Key Palau reef fish found vulnerable during spawning

By Ron Rocky Coloma

New research on one of Palau's important reef fishery species could help guide future decisions on catch limits, size restrictions and spawning protections.

Researchers found that the longnose emperor, a commercially important reef fish known scientifically as Lethrinus olivaceus, grows faster, lives shorter lives and reaches smaller sizes in Palau than populations studied in Japan, Australia and French Polynesia.

The findings are based on an analysis of 528 fish collected from markets, fishers and targeted sampling efforts between 2017 and 2025. Scientists say the data provide one of the most comprehensive looks yet at the species' growth, lifespan and reproductive biology in Palau.

"Accurate life-history information is essential for conducting data-limited stock assessments of coral reef fishes, yet this information is often lacking in small-scale fisheries," the researchers wrote.

The study found that longnose emperors in Palau reached a maximum observed age of 14 years. Females reached maturity at an estimated fork length of 43.2 centimeters, or about 17 inches, and at approximately 3.5 years of age.

Researchers also found evidence that the species changes sex during its lifetime, transitioning from female to male. Males were generally larger and older than females. The average length of males was 53.5 centimeters, compared with 45.8 centimeters for females. Males averaged 7.4 years old, while females averaged 4.3 years old.

No immature males were observed in the samples.

The study identified four fish that contained both female and male reproductive tissues, providing evidence that the species is a functional protogynous hermaphrodite, meaning it matures first as a female before transitioning to a male later in life.

Scientists found that reproductively active females were present throughout the year, with evidence of active spawning during March through June, August, September, November and December.

The species is known to form spawning aggregations around the new moon, when large numbers of fish gather to reproduce. Researchers noted that such behavior can make fish more vulnerable to fishing pressure because fishers may know when and where aggregations occur.

"Evidence of year-round spawning and functional protogynous hermaphroditism highlights potential vulnerability to fishing pressure, particularly where individuals may be selectively targeted during spawning aggregations," the authors wrote.

The study comes as Palau continues to evaluate management options for coastal fisheries.

Researchers noted that no formal management measures currently exist for longnose emperor in Palau. However, a proposed fisheries management plan for Koror includes an 18-inch minimum size limit and protection of a known spawning aggregation site.

Based on the updated maturity estimates, the researchers said a slightly larger minimum size limit of at least 18.7 inches could better protect the species' spawning potential.

The study also suggested that managers consider a maximum size limit to protect larger males. Researchers estimated that half of the population transitions from female to male at about 55.1 centimeters, or 21.7 inches.

Additional management options discussed in the study include seasonal or lunar-based fishing closures and protection of spawning aggregation sites.

Researchers found that 27.7 percent of sampled fish were below the estimated size at which females reach maturity, a finding they said could be important for future stock assessments and fishery evaluations.

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The study also underscored the importance of collecting local biological data rather than relying on information from other regions.

Compared with populations studied elsewhere, longnose emperors in Palau generally grew faster, reached smaller maximum sizes and had shorter life spans.

"The high diversity of life-history traits among locations highlights the importance of collecting locally-derived information," the authors wrote.

Researchers said the findings provide a scientific foundation for future stock assessments and could help fisheries managers develop regulations tailored to local conditions.

The study was conducted by researchers from the Palau International Coral Reef Center, University of Tasmania, University of Guam, The Nature Conservancy and the Coral Reef Research Foundation.

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