Finding Nemo…and Other Endangered Fish

Scientists have long been able to tag animals on land and follow their movements and habits. But tagging and tracking fish through vast oceans is a Herculean task. Tagging fish larvae smaller than a millimeter has been impossible.

Now an international research team has demonstrated a new technique—using the common antibiotic tetracycline and DNA fingerprinting—to track fish and determine how fish populations migrate and connect to one another. Such information is essential for identifying critical marine habitats that should be set aside to protect the estimated 70 to 80 percent of fish populations whose stocks have been overfished or whose habitats have been disrupted by humans.

WHOI biologist Simon Thorrold and colleagues Geoffrey Jones of James Cook University in Australia and Serge Planes of the Université de Perpignan in France are working in Kimbe Bay, Papua New Guinea, where anemone-filled coral reefs provide homes for clownfish—like the hero of the movie “Finding Nemo.” Like Nemo, many clownfish are harvested to stock aquariums, and their numbers are being depleted on reefs throughout the Pacific and Indian Oceans.

Tetracycline is known to darken human babies’ teeth when taken by mothers at certain stages of pregnancy, and similarly, it darkens the otoliths, or ear bones, of fish. By exposing developing embryos to tetracycline in specially designed incubation chambers, the team was able to indelibly mark the otoliths of tiny clownfish before they hatched and dispersed.

Reporting in the July 26 issue of Current Biology, the team used these telltale tetracycline-stained otoliths to track clownfish larvae to new anemones within 100 meters (330 feet) of their birthplaces.

The researchers also used DNA-fingerprinting techniques to determine if new settlers to a community were spawned by adults within it. They found that about two-thirds of the community’s clownfish were not born nearby—but perhaps immigrated from other clownfish habitats more than 10 kilometers (6 miles) away.

The new tracking methods may be adapted to help preserve other species under pressure, Thorrold said.

This research was funded by the National Science Foundation, the WHOI Ocean Life Institute, and the Australian Research Council.