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Spawning Aggregations of the Nassau Grouper: Essential Habitat, Population Structure, and Fish Movement Patterns (Year 1 of 2)
Project Number: CMRC-99-NRDE-03-00A
Principle Investigators: Eggleston, D. B., J. Hightower, P. S. Rand, C. P. Dahlgren, A. Eklund, and S. K. Bolden
Region(s): Lee Stocking Island, Bahamas |
OBJECTIVES:
We proposed to: (1) resample two spawning aggregations of Nassau grouper (Epinephelus striatus) off Long Island, Bahamas (last sampled in 1988-89), for comparisons of total fish abundance; (2) characterize the spatial structure of spawning aggregations using diver surveys, and acoustic and video images; (3) locate additional spawning aggregations of Nassau grouper in the central Bahamas, and characterize the habitats at these sites; (4) quantify the sex, size-at-age, and reproductive status of Nassau grouper harvested from Long Island aggregations; and (5) quantify movement patterns of sexually mature Nassau grouper during the spawning season using biotelemetry and external tagging methods, and possibly locate unfished aggregations.
RESULTS
Our original research plan included two research cruises in December 2000, and January 2001. Due to higher than budgeted costs associated with the research dive vessel “Sea Ray”, our study was restricted to one cruise, which targeted the full moon period surrounding 6-13 December, 2000. The full moon occurred on 11 December, 2000. The following activities were conducted during the research cruise: 1) SCUBA diver, video and acoustic surveys of spawning aggregations off of Long Island, Bahamas, 2) examination of fish behavior within spawning aggregations using biotelemetry, and 3) support of shore-based sampling of fish at Long Island fish houses. Aerial reconnaissance of possible locations of spawning aggregations was conducted during December 9-10, 2000. Nassau grouper that were landed at Long Island during 7-14 December, 2000 were sampled by our team for otoliths, gonads, length and weight. Lastly, a biotelemetry study was conducted off of Lee Stocking Island during November and December, 2000 to examine movement patterns of Nassau grouper migrating to spawning aggregations.
The general cruise track was as follows:
December 6-7: Traveled from LSI to Clarence Town, Long Island to assess where the most likely spawning aggregations were located based on local knowledge.
December 7: Traveled to the South Point aggregation site and conducted two SCUBA surveys (22o50’.847N / 74o51’.489W). This site was last surveyed by P. Colin in 1989 (Colin 1992).
December 8-11: S. Point, Long Island, Bahamas
December 12: Traveled to and surveyed three potential aggregation sites located between S. Point and Clarence Town, Long Island, Bahamas. These sites were chosen based on aerial reconnaissance and historic information, and are described herein as: 1) “Little Harbor” (23o00’.55N / 74o48’.37W; surveyed in 1989 by P. Colin; see Colin 1992); 2) “Hells Gate” (23o06’.818N / 74o55’.038W); and 3) “Hells Gate Shoal” (23o07’.620N / 74o56’.823W).
December 13: Traveled to Lee Stocking Island.
Objectives 1 & 2: SCUBA, video and acoustic surveys of spawning aggregations
SCUBA (D. Eggleston)- We conducted a total of 88 SCUBA dives to depths ranging from 20-40m, with a total bottom time of 30 hours. All dives were conducted offshore of the southern half of Long Island, Bahamas.
“South Point”: We surveyed both the “deep” (30-35 m) and “shallow” (20 m) S. Point aggregation sites sampled by P. Colin in 1987-89 (Colin 1992). No aggregations were ever observed at the shallow site. Nassau grouper (70-80 cm TL) began aggregating at the deep site on 8 December, with a peak in abundance of 72 fish on 10 December. Of these fish, ~ 4% were bi-color, ~3% dark, ~ 20% white-belly, and the remaining fish barred. Despite observing the “aggregation” during three consecutive dusk periods, no spawning behavior was recorded. We did not observe any large (e.g., 100’s of fish) schools of migrating fish, as has been the case in past observations (Colin 1992). The S. Point aggregation was fished nearly continuously during our stay by a local trap fishery. The S. Point spawning aggregation numbered 39 fish when we last located it on 11 December.
