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Spawning Aggregations of the Nassau Grouper: Essential habitat, population structure, and fish movement patterns. (Year 2 of 2)
Project Number: CMRC-00-NRDE-03-01C
Principle Investigators: Eggleston, D. B., J. Hightower, C. P. Dahlgren, P. S. Rand, A. Eklund, and S. K. Bolden
Region(s): Lee Stocking Island, Bahamas |
OBJECTIVES:
We proposed to: (1) sample at least four spawning aggregations of Nassau grouper
(Epinephelus striatus) off Long Island, Bahamas (last sampled in 1988-89 and 2000), 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.
General Results
Our original research plan included two research cruises, one in December 2001
and one in January 2002. Due to budget limitations we were restricted to one cruise
aboard the “RV Coral Reef II”, which was donated to this project by the Shedd
Aquarium. This cruise targeted the full moon period surrounding 24-31 January, 2002.
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)
support of shore-based sampling of fish at Long Island fish houses, and 3) discussion of
research program and concern over declining spawning stock of Nassau grouper with
local fishermen and government officials. Aerial reconnaissance of possible locations of
spawning aggregations was not conducted as planned due to cancellation by our charter
because of mechanical difficulties. Nassau grouper that were landed at Long Island
during 24-31 January, 2002 were sampled by our team for otoliths, gonads, length and
weight. Lastly, a biotelemetry study was conducted off of Lee Stocking Island during
December, 2001 to relocate fish tagged in 2000 and examine the timing of emigration of
Nassau grouper from the LSI area to spawning aggregations.
The general cruise track was as follows:
January 23: Arrived Georgetown, Bahamas and met with Basil Minnis to discuss the
research program and concern over status of grouper stock. Conducted acoustic
profiles in Georgetown Harbor to field trial split-beam echo-sounder.
Janaury 24: Traveled to and surveyed the Cape Santa Maria aggregation site (23o43’.660
N / 75o21’.330W). Ran acoustic profile along shelf break near Black Rocks area
(23o30’.006 N / 75o34’.300 W) east of Georgetown during cruise to Cape Santa
Maria in search of possible aggregations. Met with Delbert Smith (local
divemaster) and Alfred Knowles (local merchant and government official) in
Calabash Bay, Long Island to discuss research project, concern over status of
Nassau grouper, and confirm heavy fishing at Cape Santa Maria in December
2001.
January 25: Traveled to and surveyed three potential aggregations sites (S. Cape Santa
Maria, 23o43’.374 N / 75o18’.600 N and 23o40’.763 N / 75o16’.953 W; Hell’s
Gate, 23o06’.818N / 74o55’.038W).
January 26: Traveled to and surveyed aggregation at S. Point (22o50’.847 N / 74o51’.489
W).
January 27: Surveyed aggregation at S. Point, then traveled to and surveyed potential
aggregations at S. and N. Little Harbor ( 22o57’.058 N / 74o49’.925 W and
23o00’.425 N / 74o50’.913 W, respectively), and the “Curl” just east and north of
Hell’s Gate (23o06’.732 N / 74o55’.032 W).
January 28: Discussed our research program and concern for status of Nassau grouper
with three local fishermen in Clarence Town. Traveled to and surveyed
aggregation at Hells Gate.
January 29: Traveled to and surveyed potential aggregation at Cape Santa Maria, then
cruised to Conception Island to search for possible aggregation. Conducted diver
reconnaissance at Conception Island.
January 30: Diver reconnaissance at Conception Island. Possible aggregation at
Columbus Point, Cat Island reported by “Dive Vessel Nekton Pilot” (24o08’.466
N / 75o19’.281W and 24o08’.520 N / 75o19’.630 W).
Janaury 31: Traveled back to Georgetown.
Specific Results
Objectives 1-3: SCUBA, video and acoustic surveys of spawning aggregations
A. SCUBA (D. Eggleston)- We conducted a total of 114 SCUBA dives to depths ranging
from 20-40m, with a total bottom time of ~55 hours. All dives targeted grouper
aggregation sites off of Long Island (Fig. 1), as well as diver reconnaissance for Nassau
grouper aggregations off of Conception Island.
“N. Cape Santa Maria”: We marked this site during our cruise in 2000 based on
an extremely high number of trap floats--January 2002 was the first time this site
was surveyed. There were no traps visible in January 2002, nor did we observe
any aggregations after visually searching ~ 6,100 m2 (Table 1). Interestingly, we
provided the lat./long. coordinates to Leslie Whaylen with the Reef
Environmental Education Foundation (REEF) because they were looking for
snapper spawning aggregations during April 2002. They observed a spawning
aggregation of Mutton snapper containing 600-1,000 fish at this site on April 30,
2002.
