Scientists Study Fish Otoliths To Determine Reef Fish Migration Patterns

Ear bone of Ehrenberg's snapper serve as tree ring to determine migration patterns.

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An Ehrenberg’s snapper (Lutjanus ehrenbergii)—a commercially important snapper widespread throughout tropical and subtropical waters. (Photo credit: Simon Thorrold, Woods Hole Oceanographic Institution).
John Virata

Juvenile coral reef fishes have been known to spend their youth in seagrass beds and mangrove estuaries and then moving onto coral reefs as adults. That connection between the two habitats is more complex than what was previously understood, according to a study in the Proceedings of the National Academy of Sciences.

According to the study, scientists with King Abdullah University of Science and Technology in Saudi Arabia analyzed food webs in five regions of the Red Sea off the coast of Saudi Arabia. These areas included coastal wetlands, nearshore coastal reefs, reefs less than 60 meters deep on the continental shelf, patch reefs around a continental offshore island, and deep open water oceanic reefs. The data that was collected enabled the scientists to create an isoscape, which is a map of the unique isotope signature of each location.

The scientists then studied the otoliths, the ear bones of adult Ehrenberg’s snapper (Lutjanus ehrenbergii), which is a commercially important fish that is found throughout tropical and subtropical waters. They used a technique called gas chromatography that enabled the scientists to measure compounds in the otoliths that were then traced to each individual isoscape. This enabled the scientists to determine the habitat that the fish lived in as a juvenile.

With this data, the scientists were able to determine that some juvenile snappers settled immediately on the reef, forgoing the notion that the fish lived solely in coastal wetlands and mangrove estuaries as juveniles. The scientists were surprised in their findings, especially after having worked on the reefs for several years and not once seeing a juvenile Lutjanus ehrenbergii on the reef.

This study shows that fish take refuge in a variety of habitats and are not wed to any particular habitat. It also shows that reef fish migrate from coastal wetland habitats across deep ocean water to settle on offshore reefs. The takeaway to the study is that habitats such as seagrass beds, coastal wetlands and mangrove estuaries as well as the migration corridors that connect these habitats to reef ecosystems need protections just as much as the coral reef ecosystems themselves. In the future, the scientists are hoping that they will be able to establish the biological value of the habitats that supply the reefs with fish, and to analyze other coral reefs and seascapes to determine how general the patterns are.

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Fish · Lifestyle