“Little Harbor” – We surveyed the exact coordinates from Colin’s (1992) study, as well as a 2 km X 100m area along the shelf break surrounding this site (32-40 m depth). A total of only eight Nassau grouper were observed, one fish (50 cm TL) was swimming and six fish (30-60 cm TL) were in a fish trap.
“Hells Gate” – According to fishermen, the majority of fish being landed at Clarence Town, Long Island came from this aggregation. This anecdotal information was somewhat verified by the large numbers of fish trap floats observed by aerial reconnaissance on 10 December (see below). This site was located along the shelf break (30-40 m) where the wall makes a funnel-shape from Exuma Sound towards Clarence Town, and was characterized by a relic spur-and-groove system with approximately 10 m relief within each groove, before plunging to ~200m down the wall. A total of 45 Nassau grouper (60-70 cm TL) were observed, with 6% of fish bicolor, 21% white-belly, and the remaining fish barred. No dark fish or spawning behavior was observed.
“Hells Gate Shoal” – Aerial reconnaissance indicated a small cluster of fish traps floats at this site, so we surveyed it for a Nassau grouper spawning aggregation on 11 December. We searched an area approximately 1 km X 100m along a line of fish traps on a relatively featureless sand bottom (25-30 m depth), located ~ 1 km inshore of the shelf break. Only four Nassau grouper were observed (4-60 cm TL), all were located in one fish trap.
Although we did not visually survey spawning aggregations off Long Island in January, we were in close contact with Bahamas Department of Fishery personnel (Ms. Tameka Rahming) who sampled fish landed in Clarence Town, Long Island. Ms. Rahming reported that the January catch of Nassau grouper was the same as or lower than December, at ~10-20 fish/trip. These results suggest, at least qualitatively, that the number of fish observed during the December spawning aggregations did not increase dramatically in January. We suggest that visual surveys combined with acoustic transects and aerial reconnaissance (see below) is an effective means of surveying the reported 23 spawning aggregations of Nassau grouper in the Bahamas. Such a survey is crucial given the range of estimates in the numbers of fish at these aggregations. For example, the S. Point and Little Harbor spawning aggregations once contained 1,000’s of fish (Colin 1992), yet we only measured ~ 70 and 0 fish at each site, respectively. Similar recent declines have been reported for aggregations at High Cay, Andros (C. Ray, Univ. Virginia, pers. comm.). These findings are in contrast with recent population estimates of 10,000’s fish per aggregation by Ehrhardt and Deleveaux (1999). A comprehensive population assessment of purported spawning aggregations in the Bahamas is clearly needed for a more accurate stock assessment, which is crucial to conserving this important species.
Video Surveys (P. Rand)- We conducted video observations at South Point, Little Harbor, and Hells Gate during 10-12 December 2000. All observations were made near bottom at approximately 30m depth. Our objective was to record images on stereo-paired video cameras to allow us to measure 1) grouper length distribution, 2) enumerate number of grouper in the aggregation to compare with visual surveys conducted at the same time, 3) compute measures of aggregation attributes (e.g. nearest neighbor distances between members of aggregation, dimensions of aggregation, distance from aggregation to the bottom), and 4) record swimming and courtship behaviors, including composition of different color phases in the aggregation. We deployed two underwater cameras (two SONY CCD digital camcorders inside Ikelite housings) mounted on each end of a ca. 1 m long aluminum bar such that the camera views were parallel. The diver operating the cameras was accompanied by another diver responsible for recording visual census data. The divers swam along a transect until an aggregation was encountered. The diver that held the camera assembly was responsible for orienting the cameras to capture the aggregation on film. We recorded video data during 9 separate dives. The summary of our video records are included in Table 1. We collected approximately 28 minutes of footage of a spawning aggregation at South Point during mid- and late morning and at dusk during 10 & 11 December (Table 1, Figure 1). We have approximately 30 minutes of footage at the Little Harbor site where no free-swimming grouper were observed (12 December, Table 1). In addition, we collected approximately 30 minutes of footage at “Hells Gate”, where several small groups and solitary large grouper were observed (Table 1). Problems that we encountered included loss of stereo coverage as a result of one camera shutting off (presumably due to unintentional bumping of external control knobs on the housings), unexplained loss of focus on one camera during two of the dives, and poor visibility due to low light conditions after approximately 1730 h. Post-processing the video records from several of the dives should allow us to meet our main objectives outlined above. We conducted two separate calibration exercises in the field using a 3D quadrat to enable us to conduct the 3D analysis.