“S. Cape Santa Maria” – We observed a cluster of trap floats ~9 km south of N.
Cape Santa Maria and proceeded to visually survey 6,100 m2 of this area. This
area was interesting in that the bathymetry rose from > 40 m depth to a plateau
of15-20 m. Only four Nassau grouper were observed (~ 40-50 cm TL).
“South Point” – We observed 28 fish aggregating at 0800 on 1/26/02 at the exact
location as in December 2000. All fish were barred in coloration. No bicolor or
dark phase fish were observed. Unlike 2000, fish appeared scared by divers that
slowly approached them, and responded to divers by dispersing into nearby coral
overhangs or residing adjacent to patch reefs (see Figs. 2-6 below). The S. Point
“aggregation” was fished by local fishers using traps over the next 28 hr. of our
stay at this site. During this time the “aggregation” diminished to 12 fish. No
spawning behavior was ever observed.
“Little Harbor” – We surveyed the exact coordinates from Colin’s (1992) study,
as well as a 4,700 m2 (Table 1) along the shelf break surrounding this site (32-40
m depth). Only two Nassau grouper were observed and both of these were
relatively small (~35-40 cm TL).
“Hells Gate” – 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 4 Nassau
grouper (60-70 cm TL) were observed on 1/25 and 2 fish on 1/28. We searched ~
6,500 m2 at Hell’s Gate and the nearby “Curl”.
Although we did not visually survey spawning aggregations off Long Island in
December 2001, we discussed the relative numbers of fish landed during December 2001
with local fishers. All of the fishermen we spoke with reported extremely low numbers of
fish except for Mr. Minnis at Clarence Town. The Minnis brothers reportedly fish an
unknown aggregation off the Ragged Islands which they described as “growing in size
each year”. We were also in close communication with Kathleen Sullivan who was
sampling fish houses at Nassau during Dec. 2001 and Jan. 2002. She also reported
extremely low numbers of Nassau grouper landed. We continue to advocate the use of
visual surveys combined with acoustic transects and aerial reconnaissance (see below) as
an effective means of surveying the reported 23 spawning aggregations of Nassau grouper
in the Bahamas. Such a survey is crucial given the range in estimates of fish abundance
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 ~ 28 and 0 fish at each
site, respectively. Moreover, we measured ~ 40 Nassau grouper in the “aggregation” at
Hell’s Gate in December 2000, yet only 4 fish were observed there in January 2002.
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.
B. Video Surveys (P. Rand)- Stereo-Video System
We assembled an underwater stereo-video system for use in resolving threedimensional
attributes of grouper aggregations. The system consisted of two underwater
housings (Ikelite Model #6035.36) fitted with dome ports and mounted on opposite ends
of a stainless steel bar. The video cameras were SONY Model TRV-11 (single 1.4 type
CCD, 680,000 pixel resolution). The cameras were mounted to achieve an optical axis
separation of 60 cm. The zoom lens was fixed at wide angle (3.3 mm focal length), and
the cameras were set on auto-focus. While most of the footage was recorded in full color
mode, we did record one dive in NightShot mode to increase low light sensitivity. Video
records were archived on 60-minute DV tape format. A single diver operated the camera
system using handles placed between the two housings.
The stereo-video system was calibrated with a quadrat consisting of 1/4" welded
aluminum pipe. The quadrat dimensions were 50 cm X 50 cm X 30 cm. The pipe was
welded in a regular grid pattern on each face of the quadrat such that the nodes of
adjoining pipe were 10 cm apart. Images of the quadrat placed at 2 m and 4 m distances
from the camera assembly were captured on film. The calibration was conducted in
approximately 7-m water depth on 30 January 2002 at Conception Island, Bahamas. We
conducted calibrations with the quadrat located at 2 m and 4 m from the camera
assembly.
Because the two cameras were free running, it was necessary to synchronize them.
Frame synchronization was achieved using an underwater laser pointer (Model MBSL,
Class IIIA, max output <5mW, wavelength 635 nm) directed onto a light background. A
diver was stationed at a distance of approximately 3-4 m from the camera assembly and
repeatedly illuminated a white diver's slate. This operation was captured on film at depth
at the beginning of the dive, and again at the safety stop at ca. 7-m depth before surfacing.
