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Tiger Grouper (Mycteroperca Tigris): Profile of a Spawning Aggregation

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Tiger Grouper (Mycteroperca Tigris): Profile of a Spawning Aggregation BULLETIN OF MARINE SCIENCE, 70(1): 233–240, 2002 TIGER GROUPER (MYCTEROPERCA TIGRIS): PROFILE OF A SPAWNING AGGREGATION D. Byron White, David M. Wyanski, Brandon M. Eleby and Craig G. Lilyestrom Throughout the Caribbean several large site-specific grouper spawning aggregations have been documented, including those of Nassau grouper (Epinephelus striatus), red hind (E. guttatus), and tiger grouper (Mycteroperca tigris) (Domeier and Colin, 1997). All of these aggregations have received heavy fishing pressure, some to the extent of extermination (Aguilar-Perera and Aguilar-Davila, 1996; Sadovy and Eklund, 1999; Coleman et al., 2000; Sala et al., 2001). Unfortunately, because spawning aggregations are ephemeral events, frequently in remote places, our understanding of them is poor (Domeier and Colin, 1997). One of these poorly understood species is the tiger grouper, a moderate-sized serranid found in Bermuda, southern Florida, the western Gulf of Mexico, and throughout the West Indies to northern South America (Smith, 1971; Heemstra and Randall, 1993). Throughout their range, tiger grouper are infrequently captured; how- ever, for 2 d preceding and approximately 1 wk following each full moon during January through April, a spawning aggregation estimated to be several hundred individuals forms east of Vieques Island, Puerto Rico (Sadovy et al., 1994a). This aggregation has been heavily fished in recent years and landings have declined significantly (Matos and Posada, in press); however, most recently fishermen have reduced their efforts to harvest tiger grouper from this aggregation for reasons of safety (i.e., ‘the bends’) and to target more economically viable species (D. Matos, Puerto Rico Department of Natural and Environ- mental Resources, pers. comm.). There are currently no management regulations in place for tiger grouper. This paper provides a life-history ‘snapshot’ of tiger grouper present in this spawning aggregation on one moon, information that may be used in developing a management program. METHODS We obtained tiger grouper otoliths and gonads from 2 to 4 March 1994, 4 to 6 d following the full moon, from the commercial landings on Vieques Island. Specimens in the possession of wholesale dealers were randomly sampled, whereas all specimens in the possession of commercial fishermen were usually examined. A total of 89 specimens was collected by speargun using SCUBA or by hook and line from the spawning aggregation that occurred east of Vieques Island (18º 5'3''N, 65º10'3''W) at a depth of 35–45 m. In addition a fisherman reportedly caught several dozen tiger grouper at an aggregation south of Vieques (18º4'0''N, 65º20'3''W) but only allowed us to sample one specimen. We measured total length (mm TL) and whole weight (kg) for all specimens, and removed the left sagittal otolith from all but one specimen. Each otolith was embedded in Araldite®, and sectioned once along a dorsoventral plane through the focus with a single high-concentration diamond wheel on a Buehler® Isomet low-speed saw. Sections were mounted on glass slides using Crystalbond® thermoplastic. Otolith sections were immersed in cedarwood oil and examined under a dissecting microscope (50¥) using transmitted light. Two readers counted increments independently. If counts differed, both readers examined the otolith by projecting its image onto a video monitor. If no agreement could be reached, the otolith was removed from age and growth analysis. 233 234 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 1, 2002 The posterior portion of the gonad from each specimen was fixed in 10% seawater-formalin for 2 wks and transferred to 50% isopropanol for 2 wks. Gonads were processed with an Auto-Technicon® 2A Tissue Processor, vacuum infiltrated, and blocked in paraffin. Three transverse sections (6–8 mm thick) were cut from each sample with a rotary microtome, mounted on a glass slide, stained with double-strength Gill hematoxylin, and counter-stained with eosin-y. Two readers assessed sex and reproductive state using histological criteria (see McGovern et al., 1998), without reference to body length or date of capture. Mean observed size was compared between sexes using analysis of variance (ANOVA). Sex ratio between gear types was compared using the chi-square, c2) test. Statistical analysis was performed using SAS, and the results were considered significant if P was <0.05 (SAS Institute, Inc. 1985). RESULTS The commercial landings of tiger grouper from Vieques Island declined from 4962 kg in 1995 to 1719 kg in 1998 (Fig. 1), apparently due to decreased fishing effort. Our specimens were captured by hook and line (n = 69) and speargun (n = 21). Age estimates were determined for 80 of 89 (90%) of the sectioned