We generated several ideas that would improve our ability to conduct this work and extend the capabilities of the system. Reducing the separation distance between the cameras will make the system more manageable. This will also increase the overlap between the fields of view of the two cameras and allow stereo footage on grouper that are closer to the cameras. An external view monitor mounted on the bar would allow the diver to more easily ascertain if the aggregation is being captured on film. Frame synching the two cameras in record mode would simplify field operation and post-processing. This would allow one set of controls for both cameras. We are presently looking into whether or not this is feasible. Making records of the aggregations from a fixed, stable point would improve image stabilization and allow us to more accurately estimate fish swimming speed and bearing. It would be advantageous to fix the camera assembly on the bottom and record for extended periods of time autonomously. Longer tape lengths and a pan-tilt assembly would provide greater coverage in time and space at each aggregation site. This would also allow us to document any changes in behavior based on the presence of divers. A more mobile survey approach is also feasible, particularly in cases where fish are distributed over a larger area. Suspending the camera assembly in gimbals would ensure that the view axes of the cameras would be maintained in a fixed horizontal plane while the divers progress along a transect. Obtaining useable video records under low light conditions is a problem. We changed the record mode to "NightShot" (improving low light sensitivity by increasing signal gain), which appeared to provide better images near dusk at depth, but image quality was marginal. We have had some success with night-time infrared lighting with some salmon migration studies in an Alaskan river, but strong attenuation of IR in water limits illumination to about 1-2 m away from the light source. Powering the light source presents it's own set of problems. If cameras are deployed in an uplooking orientation, silhouettes of fish under down-welling light conditions at dusk could allow us to film past sunset and provide a way to record actual spawning behavior.
Table 1. Summary of video records archived from the December Grouper cruise off Long Island, Bahamas, December 2000.
Dive # Date Location Time Comments
1 10 December 2000 South Point 7:28-7:39 AM Camera A shut off (no stereo records). Good footage of spawning aggregation. 1 fish in trap.
2 10 December 2000 South Point 4:20-4:27 PM Good stereo footage of aggregation. Couple dark phase fish visible. Two fish in trap.
3 10 December 2000 South Point 5:28-5:33 PM Poor vis due to low light conditions, but fish are visible. However, may be difficult to do 3D analysis.
4 11 December 2000 South Point 7:14-7:16 AM Camera A shut off (no stereo records). Good footage of spawning aggregation.
5 11 December 2000 South Point 11:07-11:10 AM Good stereo footage, but unexplained blurry focus on Camera B that may limit analysis. Aggregation appears to be moving actively away from divers. 1 dark phase fish evident.
6 12 December 2000 Little Harbor ca. 11 AM Good stereo footage, but unexplained blurry focus on Camera B that may limit analysis. Couple fish in trap, no aggregation observed.
7 12 December 2000 Little Harbor ca. 12 Noon No aggregation observed
8 12 December 2000 Offshore Site 2:36 PM Camera B shut off. "Loose" aggregation of ca. 10 fish, on bottom. Some solitary fish.
9 12 December 2000 Offshore Site ca. 3:30 PM Small groups of fish, some solitary.
