Nassau Grouper

e_striatus Photo: SEAPICS

Epinephelus striatus

Summary below

Also see: Hill, Ronald L., and  Sadovy de Mitcheson, Yvonne.  2013. Nassau Grouper Epinephelus striatus (Bloch 1792), Biological Report. Report to National Marine Fisheries Service, Southeast Regional Office. June 12, 2013. 117 pp.

Description and Distribution

The Nassau Grouper is buff-coloured, with five dark brown vertical bars and a large black saddle blotch on top of the caudal peduncle, and a row of black dots below and behind the eye. It has a distinctive dark tuning-fork mark beginning at the front of the upper jaw, extending dorsally along the interobital and bifurcating on top of the head behind the eyes. It has another dark band from the tip of the snout through the eye and then curves upward to meet its fellow just before the dorsal-fin origin. Some fish have irregular pale spots and blotches all over the head and body. The colour pattern can change in a few minutes from almost white to uniformly dark brown depending on the ‘mood' of the fish (1).

nassau-map

The species is found in the western North Atlantic: Bermuda, Florida, Bahamas, Yucatan Peninsula and throughout the Caribbean (see above map - distribution indicated by dotted line). It is not known from the Gulf of Mexico, except at the Campeche Bank off the coast of Yucatan, at Tortugas and off Key West (1).

Preferred Habitat

This grouper is common on offshore rocky bottoms and coral reefs throughout the Caribbean region. It occurs at a depth range extending to at least 90 m, preferring to rest near or close to the bottom. Juveniles are found closer to shore in seagrass beds and macroalgal areas that offer a suitable nursery habitat.

Nassau groupers are typically solitary and diurnal. However, they may occasionally form schools. When threatened by predators, this fish can camouflage itself, blending in with the surrounding rocks and corals (2).

Biology

Juveniles feed mostly on crustaceans and prefer seagrass and coral clumps with macroalgae (Laurencia spp.). Adults feed mainly fishes and crabs, and to a lesser extend on other crustaceans and molluscs.

Growing to a maximum of 4 feet ( 1.2 m) and weighing over 50 pounds ( 22.7 kg), this grouper is one of the largest fish on the reef. More commonly, this grouper reaches a length of 1-2 feet (0.3 -0.6 m) and weighs 10-20 pounds (4.5 -9 kg). The species lives at least 16 years and possibly as much as 29 years (3)(10).

Sexual maturation takes place at about 5 years and at about 400 -450 mm SL in both males and females (about 500 mm TL). The species is not known to change sex (unlike many other grouper which exhibit female to male sex change) based on histological, age and size evidence, but this might be related to fishing pressure. Among fish sampled in one study, there was not evidence for functional hermaphroditism with males and females overlapping in size and age and gonochorism indicated. However, it is possible that fishing pressure has eliminated most of the larger males and that the species is naturally diandric. The largest males in this case would be secondary (derive from sex changed females) and the smaller males derived directly from juveniles. It may be just these smaller males that remain and join the spawning aggregations (4).

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Spawning aggregations of a few dozen to perhaps as many as 100,000 individuals have been reported from the Bahamas, Jamaica, Cayman Islands, Belize, the Virgin Islands and elsewhere in the Caribbean within the grey-marked area of the above map. Aggregations are not known to form elsewhere within the geographic range of the species. Many of the islands or banks in the Caribbean have, or had in the past, spawning aggregations, unfortunately many appear to have been fished to commercial "extinction" (3). See aggregation photo above.

These aggregations occur in depths of 20 to 40 m at specific locations of the outer reef shelf edge in December, January and/or February at or near the time of the full moon; the same sites are used year after year. During spawning, most fish (males and females) display the bicoloured (non-aggressive) pattern and hover above the bottom. See photo of bicolor female distended with eggs, above. Some females remain in the barred pattern, becoming very dark as mating approaches and are closely followed by bicoloured fish during courtship (3).

Spawning occurs at sunset, in groups of 3 to 25 fish. Release of gametes is preceded by various movements of the courting groups: vertical spirals, short vertical runs followed by rapidly crowding together then rapidly dispersing and horizontal runs near the bottom. Mating is initiated by a dark phase fish (presumed female) dashing forward and upward, the female is closely followed by groups of bicoloured males releasing a white cloud of sperm as the female(s) release her eggs (5).

Mariculture

The Nassau grouper has been spawned and raised in captivity and trial releases have been conducted. Survival in the wild following release of tagged individuals was at least 9 months (6).

Commercial Use

This fish is considered an important food fish throughout the Caribbean and in the West Indies. Historically it ranked top among reef fishes landed on islands of the region although landings have been much reduced throughout its range. The Bahamas appear to be the only country where significant landings are still taken but, even here, declines have been noted over the last decade.