tiger grouper otoliths. Specimens ranged in length from 320 to 642 mm TL ( x = 458; Fig. 2), and in age from 3 to 11 yrs, ( x = 7.0; Fig. 3). Males were larger than females in all age intervals, with males significantly larger than females overall (P < 0.0001, df = 86; Fig. 4). We determined sex and reproductive state for 88 of 90 specimens. No immature speci- mens were collected from the spawning aggregation. Fifty-two (59%) male and thirty-six (41%) female specimens were collected, for an overall sex ratio of 1.44:1 (Male:Female). Male tiger grouper were taken more often by hook and line (64%) than by speargun 2 (45%); however, there was no significant difference in sex ratio between gear types (c = 2.27, df = 1, P £ 0.20). Females were 320 to 480 mm TL ( x = 413 mm TL, SE =7) (Fig. Figure 1. Commercial landings of tiger grouper from Vieques Island, Puerto Rico, as reported by Matos and Posada, (in press). Landings before 1995 were recorded as ‘grouper’ (all species). No data were available for 1999 and 2000. NOTES 235 Figure 2. Length-frequency of male and female tiger grouper captured from a spawning aggregation east of Vieques Island, Puerto Rico, March 1994. 2), and 3 to 9 yrs ( x = 6.4 yrs, SE = 6.3)(Fig. 3). Males were 360 to 642 mm TL ( x = 490 mm TL, SE = 7)(Fig. 2), and 5 to 11 yrs ( x = 7.6 yrs, SE = 6.8)(Fig. 3). All females and males were in the developing, recently spawned (presence of postovulatory follicles), or running ripe reproductive states; all females had vitellogenic oocytes. Oocytes with mi- gratory nuclei or oocytes that were hydrated were present in 21 of 36 (58%) females, indicating that these specimens would have spawned in the next 24 h (Hunter and Macewicz, 1985b; Fitzhugh et al., 1993). Postovulatory follicles (>24 h old), indicating a recent spawning event, were present in only two specimens. Light yellow to yellow- brown cell masses, called gamma and delta stages of oocyte atresia by Hunter and Macewicz (1985a), were present in testes of 39 of 52 (74%) males. DISCUSSION The tiger grouper spawning aggregation near Vieques Island has been fished since its discovery in the early 1980s. It is the only aggregation that has been surveyed and con- firmed (Sadovy et al., 1994a); however, there are reports of other tiger grouper aggrega- tions in the Caribbean (Fine, 1990). Although fishing effort and landings have declined (Matos and Posada, in press), there has been no detailed assessment to determine if con- tinued harvest can be supported. Luckhurst (1996) started a tag-recapture program to examine site fidelity and to determine the distances that tiger grouper might migrate to participate in the Vieques aggregation. Their presence in fishery landings is sparse out- side of the spawning season. 236 BULLETIN OF MARINE SCIENCE, VOL. 70, NO. 1, 2002 Figure 3. Age-frequency by sex of tiger grouper captured from a spawning aggregation east of Vieques Island, Puerto Rico, March 1994. Tiger grouper aggregate to spawn around the week of each full moon in January-April. All individuals collected from the March 1994 aggregation were examined histologically and found to have gonads in developing or running ripe condition. We collected no fe- male specimens under 320 mm TL (age 3). The only immature female (examined histo- logically) and smallest mature female collected by Sadovy et al. (1994a) were 205 mm SL (ca 241 mm TL) and 255 mm SL (ca 299 mm TL) respectively, which suggests that tiger grouper attain sexual maturity between 250–350 mm TL, at age 3. It is possible that maturity is attained as early as age 2, as Sadovy et al. (1994a) found that otolith sections from nine specimens between 257 and 423 mm SL (301–495 mm TL) had two to nine opaque zones. The frequency of increment formation in tiger grouper otoliths has not been validated, but increments in two related species, Mycteroperca microlepis (Harris and Collins, 2000) and Mycteroperca phenax (Harris et al., in press) as well as another Caribbean grouper, Epinephelus guttatus (Sadovy et al., 1992), were found to be depos- ited annually. Smith (1958) reported that the size versus sex distribution (females: <370 mm TL; males: >450 mm TL) of tiger grouper off Bermuda was thought to be indicative of protogyny. Our results, as well as Sadovy et al. (1994a) and Matos and Posada (in press), indicate that males are the larger and older individuals within the Vieques population, but the extensive overlaps in size and age frequencies between sexes are not strong indicators of protogyny (Sadovy and Shapiro, 1987). The strongest indicators that we observed were membrane-lined central cavities in testes, also observed by Smith (1959) and Sadovy et al. (1994a), and sperm sinuses in the gonad wall. Based on the criteria of Sadovy and NOTES 237 Figure 4. Mean observed size-at-age by sex of tiger grouper captured from a spawning aggregation east of Vieques Island, Puerto Rico, March 1994. Error bars represent ± 1 SE.
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