Acoustic Survey (P. Rand) – There were five acoustic surveys of a Nassau grouper spawning aggregation at S. Point during 9-11 December. All acoustic surveys were conducted with a transducer mounted along-side a 6 m, flat-bottomed skiff. On 9 December, the first two transects were performed along the 30-35 m depth contour where we had observed an aggregation using SCUBA. Neither of these transects produced signs of aggregations of fish near or off the bottom, though we did count three divers. A third transect was conducted along the shelf-break in 40 m depth. This transect produced good images of bottom topography (many apparent outcroppings alternating with sand); however, the only fish that were enumerated were queen-trigger fish (Balistes vetula; verified by divers). On 11 December, a total of seven transects were run perpendicular to the shore from 20-40 m depths. These surveys were conducted at 0800 and 1200. Unfortunately, only two fish of the expected size (45 - ~60 cm) were enumerated.
We suggest that an acoustic survey is still a viable technique for large-scale assessment of fish in spawning aggregations. To insure the success of future acoustic surveys of Nassau grouper spawning aggregations, we suggest 1) that a boat be dedicated to the acoustics portion of the project, rather than being dedicated to both acoustics and divers at the same time, 2) using a larger boat that is not as affected by rough seas as a flat-bottomed skiff, and 3) using a tow-body towed off the side of the vessel with a shock cord in the support cable. Although the fathometer on the dive vessel “SeaRay” (Furuno) was very helpful in identifying habitat features, depths and possible fish schools, it was impossible to save the data or to determine whether the targets seen were actually fish, since no echo characteristics were provided. We also suggest that we mount an acoustic transducer in an ROV. George Rose (St. Johns Univ., N.S.) and his students have had some success with this type of approach on spawning cod, especially when the cod were deep in the cobble during the day on the Grand Banks. They also had video cameras for simultaneous ground-truthing.
Objective 3: Locate additional spawning aggregations (C. Dahlgren)
We used aerial reconnaissance of clusters of fish trap floats as a means to locate potential aggregation sites. Two aerial searches were conducted. On 9 December we searched an area of approximately 500 km2 by flying along the Exuma Sound coast from Lee Stocking Island south to Long Island {LSI-> Georgetown -> Black Rocks ->Cape Santa Maria, Long Island (via reef-line from Black Rocks to Long Island)], then to South Point, Long Island (along reef-line from Cape Santa Maria to S. Point), then from Long Island to Crooked Island (Pittstown Point/Bird Rock area of Crooked Island) back to S. Point, Long Island, then to Flamingo Cay and Water Cay in the Jumentos and Ragged Island chain, and returning to LSI along the “south-side” of Great Exuma. On 10 December, we conducted an aerial search from north of LSI along the western shore of Exuma Sound, from LSI to Warderick Wells to Black Rocks off of Little Exuma. The aerial search on 10 December covered an area of approximately 110 km2.
The largest clusters of trap floats were observed off of Long Island at 1) “S. Point” (surveyed in this study), 2) “Hells Gate” (surveyed in this study), 3) off of Cape Santa Maria (23o43’.806N / 75o21’.361W; not surveyed). In addition, relatively sparse lines of trap floats were observed at 1) Little Harbor (surveyed in this study), 2) southern Long Island (22o55’.600 N / 74o49’.350W; not surveyed), and just south of Lee Stocking Island (23o45’.636N / 76o03’.301W, and 23o45’.758N / 76o03’.248W; not surveyed). Aerial reconnaissance of trap floats appears to be a reliable approach for identifying the relative locations and size of spawning aggregations for Nassau grouper during winter.
Objective 4: Quantify the sex, size-at-age, and reproductive status of Nassau grouper harvested from Long Island aggregations (A-M. Eklund)
Researchers from NMFS, NCSU, and The Bahamas Ministry of Fisheries collaborated during our December sampling effort on Long Island, Bahamas. We stationed our operations at two landing sites on the southern portion of the island where fishermen were known to land Nassau groupers during the spawning season. For each fish sampled (N=119), we measured length and weight and removed otoliths, dorsal spines and rays, genetic samples, and gonads. Some fish (N=14) were landed without gonads.