Two frame synchronizations per dive enabled us to quantify frame drift over the elapsed
bottom time. We determined drift on a single dive to be no more than 3 frames over an
elapsed bottom time of 25 min (resulting in a drift of approximately 0.1 s). We
concluded that this source of error was tolerably low. To assure that we had proper frame
synchronization for a particular pair of images captured during a dive, we relied on
unique events occurring in the field of view of both cameras (diver movements, fish tail
beats or other maneuvers).
Figure 1. Nassau grouper spawning aggregations sites off Long Island, Bahamas. CSM =
Cape Santa Maria, HG = Hell’s Gate, LHN and LHS = Little Harbor north and south,
respectively, and SP = South Point.
Dive Recordings
In this report we conduct analyses of video footage from two separate dive
locations: South Point and Hell's Gate (Fig. 1) during January 2002. We present data on
nearest neighbor distances between members of a school, and determination of fork
length (FL) of individual fish. We captured an image of two Nassau grouper positioned
under a coral head at South Point on 26 January 2002 (Figure 2). During a morning dive
at South Point on 27 January 2002, we encountered a small aggregation of Nassau
grouper (numbering approximately 12 fish) in close proximity to a patch reef. We
captured paired images of 5-7 grouper as we approached the patch reef (Figures 3 & 4).
No color phases indicative of courtship or spawning were observed. Finally, we filmed a
school of Bermuda chub encountered during a morning dive at Hells Gate on 28 January
2002. The footage from these last two dives provides important verification information
for the acoustic data collected on these same fish aggregations (see Acoustic surveys,
below).
Video Analysis
Information extracted from digitizing the nodes on our calibration quadrat allowed
us to calculate the direction of lines of sight from each camera to a point of interest. We
digitized screen coordinates using a digitizer application designed for a NeXT
Dimensions computer (a NeXT cube with NeXT's video capture board). The software
used to compute real world coordinates in 3D from captured 2D screen coordinates was
provided by Lon Kelly (University of Alaska - Fairbanks). The method for 3D video
tracking is described in detail in Hughes and Kelly (1996). This approach rectifies
sources of systematic error such as perspective effects and spherical lens distortion. We
computed rectification parameters by digitizing 36 nodes from each of the near and far
faces of the quadrat captured by both cameras. We rotated the real world coordinate axes
such that x represented depth (vertical), y represented the axis perpendicular to the optical
axes of the cameras, and z represented distances running parallel to the optical axes of the
cameras (i.e. distance from camera).
We calculated real world coordinates of points midway between the nose and tail
of a fish in an aggregation to determine nearest neighbor distances within an aggregation.
For determination of FL, we digitized the nose and tail (at the fork of the caudal fin) from
each paired image (see arrows depicted on Figure 2) and computed the straight-line
distance between the two points fixed in 3D space.
Results
We computed lengths of two Nassau grouper at the South Point site at 65.7 cm
and 73.1 cm FL (#1 and #2, respectively, identified in Figure 2). We analyzed a pair of
images within seconds of identifying the aggregation. This was the point in time where
the divers had begun their initial approach of the aggregation. The aggregation was
approximately 9 m from the camera assembly. We could identify seven individual fish in
the aggregation, and we computed a nearest neighbor distance of 111 cm (ca. 1.7 body
lengths, computed using positions from individuals 1-5, identified in Figure 3). We
repeated this analysis for paired images recorded when the camera assembly was within 4
m of the aggregation. The fish aggregation appeared to respond to the presence of the
divers, and assembled into a contracted school in closer proximity to the patch reef
(Figures 4). The nearest neighbor distance of the five individuals identified in this image
pair was 92 cm (or 1.5 body lengths, Figure 4). It was not possible to get reliable
measures of FL on these fish due to coral obstructing the field of view. Finally, we
computed FL on a group of Bermuda chub (Kyphosus sectratrix) filmed at Hell's Gate.
We computed a mean FL of 50.2 cm (SD = 8.1, n = 9) and a nearest neighbor distance of
72.7 cm (or 1.5 body lengths).
We found the error in locating positions on the calibration quadrat was low (0.3
cm at 2 m, 0.8 cm at 4 m, estimated mean from the eight corners of the quadrat). In a few
cases, we found significantly higher error when we attempted to fix 3D spatial positions
in locations that departed from the position of the quadrat chosen for our system
calibration. In these cases, we chose the calibration parameters from the quadrat position
(either 2 m or 4 m positions) that most closely approximated the position of the fish in the
field of view. Designing a larger calibration quadrat for future work will help resolve this
problem.
