Fisheries Service, 74 Magruder Road Highlands, NJ 07732. Dixie Highway ...... Metairon, B. Bower-Dennis, J. Chaplin, R. Gomez, L. Hambrick,. R. Jones, C.
Melody Ray-Culp
Journal of Shellfish Research, Vol. 16, No. I , 7-18, 1997
ABUNDANCE AND DISTRIBUTION OF QUEEN CONCH VELIGERS (ST'ROMBUS GZGAS LINNE) IN THE CENTRAL BAHAMAS. I. HORIZONTAL PATTERNS IN RELATION TO REPRODUCTIVE AND NURSERY GROUNDS ALLAN VV. STONER* AND MEGAN DAVIS' Caribbean Marine Research Center 805 E. 46th Place Vero Beach, Florida 32963 ABSTRACT Veliger larvae of the large gastropod Strombus gigas (queen conch) were collected over a 7-y period at a reproductive site in the Exuma Sound, in adjacent tidal inlets, and on the Great Bahama Bank near Lee Stocking Island, Exuma Cays, Bahamas. Although the spawning season for queen conch in the region occurs from April through October, larval abundance was highest during mid summer. between late June and August. when temperatures were >28"C. Metamorphically competent larvae were most abundant in July and August and made up a small percentage of the total count, reflecting high natural mortality. Although there was considerable interannual variation, veliger densities near Lee Stocking Island (typically 1-2 individualdl0 m') were much higher than estimates made in the eastern Caribbean and Florida Keys. Because most of the collected larvae were newly hatched, regional differences in larval abundance appear to he associated with size of local spawning stock. Highest densities of larvae were found on the Great Bahama Bank directly associated with axes of tidal currents and decreased with distance onto the bank. Intensive sampling in one tidal flow field showed that total larval densities as well as densities of latc-stage veligers were highest at a well-established nursery site. Larval transport and retention may explain the general occurrence of nurseries at locations where water from the Exuma Sound flows onto the Great Bahama Bank on flood tides. Large, stable aggregations of juvenile queen conch were consistently supplied with high densities of larvae and were directly associated with tidal pathways. In contrast, more ephemeral aggregations were characterized by low or inconsistent veliger densities (particularly late-stage larvae) and were generally outside primary tidal current pathways. Queen conch distribution appears to be directly related to the horizontal supply of larvae.
k X Y WORDS: Bahamas, larval transport, oceanography, recruitment, Sti-ornbzis gigas
INTRODUCTION
1
*
1994), and from a one-time survey of 14 stations in the eastern Caribbean Sea (Posada and Appeldoom 1994). With the rapid decline in queen conch populations throughout the species' geographic range (Appeldoorn et al. 1987, Appeldoorn 1994), it is increasingly important to understand larval transport and recruitment processes on the local and regional scale. Berg and Olsen (1989) pointed out the possibility that the conch fishery in many nations might depend on upstream sources of larvae and that this should be taken into account for effective management of the species. New data, for example, suggest that recovery of the severely depleted Florida conch population will depend on the transport of larvae from Caribbean nations (Stoner et al. 1996a, Stoner et al. 1997). Here, we summarize the findings from 7 y (1988-1994) of field sampling for queen conch veligers near LSI. Our primary purpose is to provide seasonal data on the abundance of conch larvae and examine distribution over a range of habitats, from spawning sites on the island shelf to shallow regions on the Great Bahama Bank near historically important nursery grounds. We also compare annual abundance and size structure of veligers collected at some nursery areas that contain large and stable juvenile populations and at some that have been ephemeral. These data are useful in interpreting the relationships between larval supply and juvenile population size and help to explain patterns of nursery distribution.
