Black grouper aggregations in relation to protected ... - IngentaConnect

BULLETIN OF MARINE SCIENCE, 66(3): 721–728, 2000

BLACK GROUPER AGGREGATIONS IN RELATION TO PROTECTED AREAS WITHIN THE FLORIDA KEYS NATIONAL MARINE SANCTUARY Anne-Marie Eklund, David B. McClellan and Douglas E. Harper ABSTRACT During the winter of 1997–1998, we discovered an aggregation of black grouper, Mycteroperca bonaci, less than 100 m outside a newly designated marine reserve. The spawning mode and aggregation behavior of black grouper have not been reported previously, even though this species is an important component of both the recreational and commercial fishing sectors of the Florida Keys. The grouper aggregation included up to 96 individuals just seaward of a no-take area within the Florida Keys National Marine Sanctuary. We documented the seasonal and spatial distribution of the aggregation during a series of drift dives over the presumed spawning area. In January and February of 1998, the black grouper were aggregated in a spatially discrete 100-m2 area in 18–28 m of water. By April the number had declined, and their distribution had become more even, rather than aggregated. Although the marine reserve offers total protection from all fishing activity, these groupers were susceptible to being caught in large numbers, as the entire aggregation remained in waters deeper than the reserve boundary. If the reserve boundary were moved seaward beyond the slope-sand interface along the 30-m contour, the vulnerable aggregation would be protected.

Several species of groupers from the family Serranidae form large seasonal spawning aggregations (Domeier and Colin, 1997). This behavior increases their susceptibility to overexploitation, because fishing can be concentrated at predictable locations and seasons (Johannes et al., 1994; Coleman et al., 1996; Sadovy and Eklund, 1999). Exploitation of aggregated spawners can have severe consequences for grouper species, because of their longevity, slow growth, and great age and size at maturity. Examples of overexploited grouper stocks in U.S. waters include gag (Mycteroperca microlepis; Coleman et al., 1996), red grouper (Epinephelus morio; Schirripa et al., 1999), jewfish (E. itajara), and Nassau grouper (E. striatus; Sadovy and Eklund, 1999). The case of the Nassau grouper highlights the necessity for adequate protection of vulnerable aggregations, because entire spawning aggregations have disappeared in several areas throughout the Caribbean (Sadovy and Eklund, 1999). Colin (1996) believed that the loss of a threshold number of Nassau grouper led to complete lack of spawning. The failure of aggregations to reform, even after 6–9 yrs of protection, in the U.S. Virgin Islands, for example, is cause for further concern (Sadovy and Eklund, 1999). One method for conserving spawning stocks is to provide spatial reserves that protect aggregations (Plan Development Team, 1990; Roberts and Polunin, 1993). Indeed, one of the major themes in marine reserve planning has been to protect sources of fishery production, so that recruits can be exported to sinks, where the fishery may be sustainable (Pulliam, 1988; Roberts, 1997; Bohnsack, 1998). One way that a marine reserve can serve as a source for fishery biomass is to include spawning aggregations within the reserve boundary. Black grouper, Mycteroperca bonaci, a protogynous serranid, is very important to the commercial and recreational fisheries of southern Florida (Manooch and Mason, 1987; 721

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Bennett, 1996). Commercial landings of black grouper are higher than those of any other grouper species in the Florida Keys and are highest in January and February (Bennett, 1996, 1998) during the time of peak gonadosomatic indices (Crabtree and Bullock, 1998). Very little is known, however, about black grouper biology or the effects of fishing on their populations. Virtually nothing is known of their spawning behavior (Crabtree and Bullock, 1998), other than a few brief reports of spawning aggregations in Belize during January and February (Carter, 1989) and in Honduras in January (Fine, 1990, 1992). No studies have been conducted specifically to describe the aggregations (Domeier and Colin, 1997). In January of 1994, we observed a small aggregation (n = 15) of black grouper along the reef ledge–sand interface off Key Largo in 28 m of water. These grouper, along with one Nassau grouper and one tiger grouper (M. tigris) exhibited the “spawning stupor” behavior described by Johannes et al. (1999) as a lack of concern about the presence of divers. Having no other information concerning the spawning behavior of this important fishery species in the Florida Keys and realizing the proximity of the small aggregation to a proposed marine reserve site within the Florida Keys National Marine Sanctuary (FKNMS), we felt it important to find out what we could about this aggregation. We therefore describe here the spatial and temporal extent of the aggregation relative to the reserve, and we discuss the importance of establishing new reserves, and changing the boundaries of existing ones, as new biological information warrants. METHODS We conducted 54 reconnaissance dives during the winters of 1994 through 1998. Paired SCUBA divers swam transects along the outer edge of the northern portion of the Florida Keys reef tract (Fig. 1), along the reef slope–sand interface, or reef edge, from 18 to 28 m depth. The prevailing northerly current, and the occasional southerly countercurrent, allowed divers to drift-dive and cover large distances (up to 2000 m) of this reef habitat. On 12 January 1998, we discovered an aggregation of up to 96 black grouper in a 100-m2 area, along the deep forereef slope. We concentrated our dives in the aggregation area, although we also dove in reference areas of similar habitat north and south of the aggregation, and we swam additional transects from the shoreward edge of the aggregation to within the shallower marine reserve area (Fig. 1). The marine reserve is one of the Sanctuary Preservation Areas (SPAs) established in 1997 within the FKNMS. The northeastern boundary of this SPA is in less than 18 m depth, just before the reef slope steepens. One diver used a digital video camera to record the habitat, fish distribution, and fish behavior associated with the aggregation. The second recorded numbers and sizes of black grouper observed during each 5-min interval of the survey. Some aggregation dives were stationary dives, rather than drift dives, to allow the divers more time to record sizes, color patterns, and behaviors of individual fish. During each dive, surface support aboard the RV ALDO LEOPOLD followed divers and recorded starting and ending times and positions for each dive, as well as precise locations of a diver-towed surface buoy every 2 to 4 min by means of a Global Positioning System. Diver and surface support observations were synchronized, so that grouper densities could be matched to specific locations. Latitude and longitude were logged in a Geographic Information System software database (Arcview) for mapping of the aggregation area (Fig. 1). Black grouper total lengths were estimated to the nearest 5 cm by comparison of fish to a 1-m PVC rod. In addition, weather conditions, tidal state, moon phase, and water temperature at depth were recorded for each dive.

