POPULATION STRUCTURE, SEASONAL MOVEMENTS AND FEEDING OF QUEEN CONCH, STROMBUS GIGAS, IN DEEP-WATER HABITATS OF THE BAHAMAS

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1 BULLETN OF MARNE SCENCE, 51(3): ,1992 POPULATON STRUCTURE, SEASONAL MOVEMENTS AND FEEDNG OF QUEEN CONCH, STROMBUS GGAS, N DEEP-WATER HABTATS OF THE BAHAMAS Allan W Stoner and Veronique J. Sandt ABSTRACT Population structure, seasonal movement, and feeding ecology were examined for an 18- m-deep, unfished population of queen conch (Strombus gigas) in the southern Exuma Cays, Bahamas, More than 99% of the conch at the study site were sexually mature. Shell lipthickness data showed that the average age was at least 6 years; however, growth in thickness between 1988 and 1991 indicated that thickness is not a satisfactory means of aging conch once erosion of the shell begins, A 4-year survey suggested that population size was relatively constant, despite advanced age of the conch. Tag-recapture data indicated highest abundance of conch in July as a result of immigration from surrounding hard-ground feeding areas in May and June, and emigration in August and September. These population shifts were corroborated by survey data and by movements of individual conch, Densities of queen conch on hard-ground mounds and in coral rubble habitats were highest in winter, Movement to sand habitat in spring was correlated with increasing bottom-water temperature until the period of maximum reproductive activity and highest density on sand in July. Little feeding occurred in the sand habitat. Return to hard-ground and rubble occurred before a decline in water temperature, There was a significant inverse relationship between density of conch on winter hard-ground habitats and photoperiod length. The queen conch, Strombus gigas Linne, is a large gastropod mollusc important to the fisheries of the Caribbean region. The general life history is well known and has been reviewed by Randall (1964) and Brownell and Stevely (1981). After a planktonic life of 3 to 4 weeks (Brownell, 1977; Davis et a., 1988; Mianmanus, 1988), queen conch settle and metamorphose in shallow subtidal habitats where they spend much of the first year of life buried in the sediment (versen et a., 1990). After emergence from the sediment the conch move to increasingly deep water, usually from nearshore seagrass habitats to deeper seagrass or algal flats (Randall, 1964; Weil and Laughlin, 1984; versen et a., 1987). Sexual maturity is reached between 3.5 and 4 years, a few months after the flared lip is formed (Egan, 1985; Wilkins et a., 1987; Appeldoom, 1988). Longevity is at least 6 to 7 years (Berg, 1976; Wefer and Killingley, 1980) and is perhaps as long as 26 years in deep-water habitats (Coulston et a., 1988). Seasonal migrations, usually associated with summer spawning, are known for queen conch. Weil and Laughlin (1984) reported that adult conch at Los Roques, Venezuela move from offshore feeding areas in the winter to summer spawning grounds in shallow, inshore sand. Movements to shallower summer habitats have also been reported for deep-water populations at St. Croix (Coulston et a., 1988). Near Lee Stocking sland, Bahamas, deep-water conch make seasonal migrations from algae-covered hard grounds to spawning sites on bare sand (Wicklund et a., 1988; Stoner et a., 1992). Heavy fishing has reduced significantly the numbers of queen conch in many areas of the Caribbean (Adams, 1970; Hesse, 1979; Appeldoom et a., 1987; Berg and Olsen, 1989), particularly in shallow, inshore waters where conch are collected by free-diving. Most of our knowledge of the species comes from studies in those shallow-water areas. However, queen conch also inhabit continental and island shelves to depths of m (Randall, 1964; Alcolado, 1976; Brownell, 1977; 287