Threats

The Nassau grouper has been heavily fished and is vulnerable to over-fishing because of its biology and naturally slow population replenishment rates. The spawning aggregations that appear at the same site each year are easy targets for fishers who can catch large numbers of fish very efficiently. During these spawning events, many reproductively mature fish are caught and in some countries, such as Cuba and the Bahamas, a large proportion of the annual landings of the species is taken at the time of aggregation. Many of these aggregations have declined substantially in numbers very probably due to heavy fishing pressure, and few are adequately managed.

One of the best-known examples of the demise of fish spawning aggregations due to over-fishing is that of the Nassau grouper, classified as endangered primarily due to aggregation fishing. This was the first reef fish, and one of the only fully marine commercial species, to be listed as a species of concern under the U.S.A. Endangered Species Act. The species may travel more than 100 km from its resident reef to an aggregation site, where all reproduction occurs over, just a few days in a couple of months each year. Many of its aggregations no longer form, or do so with much reduced numbers and show little evidence of recovery despite various management measures; in this case, poor or no enforcement appears largely to blame (3).

Survey data from ReefCheck indicates that the Nassau grouper has virtually disappeared; among 162 reefs surveyed by underwater visual census, 142 reefs had none and 12 more had just one animal. 7 On the positive side, marine protected areas appear to be very effective in protecting this largely sedentary species for most of the year. The problem arises when fish leave protected areas to travel to spawning aggregations.

The following figure shows known spawning aggregations of Nassau grouper:

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All known aggregations reported since 1884. Each closed circle represents one, occasionally two, reported sites. In the few cases where aggregation numbers were estimated, these ranged from approximately 10,000 to somewhere between 30,000 and 100,000 fish;

Closed circles represent sites believed to exist today with fish numbers estimated at between 100 and 3,000 fish - estimates from fishing and direct observations. Open circles represent sites in Cuba still believed to produce small catches of Nassau grouper but sites have not been assessed directly (8)

Conservation and Management

Currently all harvest of the Nassau grouper is prohibited in the U.S. It is also listed as a candidate for the U.S. Endangered Species List (8). There are many regulations for this species in many countries, ranging from size limits, aggregation protection and gear controls, to seasonal controls and sales bans. However, enforcement continues to be a major challenge in almost all cases.

Conservation Status: The species is listed as ‘Endangered' on the IUCN Red List of Threatened species (9).

Information Needed

Information is needed on population structure, aggregation ‘catchment' areas, larval dispersal distances and the status of the species and its spawning aggregations in Bahamas, the only country where landings continue to be significant but for which there is very little information available for management.

Note: All lengths listed are either total length (TL) or fork length (FL) unless otherwise stated.

References

(1) P.C. Heemstra and J.E. Randall, FAO species catalogue Vol. 16. Groupers of the world (Family Serranidae, Subfamily Epinephelinae). An annotated and illustrated catalogue of the grouper, rockcod, hind, coral grouper and lyretail species known to date (Food and Agriculture Organization of the United Nations, Rome, 1993), 282 pp + XXXI plates.

(2) Florida Museum of Natural History Ichthyology department web site, www.flmnh.ufl.edu/FISH/

(3) Y. Sadovy, and A. M. Eklund. 1999. Synopsis of biological information on Epinephelus striatus (Bloch, 1972), the Nassau grouper, and E. itajara (Lichtenstein, 1822) the jewfish. National Oceanic and Atmospheric Administration Technical Report National Marine Fisheries Service 146. Food and Agriculture Organization (FAO) fisheries synopsis 157. FAO, Rome.

(4) Y. Sadovy, and P. L. Colin. 1995. Sexual development and sexuality in the Nassau grouper, Epinephelus striatus (Bloch) (Pisces: Serranidae). Journal of Fish Biology 46:961-976.

(5) P.L. Colin. 1992. Reproduction of the Nassau grouper, Epinephelus striatus (Pisces: Serranidae) and its relationship to environmental conditions. Environmental Biology of Fishes 34: 357-377.

(6) S.K. Bolden. 2000. Long-distance movement of a Nassau grouper (Epinephelus striatus) to a spawning aggregation in the central Bahamas. Fishery Bulletin ( U.S.) 98:642-645.

(7) E. Pennisi. 2002. Survey confirms coral reefs are in peril. Science 297:1622-1623.

(8) Y. Sadovy de Mitcheson, A. Cornish, M. Domeier, P. L. Colin, M. Russell, M. and K. C. Lindeman. (In press) Reef Fish Spawning Aggregations: A Global Baseline. Conservation Biology.

(9) http://www.iucnredlist.org/search/details.php/7862/all

(10) Fishbase. 2010. http://www.fishbase.org/summary/speciessummary.php?id=18