The large gastropod Strombus gigas Linne (queen conch) is an important fisheries resource in the wider Caribbean region (Berg and Olsen 1989, Appeldoorn 1994). Despite its culture in hatcheries for nearly 20 y (Creswell 1994, Davis 1994a), knowledge of the natural history and field distribution of the veliger larva is limited to the most basic facts. The spawning season lasts 24-36 wk and varies slightly throughout the Caribbean, depending on temperature and photoperiod (Stoner et al. 1992). After 3-5 days, conch veligers hatch from their benthic egg masses at 300 p m shell length (SL) (D'Asaro 1965, Davis 1994a); they then live in the water column for 18-26 days, depending on physical and trophic conditions (Laughlin and Weil 1983, Siddall 1983, Davis et al. 1993). Laboratory experiments showed that queen conch larvae are positively phototactic and negatively geotactic, suggesting that most will be found near the sea surface (Barile et al. 1994). At 0.9-1 .O mm SL, the veligers are competent to undergo metamorphosis on contact with a variety of substrata found in natural nursery areas (Davis 1994b, Davis and Stoner 1994). The morphological development of queen conch veligers was first described by D' Asaro (1965), but a comparative description of larval morphology adequate to identify different Caribbean Strombus species has only recently become available (Davis et at. 1993). The only published data on larval abundance in the field are from Lee Stocking Island (LSI), in the central Bahamas (Chaplin 1989, Chaplin and Sandt 1992, Stoner et al. 1992, Stoner et al.
METHODS Study Sites
During seven spawning seasons (1988-1 994), plankton collections were made in the vicinity of LSI, Bahamas (23"46'N, 76'06'W) (Fig. I), where there is only light fishing pressure. Large populations of queen conch juveniles occur on the Great Bahama
*Present address: Northeast Fisheries Science Center, National Marine Fisheries Service, 74 Magruder Road Highlands, NJ 07732. 'Present address: Harbor Branch Oceanographic Institution, 5600 Old Dixie Highway, Fort Pierce, FL 34946.
7
STONER AND DAVIS
8
\
NEIGHBOR CAY
L
GREAT BAHAMA BANK
Figure 1. Map showing 13 plankton collection stations in the vicinity of LSI, Exuma Cays, Bahamas. Stations were located in and around known nursery grounds. Station labels with asterisks indicate sites that were occupied by juvenile conch. Solid lines and arrows indicate the primary pathways for flood tidal currents. Dashed lines show secondary pathways. Elliptical areas represent the general locations of long-term nurseries near Shark Rock (SR2*) and Children's Bay Cay (CBC2"). Stippled areas represent shallow sand bars.
Bank (Stoner et al. 1994), and adults are abundant in the Exuma Sound (Stoner and Schwarte 1994, Stoner and Ray 1996). The islands of the Exuma chain are bordered on the west by shallow banks (mean depth, -4 m) and on the east by the deep Exuma Sound. On flood tides, oceanic waters from the Exuma Sound flow onto the bank through numerous inlets on the flood tide and mix with bank water. For the purpose of this study, we assumed that queen conch larvae were carried from the offshore spawning sites to the bank on the tidal currents. Velocities through the 5- to 8-m-deep inlets typically reach 50-100 cm/sec at maximum flood. The tide is semidiurnal with a range of approximately 1 m. Winds are predominately from the ESE during the summer spawning season, with wind speeds typically 3-6 m/sec (Caribbean Marine Research Center, unpubl. data). To observe seasonal variation in veliger density in a reproductive area, plankton collections were made at station RS, located approximately 1 km east of LSI on the island shelf in the Exuma Sound (Fig. 1). Adult conch are abundant at RS on an Ill-m-deep platform covered with sand and algae (Stoner and Sandt 1992). Sampling schedules are explained under Plankton Collections. Drifter studies have shown that tidal waters from the Exuma Sound flood through inlets north and south of LSI and into two corresponding flow fields. The north tidal system passes over conch nurseries near Shark Rock and Tugboat Rock, and the south system passes over a nursery west of Children's Bay Cay (Stoner et al. 1994, Stoner et al. 1996b) (Fig. I). Plankton collections were made to determine seasonal and geographic distribution of veligers with respect to the tidal current patterns. Plankton collections were made at four stations along the primary axis of tidal flow between Adderley Cay and Cook's Cay (stations SRl, SR2*, SR3, and
SR4) and at two stations along a secondary axis of tidal flow between LSI and Tugboat Rock (SR5 and SR6*). Plankton collections were also made at three stations in the Children's Bay Cay flow field between the inlet and Windsock Cay (CBC1, CBC2*, and CBC3). One additional station was sampled in the middle of the bank (MB), between the two primary flow fields but well outside the primary tidal currents. No juvenile conch have ever been observed at the nonnursery stations, although adults are occasionally observed over most of the Great Bahama Bank and island shelf adjacent to LSI. Stations located in known nursery grounds are indicated with asterisks in the station code (e.g., SR2*). The Shark Rock (SR2*) and Children's Bay Cay (CBC2*) nurseries have been occupied by large, stable juvenile queen conch populations that often contain between lo4 and lo5 individuals (Stoner et al. 1996b). Juvenile populations at SR6* usually contain O.S mm SL were collected in the vicinity of NBC"; however, this occurred only during peak reproductive months (June through August) (Table 5 , Fig. 9), and densities were typically an order of magnitude less
o 1989
A
1990 A 1992
* A A
MaY
Jun
A
Jul
SeP
Months Figure 6. Density of queen conch veligers collected at inlet station SRl during three spawning seasons (1989, 1990, and 1992). Each point represents the mean for two plankton collections.