In general, we found the stereo-video approach to work well for documenting 3D
attributes of fish aggregations and collecting reliable length estimates from free
swimming fish. The data on lengths of fish in the aggregation (and sex determination by
relying on color phases when filming occurs near the spawning period) should be
particularly helpful when compared to landings data to provide insights into fishing gear
selectivity (e.g. size/sex bias in catch). We feel the combination of using video and
acoustic sampling for surveying spawning aggregations represents a powerful tool for
assessing stock status and providing unique observations of fish during courtship and
spawning periods.
Figure 2. Two Nassau grouper positioned under a coral head. The paired video images
were recorded on a dive at South Point, Bahamas on 26 January 2002. Images were
recorded using a SONY video camera set in NightShot mode. Arrows indicate
approximate location of digitized points for nose and tail used to estimate fork lengths.
Fish were determined to be approximately 4 m from camera assembly. The individuals in
each paired image are identified with a unique number.
Figure 3. Paired video images captured of a small Nassau grouper aggregation at S.
Point. Distance of school from camera was determined to be approximately 9 m. Seven
individuals were identified in the frame.
Figure 4. Paired video images captured of a small Nassau grouper aggregation. Distance
of school from camera was determined to be approximately 5 m. Five individuals are
identified in the frame.
Figure 5. Paired video images captured of a Bermuda chub school at Hell's Gate on 28
January 2002. Fork lengths of individuals 1-9 were determined from digitized points of
nose and tail on each image pair. Members of the school are approximately 3 m away
from the camera assembly.
C. Acoustic Surveys (C. Taylor & P. Rand)
Background. Sonar technology, the process of transmitting and receiving sound reflected
from underwater objects, has been used in naval applications for nearly a century.
Applications of scientific sonar, or fisheries acoustics, was modified for uses in
assessments of fish population in the early 1950’s as a way to increase coverage of
sampling in the pelagic environment of large bays and oceans for herring. Since then,
applications of acoustics is as diverse as the areas of fisheries research including
enumerating migrating salmon, biomass assessments of demersal species such as cod and
haddock, and behavioral studies of fishes around hard structures such as oil rigs and dams
(McClennan and Simmonds 1992). Recent advances in fisheries sonar technology has
made it possible for portable equipment to be used on smaller bodies of water to assess
fish populations in inland waters, estuaries and coastal ocean systems. Advantages of
acoustic sampling include: 1) non-invasive method of sampling the fish community that
is less affected by size and gear selectivity, 2) collection of spatially continuous data
along a series of transects providing information on sub-meter to kilometer scales, 3) use
of “split-beam” technology, which permits collection of position of fish targets in 3-
dimensions, 4) rapid assessment of biomass and abundance through more expansive
coverage of an aquatic system, 5) reduce the number of man-hours and equipment
required to sample the populations using active and passive gear such as trawls and gill
nets. While the technology permits determination of acoustic size of fish targets,
identification of species still requires 1) supplemental sampling for species composition
and verification of size frequency distributions or 2) prior knowledge of behavior and size
distributions of resident species.
Applications of fisheries acoustics to the assessment of reef species are very rare,
primarily due to difficulties with 1) species identification and verification, and 2) an
inability to resolve fish targets that are closely associated with bottom relief. Acoustics
may be useful in situations where fishes form very large aggregations during spawning,
such as has been observed in cod as well as grouper, snapper and their allies. Formation
of very large aggregations (>1000 fish) makes visual censuses extremely difficult due to
limitations in bottom time, especially at depths near 30-m. In such cases acoustics
provides an advantage of increasing coverage of the survey by several fold.
Methods
We used a 200-kHz HTI Model 241 split-beam echosounder (Hydroacoustic
Technology Incorporated, Seattle, WA) coupled with a circular (6o nominal beam
dimension) transducer. The transducer was mounted to a 1.2-m long towbody (HTI
Model 621) rigged from a 2.5-m boom attached mid-ship on the starboard side. Towbody
depth was maintained at 0.5- to 1.5-m below the surface depending on sea conditions.