2 288 BULLETN OF MARNE SCENCE, VOL. 51, NO.3, 1992 EXUMA SOUND...."' 'Shark Rack EXUMA BANK 1 km Figure. site (*) Map showing the southern Exuma Cays, Lee Stocking sland, and the location of the study Appeldoorn et ai., 1987). Given the possibility that these deep-water populations now sustain shallow-water fisheries through their reproductive efforts (Wicklund et ai., 1988; Stoner et ai., 1992), it is critical that we learn more about the abundance and ecology of queen conch in deep waters. This study was designed to examine the population structure, distribution, and seasonal movements of queen conch in an 18-m-deep site typical of unfished, deep-water habitats in the Bahamas. Movements are discussed in terms of reproductive biology, feeding ecology, seawater temperature, and photoperiod. STE DESCRPTON This investigation was conducted near Lee Stocking sland in the southern Exuma Cays, Bahamas (Fig. ). The islands and cays of the Exuma chain are bordered on the west by the shallow Great Bahama Bank (locally called the Exuma Bank) and on the east by the deep Exuma Sound. Approximately 1.0 km to the northeast of Lee Stocking sland is a coral ledge where depths increase rapidly from 10 to 18 m. A l.o-km-wide platform with gradual slope, ranging from 18 to 24 m, exists below the ledge. Seaward from the platform, depth increases rapidly to the deep basin Exuma Sound. This geomorphology is typical of the western side of the Exuma Sound. Highest abundance of adult queen conch occurs in sand and rock habitats between 10 and 20 m, and few are found in depths greater than 25 m (Stoner and Schwarte).' A 500-m by 600-m site on the 18-m-deep platform off Lee Stocking sland, previously described in an analysis of conch reproductive behavior (Stoner et a., 1992), was mapped by divers (Fig. 2). Because of consistent presence of adult queen conch in the area, emphasis was placed on a 12-ha surface area encompassed by mound M5 and an area oflow hard-ground and sand (H ) to the north, the coral ledge (R) to the south, and transect line S- to the west. Mounds through 5 and area B were mapped by taking compass bearings and distances along the mounds at the base ofthe rock and along the rubble edge ofb and the coral ledge. Surface areas were then determined from scale drawings Stoner, A. W. and K. C. Schwarte. Source and significance of deep-water queen conch populations in the central Bahamas. n prep.

3 STONER AND SANDT: ECOLOGY OF DEEP-WATER QUEEN CONCH ,, film 5.M3 / ', , _M4// N t " " " N..-- (f),,------, M2 51, H1 23 m ---,,, 100 m Figure 2. Map of the primary habitat types within the study site near Lee Stocking sland, Bahamas. B = boulder, H = hard bottom, M = mounds, R = rubble, S = sand. (Table ). The site is isolated from surrounding habitats only by the coral ledge which is an effective barrier to conch movements. Conch were present in low numbers in all of the adjacent non-coral habitats, and the population investigated should be considered open. Surveys were made in three kinds of habitat within the study site: hard-bottom mounds, sand flats, and rubble and boulder areas. Hard-bottom areas were defined by five discrete mounds ranging in depth from 13 m on top to 18 m at the base (Fig. 2). Sand habitats were divided into two major regions: an extensive sand flat between the lo-m coral ledge and the mound zone (S), and a sand area within the mound zone (S2) (Fig. 2). Rubble and boulder areas were located in a narrow band at the base of the lo-m reef(r) and in an extensive rubble field (B) at the southeast end of the study site (Fig. 2). The mounds and rubble, particularly in the B area, were covered with a turf of filamentous green algae (Cladophoropsis sp.) interspersed with abundant erect forms such as Halimeda incrassata. METHODS AND MATERALS Physical Measurements.-Seawater temperature data were recorded with a Ryan nstruments Temp Mentor placed at 17-m depth near the base of the coral ledge in zone R. The thermograph recorded temperature every 30 min, with a precision of 0.2 e. Seven-day averages were generated from the data. A photoperiod curve was generated from the Nautical Almanac by determining the number of hours and minutes between sunrise and sunset for the local latitude every 9 days for the first year of study.