HORIZONTAL DISTRIBUTION OF QUEENCONCH LARVAE
13
SR1 8r
0 newly-hatched
= SR2*
12
-
10
*
mid-size late-stage
SR2*
l2
r
SR2*
0 6 4 2
0
SR4
n
SR6* 4
SRV 4
Figure 7. Mean density of queen conch veligers collected at seven stations in the Shark Rock flow field during three spawning seasons (1992, 1993, and 1994). Densities are reported by size category. Each column represents the mean for two plankton collections made on 13-14 sampling dates between May and September. Newly hatched veligers were 300-SOO pm SL, midsize veligers were 500-900 pm SL, and late-stage veligers were 2900 pm SL.
than those at the sites with larger juvenile populations. In the 2 y of sampling at SR6*, late-stage larvae were common (20.1 veliger/l0 m3) only on single dates in both 1992 and 1993 (Table 3). The ephemeral nursery at CHB" was sampled during only one season (1992) and yielded the lowest mean density of veligers among stations sampled that year (Table 1); midsize veligers were never collected, and only two late-stage veligers were collected (Fig. 9). DISCUSSION
The reproductive season for queen conch at LSI extends from mid-April to early October (Stoner et al. 1992); however, veligers
were collected only between the end of May and late September, with the vast majority occurring in a relatively narrow period between June and August. Although no correlation has been found between water temperature and reproductivity of queen conch at the study site (Stoner et al. 1992), seasonality of larval production associated with high summer temperature may be an adaptive strategy to shorten the time to metamorphosis and improve survivorship through the planktonic stage (Scheltema 1986). In laboratory culture, growth rates of queen conch veligers were highest in temperatures between 28 and 32"C, slowed at 24"C, and rapidly declined to near zero at 20°C (Stoner and Davis, unpubl. data). It is possible that production or types of phytoplankton food avail-
STONER AND DAVIS
14
TABLE 1. Density of S. gigus veligers (no. of veligers/lO m3) during the peak reproductive months (July and August) of 5 y at 13 stations near LSI, Bahamas. Years Sites Shark Rock flow field SR1 SR2” SR3 SR4 SR5 SR6* Children’s Bay Cay flow field CBC 1 CBC2* CBC3 Middle Bank (MB) ephemeral nurseries NPC* NBC” CHB”
1988
1989
1992
1993
1994
1.99i0.33 (4)
2.41 i 0.82 (4) 4.20 i 1.77 (5) 2.31 i 0.80 (4) 2.52 i 1.06 (4)
1.94 i 0.70 (7) 3.61 i 1.00 (7)
1.61 10.39 (7)
3.89 i 1.22 (7)
1.1210.37 (7)
0.43 rt 0.02 (2)
1.90 i 0.91 (4) 0.98 i0.15 (3) 0.23 50.06 (2)
1.98 i 0.41 (7)
0.50 i 0.34 (7)
2.00 rt 0.46 (7)
1.81 i-0.87 (7)
0.68 i 0.16 (7) 0.16 i 0.05 (7)
0. I7 i 0.08 (7)
1.00 i0.42 (7)
0.01 i o . O 1 (3) 2.56 i 1.03 (3) 0.47 i 0.18 (2)
Stations located in known nursery grounds are indicated with asterisks. Values are mean I SE (n = number of sampling dates used to calculate the mean; two replicate tows were collected on each date).