EKLUND ET AL.: BLACK GROUPER AGGREGATION IN FLORIDA

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Figure 1. The spatial distribution of black grouper, Mycteroperca bonaci, observed on 37 dives between 3 December 1997 and 4 August 1998, in proximity to a Sanctuary Preservation Area that was designated a marine reserve within the Florida Keys National Marine Sanctuary in July 1997. Circles and squares mark beginning dive locations, determined by Global Positioning System units on a boat above the divers. Solid circles represent dives during which the density of black groupers exceeded 15 per 50 m. Open squares represent dives with densities of 0–15 black groupers per 50 m. Note that no Geographic Information System benthic habitat maps were available beyond the 20-m depth contour.

RESULTS The grouper aggregation was spatially discrete, lying wholly within a 100-m2 area on a deep forereef slope between 18 and 28 m depth (Fig. 1, Table 1). Dives outside the 100-m2 area revealed much lower densities of grouper (Figs. 1,2). The benthic habitat has not yet been mapped or included in Geographical Information System databases beyond the 20-m depth contour, but this habitat may be essential spawning habitat for black grouper and many other species. At 20 m, the forereef slopes steeply to a sand plain at 28 m. An area of structurally complex, fossilized coral reef/rock occurs at the sand bottom and contains several large caves, which provide shelter for large fishes. Fish were not restricted to the caves, however. We observed many groupers (> 20–30) in the water column at any given moment, along with 20–40 other individuals more closely associated with the slope face. The black groupers exhibited typical spawning-aggregation behavior, in that they were not wary of divers and did not swim away when approached. This behavior is in marked contrast to that of solitary black groupers, which are easily spooked by divers. Large schools of scad (Decapterus spp.) and grunts (Haemulon

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Table 1. Number of black grouper, Mycteroperca bonaci, at each of three locations in Florida Keys National Marine Sanctuary (FKNMS). A = Aggregation site (the specific location where M. bonaci were observed aggregating in January and February 1998). R = Reference areas (500 m from the aggregation site and along the deep forereef slope-sand interface from 18 to 28 m). S = Sanctuary Preservation Area (designated a no-take area within the sanctuary on 1 July 1997 and encompassing the forereef area between 1 and 18 m of water). Date 13 Jan. 94 01 Feb. 95 12 Dec. 96 08 Jan. 97 21 Jan. 97 08 Feb. 97 09 Feb. 97 03 Dec. 97 12 Jan. 98 13 Jan. 98 29 Jan. 98 30 Jan. 98 06 Feb. 98 10 Feb. 98 26 07 09 04

Feb. 98 Apr. 98 Jun. 98 Aug. 98

Location A S A R A A A R A R A R A S A R A A/S A R A R A A A A

No. of dives 1 1 1 3 4 2 4 1 1 4 3 2 4 1 4 1 3 1 2 1 4 2 1 1 1 1

M. bonaci observed 15 0 2 4 10 4 6 6 1 19 140 21 135 0 186 3 139 6/0 20 9 40 1 10 8 21 13

Moon phase New New New New Full New New Quarter Full Full New New Quarter Full New Full Full Full

aurolineatum and juvenile Haemulon spp.) were associated with the grouper aggregation. Jewfish, mutton snapper (Lutjanus analis), and hogfish (Lachnolaimus maximus) were also using the reef-slope habitat. The density of grouper was low in December but peaked in January and early February and declined in late February (Fig. 2). Density remained low (