so distinctly different, one from the other, in terms of their habits and behaviors."

As the researchers monitored mandarin fish populations, they found that each evening, within 20 minutes of sunset, groups of three to five females gathered. Each group had its own "street corner" to which its members returned night after night. Males visited the sites and displayed courtship behavior, hoping to attract females.

To spawn, a female joins the male, resting on his pelvic fin, and the pair rises slowly about three feet (one meter) above the coral reef. At the peak of their rise, a cloud of eggs and sperm is released.

Larger male mandarin fish are far more successful at mating than smaller ones. Yet the researchers discovered that the smaller males don't appear to have a strategy for enhancing their reproductive opportunities, which may affect the viability of populations in the wild.

"Usually when you have great differences in mating success among males, smaller males develop alternative mating tactics, like 'sneaking' or 'streaking,' in which smaller males rush up to a spawning pair and release sperm in the hope of some fertilizations," Rasotto explained.

The researchers found that when the larger males are removed, female mandarin fish spawn with smaller males—but reluctantly.

Courtship takes longer. The females are more inclined to end the mating dance before spawning has occurred, requiring a second or third attempt. In addition, the mating pairs are more vulnerable to predators as they rise to complete the act.

Declining Populations

Aquarium owners and fishermen are usually interested in catching larger specimens of mandarin fish and other ocean spawners. Besides directly depleting the fish populations, this makes fewer males available for spawning, which leads to lower reproduction rates.

The researchers say their findings show the importance of understanding the reproductive biology of a species to better predict how it will be affected by harvesting pressures. "It was only by looking in detail at both the reproductive biology as well as the ways in which the fishers actually target the fish that we realized what was happening and the implications for the targeted population," Sadovy said.

Thousands of fish are removed every month from very limited areas, she noted. Many aquarium collectors, Sadovy said, incorrectly believe that the fish they purchase won't live long and can be replaced easily because there are plenty of fish in the sea.

"Many of the lovely reef fishes in the marine aquarium trade are treated like cut flowers, dying soon after they are purchased, then simply being replaced," she said.

But many reef fish can live for years or even decades, said Sadovy. It is the methods by which the fish are caught, transported and ultimately cared for by consumers that is often to blame for high mortality rates among aquarium fish, she explained.

Some environmentally aware businesses have sprung up to supply the aquarium trade while minimizing fish mortality.

"Whether or not the marine aquarium trade is a benefit, by creating interest and concern in reefs and reef fishes, or a menace to reef fish populations of desired fish species, will depend on using our understanding of these populations and managing the fisheries," Sadovy said.

"There is much promise, but this has yet to be realized," she added. "Traders and consumers have an important role to play by demanding fish (be) caught only using sustainable practices."

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