able to larvae have affected the seasonality of reproduction in queen conch; however, recent measurements of chlorophyll a near LSI and throughout the Exuma Sound, in November 1993 and June 1994, have shown that concentrations of chlorophyll change little with season (A. Stoner, unpubl. data). The midsummer reproductive strategy in queen conch appears to be linked primarily to physical cycles in the environment, especially temperature and photoperiod, as suggested by Stoner et al. (1992), and not variation in phytoplankton biomass. Because adult conch are abundant in 10- to 20-m depth all TABLE 2. Mean density of midsize (500-900 pm SL) and late-stage (>900 pm SL) veligers of S. gigus collected during three reproductive seasons in the Shark Rock flow field at nursery site SR2”. Density of Veligers (no. * 10 m-3) 1992
1993
1994
TABLE 3.
Date
Mid
Late
Date
Mid
Late
Date
Mid
Late
5/20 611 619 6/18 711 718 7/20 713 1 815 8/18 8/29 914 9/16
0 0 0.047 0.029 0.768 0.076 0 0
0 0 0 0 0.530 0.025 0 0 0 0.062 0.025 0 0.027
5/28 614 6/10 612 1 6/27 716 7/14 7/23 713 1 8/10 8/19 8/26 919 9/16
0
0 0 0 0 0 0 0 0.039 0 0.038 0.016 0 0 0
5/27 612 6/10 6/19 6/30 719 7/15 7/25 811 8/12 8/22 8/29 917 9/14
0 0 0 0.020 0 0 0.016 0.020 0 0 0 0 0.022 0
0.053 0 0.295 0 0 0.017 0 0 0 0 0 0 0 0
0 0.031 0 0 0
0 0 0.024 0 0.014 0 0.058 0.015 0.019 0 0.019 0 0.041
along the Exuma Island chain (Stoner and Schwarte 1994, Stoner and Ray 1996) and because of the typically southeast to northwest alongshore drift (N. Smith, unpubl. data), it is likely that LSI receives larvae from spawning stocks to the south. Densities of midsize and late-stage larvae, therefore, are subject to events that are kilometers to tens of kilometers upstream in the Exuma Cays, whereas newly hatched veligers represent the local spawning stock. The significance of alongshore larval drift for the recruitment of dungeness crabs on the Pacific Coast of Washington was reported by McConnaughey et al. (1992). A large literature has developed with respect to how larval fishes (Rowe and Epifanio 1994) and invertebrates (Heron et al. 1994) enter estuarine nursery areas by selecting particular strata on the flood tides (i.e., selective tidal stream transport, scmu Boehlert and Mundy 1988); however, there are very few data for molluscs (Mann 1988), and the mechanisms in nonestuarine systems may be quite different. Most likely, in the Exuma Cays, conch veligers are
Overall abundance data are shown in Figure 7.
Mean density of midsize (500-900 pm SL) and late-stage (>900 pm SL) veligers of S. gigus collected during two reproductive seasons in the Shark Rock flow field at the small nursery site SR6*. Density of Veligers (no. * 10 m-’) 1992
1993
Date
Mid
Late
Date
Mid
Late
711 718 9/16
0.080 0.050 0.025
0.135 0.030 0
612 1 7/23 713 1 8/19 8/26
0.016 0.019 0.084 0 0.028
0.100 0.038 0.084 0.018 0.048
Sampling dates with only newly hatched veligers were not included. Overall abundance data are shown in Figure 7.