Rigging of the towbody included a shock-dampening system that minimized the
oscillations due to pitch and roll of the vessel. Vessel speed was about 2 m s-1. Data
collection parameters were specified by the user and controlled using a laptop computer
networked to the echosounder. Ping rate was 5 pulses s-1 and the pulse width was 0.18
ms during all transect runs. Maximum range of echo detection was set at 75 m, the
maximum range allowed given ping rate and sound velocity. Target resolution was
calculated based on pulse width and sound velocity and found to be approximately 0.1 m;
however, due to significant bottom relief, fish targets were not resolved from reefs at
distances less than about 1 m. At the beginning of the cruise we conducted an in situ
system calibration using a tungsten-carbide reference sphere of known target strength
placed greater than 5-m from the transducer. Gain parameters were adjusted accordingly
based on calibration results. Received acoustic signals were simultaneously adjusted for
spreading loss by applying 40-logR and 20-logR time-varied gain for split-beam and
echo-integration processing, respectively. The data were processed in real-time for splitbeam
and echo-integration (HTI DEP v. 3.53, HTI Seattle, WA) and stored as text files
on a laptop computer for later data analyses.
Acoustic data were processed using split-beam and echo-integration analyses.
Split-beam analysis was used to determine acoustic size (target strength) of individual
fish targets in decibels (dB). Algorithms were used to accumulate several consecutive
echoes from individual fish to produce an average acoustic size and position within the
water column (HTI Echoscape v. 2, HTI, Seattle, WA). Target strength is proportional to
fish size, and using established equations for reef species encountered during previous
studies (McClennan and Simmonds 1992, Ehrhardt and Deleveaux 1999), target strengths
were converted to fish size and verified during diver surveys. Only fish targets between –
50 and –30 dB were used for split-beam analysis, representing the range of fish sizes
observed by divers. Due to the limited number of fish aggregations observed, split-beam
target tracking was used to enumerate fishes and provide estimates of average target
strengths and sizes.
Surveys were conducted at five of the sampling sites, Hells Gate, South Point,
Little Harbor, N. Cape Santa Maria, and S. Cape Santa Maria (Table 1, Figure 1).
Surveys were attempted during morning, afternoon and evening hours, to correspond with
dive surveys at the same locations (Table 1). Acoustic survey design was a grid of 4 to 8
parallel transects 0.5 to 1.0 km in length and spaced approximately 100-m apart (e.g.,
Figure 6). In general, transects ran nearly perpendicular to shore from the 10- to 30-m
depth contour nearshore to >100-m offshore. Exact direction of the transects were
slightly modified during each survey to account for changes in wind, waves and swell that
would have produced pitching and rolling of the boat and cause significant oscillations in
the towbody. In all cases, amplitude of the oscillations of the towbody was kept to less
than 1-m. A second survey design was employed at the South Point location that
followed the 20 to 30-m depth, targeting the depths where fish traps were observed.
Acoustic survey spatial coverage ranged from 31,790 m2 for the contour transect at South
Point to over 760,000 m2 at Cape Santa Maria (north & south) (Table 1). These
coverages were two orders of magnitude greater than diver-surveyed spatial coverage at
the same sites, which ranged from 2000 to 7000 m2 (Table 1).
Results
No significant difficulties were encountered during acoustic data collection, even
during periods of relatively high winds and heavy waves and swell. We were able to
collect reasonable data in sea conditions as severe as 1.5 to 2-m swell by reorienting the
transects parallel to the direction of the swell or waves.
Surveys at all five sites failed to produce evidence of large aggregations (>100) of
fishes in the range of acoustic sizes that would likely be Nassau Grouper (Ehrhardt and
Deleveaux 1999). Only two sampling occasions resulted in observations of 20 or more
fish in a single “aggregation”. At South Point on 26 January, a group of 23 fish were
enumerated at 30 to 40 m deep (Figure 7). Average target strength of fish was -40.9 dB
(range: -45.4 dB to -34 dB). Using TS-length relationships from Ehrhardt and Deleveaux
(1999) resulted in estimated fish sizes of 68.0 cm. Diver observations confirmed
identification as Nassau Grouper and estimated sizes ranging from 50 to 70-cm TL. At
Hells Gate on 28 January 2002, a small aggregation of 26 fish was enumerated near the
bottom at approximately 30-m (Figure 8). Average target strength (TS) of these fish was
–40.5 dB (range = -36 to -45 dB) and estimated fish size using relationships for grouper
were on average 68.6 cm. However, two dive surveys of the same area confirmed that the
fish were Bermuda chub (Kyphosus sectratrix) ranging in size from 35 to 50 cm TL.