Genetic samples have been sent to Dr. Bob Chapman for population analysis. The ratio of mature females to mature males was 4:1, and GSI’s ranged between 0.52 and 12.03 (mean = 4.68). Histological examination of gonad samples revealed that all females were in vitellogenesis, no samples presented hydrated oocytes. Dorsal spines and rays and otoliths will be sectioned and used to determine age. Further analysis will yield size-at-age, sex-at-age, sex-at-length.
Research objectives for subsequent sampling events are: (1) continue the resampling of spawning aggregations of Nassau grouper off Long Island, Bahamas, (2) conduct fishery- independent sampling (around the clock) in the forthcoming year to better elucidate the timing of spawning and sexual strategy (i.e. gonochoristic vs. hermaphroditic), (3) decrease post-mortem decay in our samples by updating our sampling methodology, (4) continue working with the Bahamas Ministry of Fisheries in support of concurrent research initiatives.
Collaboration with the Bahamas Ministry of Fisheries has been paramount to the success of these operations. We would like to continue working with them to increase the scope and scale of our sampling regimes. During this past winter, we were able to increase our N of 119 to an N of 220 by combining our sampling efforts over time and space.
We will contribute: (1) a section of the final CMRC project, presenting the results of our resampling of the two spawning aggregations of Nassau grouper off Long Island, Bahamas, (2) a manuscript detailing the histological characteristics of the species’ gonads including interesting findings from subsequent sampling from Bahamas Ministry of Fisheries in January 2001, (3) a manuscript on age and growth and the comparison of hard parts (i.e. otoliths and dorsal fin spines and rays) – this may be published along with results from an age and growth study on Jewfish (E. itajara).
Objective 5: Fish movement patterns
Movement patterns near LSI (J. Hightower)- Nassau grouper were captured near Lee Stocking Island using traditional Bahamian fish traps that were fished at depths ranging from 6.1 to 35.1 m. Traps were checked daily and rebaited unless weather conditions proved to be unworkable. Fish large enough for tagging (primarly those that appeared to be sexually mature) were anesthetized and surgically implanted with VEMCO ultrasonic transmitters. Fifteen of the transmitters were model V16-6H tags that measured 16 mm by 90 mm and weighed 14 g in water, and six were V16-4H tags that measured 16 mm by 65 mm and weighed 10 g in water. The tags were programmed to have a 90-day ON, 275-day OFF duty cycle to last for two winter field seasons (ON during November 2000-January 2001, November 2001-January 2002). We recorded the total length and sex for each captured fish, as well as the latitude, longitude and depth at the capture site. Fish were relocated by searching alongshore transects near LSI and surrounding cays, and by searching reef areas near capture sites. At each relocation site, we recorded latitude, longitude, depth, and general habitat characteristics.
A total of 64 Nassau grouper were captured between 16-November and 12-December 2000, with many of the captures occurring at “Bock Wall”, “Whitehorse”, “Horseshoe”, and “South Perry” reefs. Thirty-three fish ranging in size from 22.0 to 70.2 cm total length (TL) were given external tags only. Twenty-three fish (2 males, 11 females, and 9 of unknown sex) ranging from 42.6 to 72.2 cm TL received transmitters in addition to external tags. An additional eight fish were sacrificed to obtain gonad and otolith samples for the age, growth and reproductive biology component of this study. Four of the telemetered fish were not relocated following release. The remaining 19 fish were relocated a total of 184 times, with no more than one relocation per individual per day. The number of relocations for each fish ranged between 1 and 20. Nine of the fish were relocated on at least 10 days. With one exception (a 70.4 cm female), the fish that remained in the LSI area throughout the study period were relatively small, ranging in size from 42.6 to 59.5 cm TL. Several telemetered Nassau grouper were observed by SCUBA divers 2-74 days post-capture. These telemetered individuals were found occupying the same habitats as untagged fish and exhibiting normal behaviors (e.g. taking position at wrasse cleaning stations). This suggests that the impact of tagging is minimal on this species.
