Goliath Grouper (Epinephelus itajara)

Biologist Eric Fernandez and I encountered this fish during a day of sampling in Placencia Lagoon. The man who had caught it was a Garifuna fisher from that village of Seine Bight. We found him happily paddling back home from drop fishing and checking set lines tied to the mangrove the night before. The hook from the set line was still in the grouper's mouth.

We paid the fisher to take a tissue sample from this fish. From the stable carbon isotopes in that sample, we found this fish was feeding from a foodweb based primarily on mangrove. In some senses, this fish was unique. Most of the fish we tested in the lagoon had used a much higher proportion of carbon from sea grass and single celled algae. Almost all the tissue in this fish came from animals that had been feeding on mangroves. I suspect crabs were its' primary prey.

The link between mangroves and goliath grouper is well established. Recently scientists have discovered that as juveniles, they live among the prop roots of red mangrove. Later, they migrate out to the Mesoamerican Reef where they can attain sizes well over 100kg. Fishers in Seine Bight have targeted Placencia Lagoon goliath groupers (formerly known as "jewfish") for generations. That fishery is still active and Seine Bight fishers still actively harvest goliath grouper in gill nets and with handlines.

Considered alone, the long standing goliath grouper fishery is probably not a threat to the sustainability of this species. Certainly some economic benefits arise from their harvest. The fish in the picture above represents a modest economic boon for the man who caught it (US 2$ per kg at approximately 10kg). However, coupled with the large scale changes that are occurring in coastal Belize, there are concerns about the conservation status of this species. After all, the fish in the picture above is still very much a baby. How many of those "smaller" fish are maturing to become adults and reproduce? When coupled with the wide spread removal of mangrove by developers, and the reduced water quality from effluent sources is there reason to be concerned about goliath groupers in Placencia Lagoon?

An additional concern is raised by a recent study by the Biodiversity Institute found that tissues in just over half goliath grouper they sampled in Southern Belize contained methyl-mercury levels above the USEPA threshold for health advisories. Children and pregnant women feeding on goliath grouper more frequently than twice a week are at risk are at risk for neurological damage.

http://www.briloon.org/pub/doc/posters/07Grouper.pdf

Some data from this study seems to indicate that the higher a goliath grouper feeds in the food chain, the more mercury it is likely to contain. Mercury increased with delta-15N levels in the tissues of this species. This pattern fits well with the observation that mercury bioaccumulates, or concentrates in the tissues of animals that feed on other animals. Delta-15N also tends to rise as fish feed higher in the food chain.

Because larger fish tend to consume larger prey that are often higher in the food chain than smaller prey, it is possible that consuming larger goliath groupers poses a greater health risk than smaller ones. These researchers will examine this and other questions over time.