4 290 BULLETN OF MARNE SCENCE. VOL. 51. NO Table 1. Numbers and density (numbers/l 00 m') of queen conch in the deep-water habitat offshore from Lee Stocking sland, Surveys were made in January of each year. Surface areas of the sand transects varied depending upon horizontal visibility. Totals for B were extrapolated for the total surface area of the habitat Reef front Site Area (m') L990 L99L 3, (0.61) 20 (0.56) 53 (.47) 16 (0.44) Rubble field B (2.83) 27 (3.81) 32 (4.53) 38 (5.38) B (1.59) 15(4.78) 23 (7.32) 20 (6.37) B (0.99) 13 (1.84) 17 (2.40) 11 (1.56) B total 6, Mounds M 6, (3.25) 194 (2.98) 41 (0.63) 111 (1.70) M (2.52) 46 (5.52) 0(0) 27 (3.24) M (0.39) 6 (1.16) 0(0) 2 (0.39) M (7.25) 68 (10.36) 12 (1.81) 50 (7.54) Sand transects S- (Var.) 0(0) 3 (0.038) 8 (0.080) 2 (0.033) S-2 (Var.) 0(0) 5 (0.069) 0(0) 16 (0.296) Estimated total (rubble and mounds) Year Distribution and Population Structure.-Preliminary observations of the deep-water population showed that conch were concentrated on hard ground and rubble during the winter and were widely dispersed on sand during the summer reproductive season (Stoner et a., 1992). Therefore, to facilitate longterm analysis oflocal population size, surveys in selected hard-ground and rubble sites were conducted every January between 1988 and To provide data on seasonal movement patterns during the first year of study, surveys were also made on a monthly basis from March to October 1988 and in December Survey effort was greatest during spring through fall when conch locomotion was greatest and substratum changes were most frequent. During each survey period several different measurements were made according to habitat type. Total numbers of queen conch were counted on M, M3, M4, and M5 (few conch were found on M2, and this mound was abandoned early in the investigation). Total numbers of conch found within 10 m of the base of the coral ledge were counted in area R. Conch density in area B was determined by counting all conch within fixed radii from three permanent station markers in the area. Stations and radii were set so that the circles fell within uniform rubble areas. Two circles had radii of 15 m (B-, B-3) and one circle had a radius of 10 m (B-2). Numbers of conch in the entire B region were extrapolated for the total surface area of the habitat using mean density of conch within the circles. Total numbers of conch were counted along two parallel transects from the coral ledge to M (S-) and from the ledge to M2 (S-2) (Fig. 2). The transects were made by towing a diver 5 m above the sediment after determining horizontal visibility with a tapc measured in meters. Based on this measure, the total survey area was calculated for each transect. High water transparency resulted in a mean transect width of 29 m (SD = 6; range = m). Additional dives were made during each survey in attempts to find most tagged conch in the 12-ha study site described above. Use of diver propulsion systems facilitated surveys of the extensive sand plain. Between the beginning of the survey and October 1988, every conch encountered was marked with a spaghetti tag (F1oy Tag and Manufacturing Co.) tied around the spire of the shell. At the time of tagging, each conch was measured for shell length (tip of spire to siphonal groove) using large calipers, and most shells were measured for lip thickness. The latter measurement was made in the area of greatest thickness, approximately two-thirds of the distance posteriad from the siphonal groove. Shell length and lip thickness were measured with a precision of ± mm. Because lip thickness increases with age after sexual maturity but shell length does not, thickness measurement provides an approximate, or at least relative, age of the animal and provides data for comparison of population age structure with that of other studies (Appeldoorn, 1988). Several conch without developed shell lips were encountered during the survey. These individuals were tagged, measured for length, and recorded

5 STONER AND SANDT: ECOLOGY OF DEEp WATER QUEEN CONCH a n 1966 a ::l ::l Q. Q. a Q. 15 a Q. '5 25 n C- C- o 10 a '0 t- t- W W U U '" w W Q. Q. '" B J SHELL LENGTH (em) SHELL THCKNESS (mm) Figure 3. Length-frequency distribution of the queen conch population at the study site. Figure 4. Distribution of shell lip-thickness for the adult queen conch population at the study site. as juveniles. At the time of tagging and in all subsequent recaptures, the location and substratum type (hard bottom, sand, or rubble) were recorded. Between January and March 1991, an extensive search for tagged individuals was made. Those found were re-measured for shell length and tip thickness to provide estimates of growth. dentification of sex in a queen conch can be accomplished by turning the shell on its side; the genitalia are revealed when the animal rights itself. However, this process is painstaking, and it is not