Results such as these emphasize the need for species verification and ground-truthing of
all acoustic surveys where species may have overlapping acoustic sizes and exhibit
similar aggregating, shoaling or schooling behaviors.
Table 1. Summary of acoustic sampling dates and estimated survey coverage for acoustic surveys and dive
surveys.
Date Time Site Transect
Acoustic Area
Searched (m2)
Dive Area
Searched (m2)
1/25/2002 Afternoon Hell's Gate Grid 91,000 3,500
1/26/2002 Morning South Point Grid 145,230
1/26/2002 Morning South Point 30-m contour 31,790
1/26/2002 Evening South Point Grid 145,230
1/26/2002 Evening South Point 30-m contour 31,800
1/27/2002 Morning South Point Grid 145,230
1/27/2002 Afternoon Little Harbor Grid 550,458 4,700
1/28/2002 Morning Hell's Gate Grid 374,832 3,000
1/28/2002 Afternoon Cape St. Maria S. Grid 761,120 4,600
1/29/2002 Morning Cape Santa Maria N. Grid 418,750 6,100
#
#
#
#
Cape
Santa Maria
CR II
Hells
Gate
Little
Harbour
South
Point
Long Island
#
N
0 10 20 Kilometers
Hells Gate
28 January 2002
0 100 200 300 Meters
0 100 200 300 Meters
South Point
26 January 2002
N
N
Figure 6. Map of Long Island, Bahamas and 5 sites sampled using acoustics. Example transects
for Hells Gate and South Point are shown in each of the squares. Arrow indicates direction of
vessel travel. Rectangles highlight areas
Figure 7. Excerpt of the echogram produced during acoustic data collection at South Point on 26
January 2002 and represented by the rectangle in Figure 6. Thick gray line and dark
surrounding echoes represent bottom. Box and arrow highlights small fish aggregation.
Statistics of fish are described in the text.
Figure 8. Excerpt of the echogram produced during acoustic data collection at Hells Gate on 28
January 2002 and represented by the rectangle in Figure 1. Thick gray line and dark
surrounding echoes represent bottom. Box and arrow highlights small fish aggregation.
Descriptions of fish may be found in the text.
Objective 4: Quantify the sex, size-at-age, and reproductive status of Nassau
grouper harvested from Long Island aggregations (A-M. Eklund & J. Schull)
To expand on our sampling regime from last year, we resampled Nassau grouper
landed in Long Island, Bahamas with the added objective to comparatively sample
landings from the central versus northern areas of the island. A total of 71 fish were
sampled on Long Island (16 from northern Long Island and 55 from Clarence Town) from
25-30 January, 2002. Fish landed whole were measured (TL and SL - mm) and weighed
with a Stren digital scale (lbs. and oz.) that had been calibrated prior to the trip. Entire
gonads were collected from each fish, as well as the third dorsal spine (for age analysis
and genetics), and otoliths (for age analysis). Gonad and spine samples were
immediately placed on ice. Gonads were later weighed to the nearest 5 grams (limitation
of Acculab digital balance) and a section of gonad was placed in a ziplock bag with 10%
formalin buffered with seawater. Spines were stored frozen until genetic tissue samples
could be taken. The spines were then dried, sectioned with a Beuhler Isomet saw and
used for ageing analysis. Otholiths were stored dry in glass vials. They were then
imbedded in epoxy and sectioned using a Beuhler Isomet saw. Sections were cleaned and
fixed to microscope slides using Protex mounting medium. Annuli will be counted using
a light microscope with emitted light (Summer 2002).
Results and discussion
The minimum total length of fish collected (N=71) was 325 mm and maximum
TL was 812 mm (mean = 587.4 mm) as compared to the 2000 sampling (N = 119, min
TL = 398 mm, max TL = 800mm, mean = 629mm). A total of 68 fish were landed with
gonads, but gonadasomal indices (GSI’s) could only be determined for 42 of these fish.
The remaining gonads were not weighed, but all were less than 5 grams (the balance used
to weigh the gonad samples was not calibrated properly). GSI’s ranged from a minimum
of 0.15 to a maximum of 11.26 (mean = 3.03) as compared to the 2000 samples which
ranged from 0.52 - 12.03 (mean 4.68). These GSI’s for 2002 were somewhat skewed due
to the fact that the 16 gonads < 5 g. could not be weighed due to the limitations in our
scale equipment.