HORIZONTAL DISTRIBUTION OF QUEEN CONCHLARVAE
22
3
7
0
$2
1992
1993
0newly-hatched
32
; $
9
mid-size
=
15
z
;
$
1994 late-stage
Figure 8. Mean density of queen conch veligers collected in the Children's Bay Cay nursery area (CBC2*) during three spawning seasons (1992, 1993, and 1994). Densities are reported by size category. Each column represents the mean for two plankton collections made on 13-14 sampling dates between May and September. Newly hatched veligers were 300-500 pm SL, midsize veligers were 500-900 pm SL, and late-stage veligers were >900 pm SL.
transported northwest on the alongshore current and then through passes between the islands on flood tide. Whether or not the larvae use behavioral processes to enter the inlets or remain on the bank is unknown; however, queen conch larvae migrate vertically over a few meters or tens of meters on a diurnal periodicity (Barile et al. 1994, Stoner and Davis 1997), and they may respond to salinity or temperature gradients that occur in the inlets. The most parsimonious explanation for the presence of large queen conch nurseries along the Exuma Cays island chain is the net hankward flow of shelf water (Smith and Stoner 1993). Data from surface drifters indicate that once the veligers are drawn through the numerous inlets, they will be transported to nursery sites on the shallow bank (Stoner et al. 1996b). Plankton data presented in this study confirm the hypothesis that larval densities were high in the primary tidal streams, low in secondary
branches, and near zero in areas that do not receive regular incursions of water from the Exuma Sound. These new data provide an explanation for the observation that queen conch juveniles are absent from large, seemingly appropriate benthic habitats of seagrass outside major tidal currents in the Exuma Cays (Stoner et al. 1994). In fact, the highest larval densities occurred not only in association with the flow fields, but directly over the primary nursery ground at Shark Rock. Although significant densities of conch larvae were collected at sites farthest from the Exuma Sound (e.g., 5 km beyond the nursery), no rnidstage or late-stage larvae were ever found at locations beyond the nurseries. Comparable to our observations with queen conch, Field and Butler (1994) found that the postlarvae of spiny lobster (Panulirus argus) rarely settled beyond the emergent banks that ring Florida Bay. They concluded that the postlarvae were not regularly transported to the interior of
NBC* 4~
21-
NBC*
n
IIn
4 1
2t
*t
0 newly-hatched mid-size late-stage
m
a
a
d
1992 Figure 9. Mean density of queen conch veligers collected at the ephemeral nursery sites, Neighbor Cay (NBC*) and Charlie's Beach (CHB*), during two spawning seasons (1992 and 1993). Densities are reported by size category. Each column represents the mean for two plankton collections made on 13-14 sampling dates between May and September. Newly hatched veligers were 300-500 pm SL, midsize veligers were 500-900 pm SL, and late-stage veligers were >900 pm SL.
STONER AND DAVIS
16 TABLE 4.
Mean density of midsize (500-900 pm SL) and late-stage (>900 pm SL) veligers of S. gigas collected during the three reproductive seasons in the Children's Bay Cay flow field at nursery site CBCZ". Density of Veligers (no. * 10 m-3) 1992
1993
1994
Date
Mid
Late
Date
Mid
Late
Date
Mid
Late
5/20 6/l 6/9 6/18 711 7/8 7/20 7/31 8/5 8/18 8/29 914 9/16
0 0 0 0 0073 0 105 0019 0022 0 0 0337 0 0
0 0 0024 0 0 176 0 0 0 0 0 0 0 0
5/28 614 6/10 6/21 6/27 7/6 7/14 7/23 7/31 8/10 8/19 8/26 919 9/16
0 0021 0 0103 0015 0016 0 0206 0 176 0 0411 0046 0018 0040
0 0 0 0031 0 0 0 0018 0 059 0 0629 0 0036 0
5/27 612 6/10 6/19 6/30 7/9 7/15 7/25 8/1 8/12 8/22 8/29 917 9/14
0 0 0 0081 0 0 0 0022 0 0 0 0 0023 0
0019 0 0 609 0181 0 0 0 022 0 0 0 0 0 020 0045 0
Overall abundance data are shown in Figure 9