possible to identify the sex of all individuals. Consequently, determination of the sex of tagged conch in this study was based on reproductive behavior (i.e., pairing, copulating, and egg-laying). Seasonal Movement Patterns. -Seasonal movements of tagged conch were analyzed on the basis of reproductive activity observed at the study site during (Stoner et a., 1992). Movements over four different periods were examined: 1) during the peak reproductive season (April-August 1988)2; 2) during the non-reproductive season (September 1988-March 1989); 3) between the peak reproductive season and the non-reproductive season; and 4) between the non-reproductive season and the subsequent reproductive season (April-August 1989). Movements of tagged conch were examined in terms of shifts in location (e.g., B, R, M5, etc.) and substratum type (i.e., sand, rubble, hard ground). For final analysis, the mounds were pooled because of few movements to and from mounds with relatively low numbers of conch (M3, M4, and M5). Total population size in the study area was estimated by the Jolly-Seber multiple mark-recapture analysis (Seber, 1982) for each of the months between April and October A total of 1,966 conch were tagged during the survey, and 1,647 recaptures were recorded. Feeding Ecology. -Adult conch were collected for stomach analysis from open sand and from the hard-ground substratum of M in February and August Fifteen animals were collected from each habitat on each collection date. An additional 15 conch were collected from the rubble area (B ) in October 1988 after it became apparent that this area might be an important habitat for feeding. Conch taken for stomach analysis were frozen. After thawing, soft tissues were drawn carefully from the shell so that the stomach was retrieved intact. Stomach fullness was recorded on a scale from 0 (empty) to 5 (greatly distended). The gut contents of the S animals from each site and date were pooled, preserved in 70% ethanol, and stained with rose bengal. The sieve gravimetric method of Carr and Adams (1972) was used to examine stomach contents. This method has been employed successfully in other studies of queen conch (Stoner, 1989; Stoner and Waite, 1991). Food items were fractionated using sieves with meshes of2.0, 0.85, 0.425, 0.250, O. 50, and mm. The relative proportion of different kinds of foods in each fraction was determined using a dissecting microscope. At least 250 particles were examined except where numbers of particles were fewer. Foods were separated into major taxonomic groups, most of which were mutually exclusive. Each sieve fraction was then dried at 80 C and weighed. Because each fraction was comprised of particles of similar size, the relative contribution of each food type was determined on the basis of count. This method was particularly useful given the heavily macerated nature of the stomach contents. RESULTS Population Structure. -Only 23 juvenile conch were found in the entire survey. Given that 1,966 conch were tagged during the first year of study and more than 800 untagged conch were observed in subsequent winter surveys, juveniles com- ;! First copulation and egg-laying were observed in April n September no copulation was observed and only one egg mass was seen. The last egg mass was observed in early October L988.

6 292 BULLETN OF MARNE SCENCE, VOL. 51, NO.3, 1992 Table 2. Estimates of total conch population size in the study site and calculated immigration/ emigration rates based upon tag-recapture data in 1988.(Area of the study site was 12 ha) Recaptures Total Oensity mmig.rationl Month Tagged Untagged population (No.lha) emigration May June July , August September prised no more than 0.9% of the total conch population at the deep-water site. The few juveniles were large; mean shell length was 209 mm (SD = 34, range = ). All other conch were mature adults with fully developed shell lips. Mean shell length of the adults was 219 mm (SD = 24, range = ) (Fig. 3), and mean shell thickness was 28 mm (SO = 5, range = 2-44) (Fig. 4). More than 90% of all conch had lip thicknessess between 20 and 34 mm. There was no correlation between shell length and shell thickness (r = 0.159, N = 860). The mean length of female conch (227 mm, SD = 25) was slightly larger than that of males (220 m, SD = 19), but length-frequency distributions of the sexes were not significantly different (Kolmogorov-Smimov test, P = 0.857). The distributions of shell lip thickness also were not different (P = 0.821) (mean = 28 mm, SO = 5, for females; mean = 27 mm, SD = 4, for males). Table 3. Changes in shelliength and shell lip thickness in adult conch tagged at Lee Stocking sland, (All final measurements were made in January 1991) Length Thickness Growth/year Growth period (mm) (mm) (mm) (mon.) Orig. Final Orig. Final Length Thickness Mean length, -1.1; thickness SD length, 1.2;thickness 1.6.n length, 26; thickness 26.