The sexual strategy for Nassau groupers has still not been determined beyond
doubt. While some researchers believe the Nassau grouper is both bisexual and
gonochoristic with the potential for sex change (Sadovy and Colin 1995), others believe it
is essentially a protogynous, similar to many other epinepheline groupers (Smith 1959,
Garcia-Cagide and Espinosa 1991). This year’s sampling yielded much more diverse
samples than last year. We sampled fish from different landing sites (55 from Clarence
Town dock, and 16 from the Glynton’s area), and from spear caught and trap caught fish
(N = 12 and 59 respectively). The low catches and the small observed size of both fish
and gonads lead us to believe that fish taken in the northern area were not aggregating to
spawn. Shifting our effort to the Clarence Town area allowed us to maximize our effort
and sample as many fish as possible. Many of these fish did seem to be involved in
aggregative behavior. Of the samples collected, 41 could be sexed definitively (27
female, 14 male). Due to the questionable sexuality of these fish, 27 samples will need
further investigation before a sex ratio can be calculated (should be completed in June
2002). The presence of possible transitional individuals and small males is notable and
should add much information to the current body of knowledge on Nassau grouper
biology and grouper biology in general. Sampling in 2000 yielded a sex ratio of 4:1
(F:M), consistent with results from Colin (1992). Figure 9 depicts the sex at length
frequencies for both the 2000 and 2002 sampling.
In general, catches were poorer than our previous trip. Fishermen in general
thought the January 2002 full moon was “weaker” than November or December, with less
fish present and/or less “schooling behavior”. They also felt that although the November
2001 and December 2001 full moons were better than the January 2002 moon, catches, in
general, were poor this year. Many attributed the depressed catch to bad weather and
their inability to fish; many also blamed the researchers (both Pat Colin’s group in the
early 90's and Eggleston et. al from 2000) for spooking the fish and scaring them off. Our
group spent a considerable amount of time “mending these fences” by interacting with the
local fishermen and communities and explaining our research program.
Size Frequency of Nassau grouper (Dec 2000)
0
5
10
15
20
25
30
540-569
570-599
600-629
630-659
660-689
690-719
720-749
780-809
TL (mm)
Number of individuals
unknown
male
female
Size Frequency of Nassau Grouper (Jan 2002)
0
2
4
6
8
10
12
360-389
390-419
420-449
450-479
480-509
510-539
540-569
570-599
600-629
630-659
660-689
690-719
720-749
780-809
810-839
TL (mm)
Number of Individuals
unknown
male
female
Objective 5: Fish movement patterns
Nassau grouper were captured using traditional Bahamian fish traps 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 (primarily 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 during 17 November-26 December 2000 and 17-21
December 2001 by searching transects from the shelf edge to the beach 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 (Figure 10). Thirty-three fish ranging in size from 22.0
to 70.2 cm total length (TL) were given external tags only. Twenty-three fish (1 male, 13
females, and 9 of unknown sex) ranging from 42.6 to 72.2 cm TL received transmitters in
addition to external tags (Table 2). An additional 10 fish were sacrificed to obtain gonad
and otolith samples for the age, growth and reproductive biology component of this study.
Three of the telemetered fish were not relocated following release. The remaining
20 fish were relocated a total of 183 times during the 2000 field season, 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 2-14 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 between late November and early December 2000. 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.
During the 2001 field season, limited searches were made from between 17-21
December. Eleven fish tagged during the previous field season were relocated during
these searches for a total of 46 relocations. Searches were limited in scope and duration
due to unfavorable sea conditions. None of the large fish which disappeared the previous
field season were relocated during search efforts. In general, it appears that most fish
remained in the same areas as they were encountered the previous sampling season
(Figure 2). One female fish (#1447) which was last relocated on 17 December 2000 was
recaptured by a commercial fisherman using a fish trap. This fish had moved a
considerable distance (>20 km) when it was harvested on 20 November 2001.
Telemetered grouper typically remained near their tagging locations, 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. One exception to this was a 55.6 cm
female (#1450B) which was tagged along the shelf edge and then moved inshore in the
general area of the Hill House. This fish was relocated in 2000 and 2001 in nearshore
reef habitat several kilometers from where it was captured and tagged in 2000.