Four of the larger fish (a 72-cm male, 3 females 62-69 cm) either were harvested or migrated out of the LSI area in late November through early December. The three females disappeared from the LSI area during a period of bad weather when searches could not be conducted. Despite searches along the reef edge over distances up to 100 km from LSI, we were unable to relocate these fish, which could have moved to distant spawning aggregations. Telemetered grouper typically remained near the tagging location, although movements of up to 10 km away were recorded. In most cases, fish remained on or near (<0.5 km) the patch reef where they were tagged. Movements within an individual’s home range were limited to high-relief reef habitat and we did not observe use of low-relief sand areas between reefs. We plan additional analyses of telemetry data including characterization of home range and movement between reefs.
The use of ultrasonic telemetry for documenting large-scale movement patterns is not without a few inherent problems. Prolonged periods of inclement weather encountered in Exuma Sound during the winter spawning season make tracking individual fish difficult. When a fish is not relocated in the LSI area, it could indicate migration to a spawning aggregation (in which case, alongshore searches would be warranted) or harvest (in which case, alongshore searches would be wasted effort). One transmitter was returned to us by a fisherman and some trap fishing was observed in the area surrounding LSI. Rewards were offered for the return of transmitters and tags, but it is highly likely that some would not be returned. In addition, the behavior of Nassau grouper (crevice dwellers) makes long-range (>1 km) relocation of tagged fish difficult. In many instances, we lost contact with individual fish when they moved behind reef structures which precluded signal passage. Because of these practical problems, future studies on Nassau grouper migration should focus on lower-cost and simpler approaches such as fish capture and tagging with external tags over large-scale areas. Sampling for these tagged fish could then be carried out at fish houses and or tag returns might be obtained.
Movement patterns within S. Point aggregation (S. Bolden) – The goal of this portion of the study was to obtain fine-scale behavioral information on possible sex-related participation rates by fish in discrete spawning rushes, diel activity patterns, and time spent at an aggregation site. Three buoys containing a hydrophone and receiver were arranged in an equilateral triangle (~ 70,000 m2) surrounding the South Point aggregation site during 8-11 December. As an initial test, the buoy tracking system accurately tracked divers carrying sonic transmitters. We then deployed two fish traps to catch and tag 10 grouper with internal sonic transmitters as described above. Unfortunately, we did not realize until near then end of our study that fishermen were fishing our traps, such that we never caught any Nassau grouper. We purchased one live Nassau grouper from a fishermen and internally tagged and released it within the buoy system. The fish was not relocated within a 12 h period before we had to remove the buoys for transport back to LSI. Although this technology has proven successful in similar studies, it will be crucial to arrive at an aggregation site early enough and with a large enough complement of traps (> 6 traps), to insure adequate numbers of spawning-phase fish are captured and tagged.
PAPERS PRESENTED BASED ON 2000 WORK:
Although no papers have been presented, several are planned.
MANUSCRIPTS AND PUBLICATIONS:
A total of four manuscripts are planned once data analyses are complete. We have, however, used some of the information from this study (i.e., visual surveys of Long Island aggregations), in concert with the Bahamas Department of Fisheries and the IUCN committee on threatened and endangered species, to push for a pre-cautionary management approach for Nassau grouper through seasonal fishing closures during wintertime spawning periods.
REFERENCES
Colin, P. (1992) Reproduction of the Nassau grouper, Epinephelus striatus (Pisces: Serrandidae) and its relationship to environmental conditions. Env. Biol. Of Fishes 34:357-377.
Ehrhardt, N. M. and V. Deleveaux. (1999). Report on the 1999 Nassau grouper stock assessment in the Bahamas. University of Miami, 38 p. |
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