7 STONER AND SANDT: ECOLOGY OF DEEP-WATER QUEEN CONCH 293 N E 12 R 2.0 a 0-0 M1 E.-. S1-1.:::: M S1-2 /4 ci 6-/> M~.:::: M ci / 3 U1 0 6 ~ 4/ =:l z 0 <: ~.-.. U1 ::;: ~/' / ' ~~@l : J: J: ~., /> 6----/> U U />_/>_~~.=::; z z a J/F A M J J A S a N J/F U J/F A M J J A S 0 N U MONTH MONTH Figure S. Density of conch on four hard-ground habitats (mounds) between January 1988 and February Figure 6. Density of conch on two sand-bottom transects between January 1988 and February Examination of tag returns between April and October 1988 yielded estimates of the size ofthe conch population in the study site and estimates of immigration and emigration between May and September 1988 (Table 2). The highest population estimate occurred in July with 1,216 conch (101.3 conch/ha). Lowest values, between one-half and one-third of the July value, occurred in May and September. Long-Term Analysis of Population Size and Shell Thickness. -Estimates of the total number of conch in the study site based upon January surveys between 1988 and 1991 (Table 1) were relatively consistent from year to year, ranging from 399 conch in 1990 to 549 conch in Confidence in the values is high because counts were direct in all of the areas except B1. (Numbers of conch on sand during the winter were negligible.) Also, the winter population estimates correspond well with estimates from tag returns in May (Table 2), given the known immigration and emigration of conch in spring and fall, respectively. Only 26 of the conch tagged in 1988 were found in the study area in January 1991; however, remeasurement revealed changes in both length and lip thickness over an approximately 2.5-year period (Table 3). All except one conch demonstrated either zero or negative growth in total shell length, with a maximum decrease of 4.4 mm' yet (mean = mm' yr- ). Lip thickness increased in 17 of the 26 conch and decreased in four conch. With few exceptions, the increases in thickness were small. ndividuals with shell lip thicknesses less than 20 mm in 1988 showed maximum increases in thickness (4.4 and 5.2 mm yc l ). Lowest increases in lip thickness tended to occur in the thickest shells, and there was a significant negative correlation between growth in the lip and initial lip thickness (r = , F = 21.33, P < 0.001). Most of the thick-lipped shells were eroded extensively on the outside, many with severely reduced spines and spires. Consequently, the younger, thin-lipped conch tended to have larger shells with more pronounced spines. Seasonal.Movement Patterns. - Highest density of conch was found on the hardground substratum of the mounds, particularly on M5, where density exceeded 4.0 conch 100 m-2 except in June (Fig. 5). Overall densities were lower on the other three mounds, which had poorer access for conch. A gradual slope from sand to rock on all sides of M5 permitted passage by conch whereas the other mounds had steep sides in certain locations. Seasonal trends in density were similar on all of the mounds, with highest densities occurring in fall and winter months.