Preliminary analyses suggest that movements of telemetered fish were limited to
high-relief reef and low-relief hard-bottom habitat, with no detected 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. When a fish is not relocated in the LSI area, it could
indicate migration to a spawning aggregation or harvest. Searching is difficult because of
the potentially great distances involved, the highly variable weather (listening) conditions,
and the possibility that the telemetered individual has been harvested. Two transmitters
(1450, 1451) were returned to us by fishermen and trap fishing was observed in the area
surrounding LSI during both field seasons. 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 temporarily lost contact with individual
fish when they moved behind reef structures. An area should be searched several times to
ensure that fish are not merely inhabiting crevices. Because of these practical problems
coupled with successful tag returns by commercial fisherman, 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 through
a reward program. High rewards should be used to insure that the tag reporting rate is
essentially 100 percent.
Table 2. Characteristics of Nassau grouper receiving sonic transmitters during November-
December 2000. Transmitters 1450 and 1451 were returned by commercial fishermen and used
to tag two additional fish (1450B and 1451B).
Fish # Date
Collected
Size
(cm)
Sex Tag Frequency
(kHz)
Tag Interval 2000 Relocations 2001 Reloca
1435 11/17/00 51.5 Unknown 50.0 1006 19 5
1436 11/26/00 59.5 Female 50.0 1104 14 5
1437 11/30/00 69.4 Female 50.0 1200 0 0
1438 11/20/00 72.2 Male 50.0 1296 5 0
1439 11/26/00 54.4 Female 50.0 1392 11 5
1440 12/12/00 67.2 Female 50.0 1488 1 4
1441 12/10/00 54.6 Unknown 50.0 1584 2 0
1442 11/21/00 56.8 Female 67.3 1058 16 0
1443 11/30/00 62.2 Female 67.3 1159 4 0
1444 11/29/00 64.3 Female 67.3 1260 0 0
1445 11/25/00 66.0 Female 67.3 1361 4 0
1446 11/30/00 61.6 Unknown 67.3 1462 9 1
1447 12/12/00 62.2 Female 67.3 1562 3 0*
1448 11/16/00 44.5 Unknown 67.3 1663 19 0
1449 11/28/00 49.3 Unknown 76.8 1050 10 5
1450 11/29/00 50.2 Female 76.8 1150 6 0
1450B 12/14/00 55.6 Female 76.8 1150 2 5
1451 11/21/00 47.4 Unknown 76.8 1250 3 0
1451B 12/12/00 63.4 Female 76.8 1250 0 0
1452 11/21/00 53.0 Unknown 76.8 1350 15 5
1453 11/25/00 70.4 Female 76.8 1450 13 2
1454 12/02/00 57.3 Unknown 76.8 1550 7 3
1455 11/16/00 42.6 Unknown 76.8 1650 20 5
*Fish harvested by commercial fisherman using fish trap on 11 November 2001
20
Figure 10. Locations where Nassau grouper were tagged with either an external tag only or an
external tag and sonic transmitter during November-December 2000.
.
(
(
(
(
(
(
(
(
(
(
((
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(
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)
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Bock Wall
Bock Cay
Whitehorse
BB Mooring
Batcaves
Horseshoe
South Perry
0 4 8 2 km
) Captures w/o sonic tag
( Captures w/ sonic tag
±
21
Figure 11. Sites where telemetered Nassau grouper were located in November-December 2000
and December 2001.
(((( (((( (((((
(( (
(
(
(
(( ((
((
(
( (
(((( (
(
(
( ( (( ((
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(((((((
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(
( (((
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( ((
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((((((
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(((( (((((
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)
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)
)
Bock Wall
Bock Cay
Whitehorse
BB Mooring
Batcaves
Horseshoe
South Perry
0 4 8 2 km
( Relocation 2000
) Relocations 2001
±
PAPERS PRESENTED BASED ON 2001-02 WORK:
Eggleston will be presenting this research at the GCFI meeting in Mexico in Fall 2002. Several
other presentations are planned.
MANUSCRIPTS AND PUBLICATIONS:
A total of five manuscripts are planned once data analyses are complete (Eggleston et al.; Fox et
al.; Eklund et al.; Taylor et al.; Rand et al.). We have 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 precautionary
management approach for Nassau grouper through seasonal fishing closures during
wintertime spawning periods.
22
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.
Hughes, N.F. and L.H. Kelly. 1996. New techniques for 3-D video tracking of fish swimming
movements in still or flowing water. Can. J. Fish. Aquat. Sci. 53:2473-2483.
MacLennan, D.N. and E.J. Simmonds. 1992. Fisheries Acoustics. Chapman and Hall Fish and
Fisheries Series 5, New York.
Ehrhardt, N.M. and V. Deleveaux. 1999. Report on the 1999 Nassau grouper stock assessment in
the Bahamas. |
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