the bay, analogous to our findings for late-stage conch larvae on the Great Bahama Bank. There is now considerable literature indicating that spatial and interannual heterogeneity in larval supply has an important influence on the settlement and recruitment of invertebrates (Yoshioka 1982, Gaines et al. 198.5, Olmi et al. 1990, Bertness et al. 1992, Peterson and Summerson 1992, Martel et al. 1994) and fishes (Milicich et al. 1992). Sites with ephemeral juvenile conch populations had more sporadic densities of larvae than did the larger and more stable nursery sites near Shark Rock and Children's Bay Cay. For example, the ephemeral Tugboat Rock population (SR6*) had a lower mean density of veligers than the Shark Rock nursery (SR2*). SR6* lies in a secondary tidal branch associated with the inlet north of LSI and probably does not receive oceanic water on every tide. as does station SR2*. Low tidal current velocities would also reduce the flux of larvae to a site. Densities of larvae observed at NBC* and CHB" were typically much lower than
TABLE 5. Mean density of midsize (500-900 pm SL) and late-stage (>900 pm SL) veligers of S. gigas collected during two reproductive seasons at the ephemeral nursery site NBC". Density of Veligers (no. . 10 m-3) 1992
1993
Date
Mid
Late
Date
Mid
Late
711 718 7/20
0.025 0.025 0.040
0.025 0.025 0
6/27 713 1 8/10 8/26
0 0.030 0.016 0.016
0.016 0.058 0.033 0.017
Sampling dates with only newly hatched veligers were not included.
those at the more stable nursery sites, and densities of midstage and late-stage larvae were very low and erratic. It is likely, therefore. that population size and stability are related to the quantity and regularity of larvae arriving at a nursery. These results indicate that the importance of presettlement processes should be considered in the distributional ecology and management of queen conch populations. Transplant experiments with juvenile conch near LSI (Stoner and Sandt 1992, Ray and Stoner 1994, Stoner et a]. 1994) have shown that: (1) some habitats without resident conch can support juveniles, (2) conch nursery grounds are probably not saturated with juveniles in most years, and (3) recruitment probably limits the number of individuals in a nursery ground. We have also concluded that the settlement of conch larvae is not random. The Shark Rock nursery area possesses specific biological cues that induce a higher settlement rate than areas with seemingly similar general features both upstream and downstream from the nursery (Davis and Stoner 1994). Therefore, long-term queen conch nurseries, whether supporting large, stable populations of juveniles or small and ephemeral populations, are associated with a combination of important attributes: (1) hydrodynamic properties that supply and retain larvae, and (2) unique benthic characteristics that attract settlement of competent larvae and provide food and shelter for juveniles. Although densities of larvae, particularly late stages, probably affect the distribution and abundance of juvenile conch in the nursery habitats, the abundance of early-stage larvae is undoubtedly influenced by the density or abundance of nearby spawners. Although few data exist for densities of queen conch larvae, some comparisons can be made on a regional scale. Densities of queen conch veligers were typically 1-2 larvae/10 m3 during the primary reproductive season near LSI, with some densities as high as 10 veligerd10 m3. In the Florida Keys, densities rarely exceeded 0.5 veligers/10 m3 between 1992 and 1994, even near important spawning sites (Stoner et al. 1996a, Stoner et al. 1997). The mean density of queen conch veligers in surface tows made by Posada and Appeldoorn (1994) in a July 1989 cruise along the islands of the eastern Caribbean Sea from Martinique to the Grenadines was 0.18 larvae/lO m3 (SD = 0.33, n = 19). The highest value in a single tow was 1.22 veligerdl0 m3, found downcurrent from the important conch-producing banks of the Grenadines. Densities higher than those near LSI have been found only in the northern Exuma Sound, Bahamas. Stoner and Ray (1996) reported values commonly between 2.5 and SO queen conch veligers/10 m3 in repeated samplings in the Exuma Cays Land and Sea Park during 1993 and 1994. Most larvae in collections just described have been newly hatched individuals; therefore, regional variation in observed densities is probably a direct function of spawning stock size or density in the surrounding areas. The density of adult conch in the Florida Keys was
