A study by NCCOS-funded researchers evaluated the accuracy of determining a blacktip grouper’s sex and maturity stage “by eye” vs. “under a microscope” and found that visual assessments were incorrect over 50% of the time. Commercial fisheries with limited data often use quick visual assessments of reproductive parameters (e.g., size at maturity and sex ratios) to estimate a fish stock's reproductive potential. Determining the accuracy of these visual assessments was the goal of this research as getting this measurement wrong can lead to overfishing or underutilization.

Blackfin grouper, Epinephelus fasciatus, is commonly found throughout the Indo-Pacific Ocean. Photo Credit: Jack Randall, Bishop Museum

This study focused on blacktip grouper, Epinephelus fasciatus, in Indonesia, a data-limited fishery. Blacktip grouper are widely found across the Indian and Pacific Oceans. It is a common shallow-water grouper species and an important food source for local populations. Blacktip groupers are all females at birth, with some later becoming male. Because of its importance in local diets, understanding the accuracy of sex and maturity stage measurements is key to ensuring the future of the fishery.  In general, groupers are slow-growing and mature late, and form large schools during breeding season, making them susceptible to overfishing.

Using fish purchased from fishers on the northeast coast of the island of Buru, Maluku Province, Republic of Indonesia, scientists visually and microscopically assessed 406 blacktip grouper. Just like the old adage, “you can’t judge a book by its cover,” researchers found that, over 50% of the time, you can’t determine the sex and maturity of a blacktip grouper by simply looking at its gonads. Visual assessments incorrectly identified fish as immature females when they were mature males, leading to an underestimate of males in the population and a massive overestimate of female size at maturity. 

The study, led by the Bishop Museum in Honolulu, Hawaii, was funded in part by NCCOS. The overarching project is investigating American Samoa’s mesophotic coral ecosystems (40–100 meters or ~130–330 feet deep) to better understand them and their relationship to shallower reefs.