8 294 BULLETN OF MARNE SCENCE, VOL. 51, NO.3, r , /.... '...'. '.., ". '.., '. "- 25./.. ' '...,.'..." ,--,.---,----,r---r--"'t--r---r-r----,----.,r---r--"'t-+ 10 J FMA MJ JASONDJF "'U o --l o "'U ftl :;;0 o,... :Y o C..., en ~ MONTH Figure 7. Bottom water temperature (solid line) and photoperiod (broken line) at the study site between January 1988 and February 1989, Values for water temperature are 7-day means for temperature recorded every 30 min. Photoperiod is number of hours between sunrise and sunset. Lowest densities were observed between May and August when the conch had moved to sand substrata. Density of conch on sand was less than 0.5 conch, 100 m- 2, except during the mid-reproductive season of June through September Sand transect 1 had one maximum of conch density (1.76 conch, 100 m- 2 ) between the two maxima observed on transect 2 (Fig. 6). This high density appeared to be related to large numbers of conch moving to the northwest out of the rubble area and across the sand flat in the early reproductive season. Conch then moved back across transect 2 toward the end of the reproductive season in September. Tag return data for individual conch (see below) confirm the movement patterns from sand to hard substrate. The density of conch on sand increased as a direct function of bottom water temperature from January to July 1988 (r = 0.976, F = , P = 0.004). Conversely, density of conch on the most populated mound (M) decreased over the same period as an inverse function of water temperature (r = , F = , P = 0.035). Although the density of conch on sand was greatest during summer months, correlations between water temperature and numbers of conch on sand (r = 0.434, F = 1.861, P = 0.210) and on rubble (r = 0.196, F = 0.320, P = 0.587) were not significant when all months were included in the analyses because conch returned to the hard-ground habitats prior to the fall decline in temperature (Fig. 7). For example, the density of conch on sand decreased to only 0.061,100 m- 2 in October while bottom temperature was 27.9 C, only 1.0 C lower than the summer maximum. High winter-like densities on all of the mounds in October indicated that conch had returned to hard-ground habitat. Photoperiod (Fig. 7) proved to be a better predictor of queen conch migration. Observations during the first year of study showed a highly significant inverse relationship between length of day and density of conch on Ml (r = , F = , P < 0.001) and on M5 (r = , F = , P < 0.001). However, the correlation between day length and conch density on sand was not significant (r = 0.559, F = 3.644, P = 0.093).

9 STONER AND SANDT: ECOLOGY OF DEEP-WATER QUEEN CONCH N Ea a.... ""-. o '-" W ---J CD CD ::::> 0::: z () o () o J/F A M J J A SON J/F MONTH Figure 8. Density of conch in two rubble habitats between January 1988 and February B1 is a large rubble area where three measurements of density were made (mean ± SD shown). R and R2 arc narrow bands of rubble at the base of the coral ledge where total numbers of conch were counted. Other habitats at the study site showed less dramatic seasonal changes in conch density. n the rubble habitat of area B, densities were high throughout the year ( conch 100 m- 2 ), with large variation over the three circles surveyed (Fig. 8). The total numbers of conch found in rubble at the base of the reef (R) were highest in January of 1988 and 1989 but were also high in August and October. These rubble habitats had conch densities similar to those of the mounds, but with less seasonal variation. ndividual Movement Patterns. - During the peak reproductive season (April- August), reciprocal movements between Sl and the rubble site (Bl) and between Sl and the reef front were most numerous (Table 4). However, with 50% of all movements to Sl (16% from the mounds), there was a 25% net movement to the sand zone. Patterns of movement between different substrata showed that conch frequently moved between sand and hard bottom during the reproductive season (Table 5). During the non-reproductive season, migration between S and other sites yielded a 20% net movement of conch away from the sand habitat. Movements from the reef front and area Sl to the rubble field (Bl) occurred frequently; 53% of all movements were to B. These movements were associated with large numbers of conch moving from sand to hard substrata (Table 5). Tag return data for the transitions between reproductive and non-reproductive periods confirm survey data. Tag recaptures between the reproductive season and the fall/winter non-reproductive season indicated highest frequencies of movement from area Sl to mounds and the rubble field (Table 4). Only 16.3% of all tag movements were into area S1. During the fall, 83.9% of the movements were from sand to hard-bottom habitat (Table 5). The reverse trend was evident in the

10 296 BULLETN OF MARNE SCENCE, VOL. S, NO.3, 1992 Table 4. Summary of changes in locations of tagged conch, 1988 to (See text for definition of the periods.) Values are the number of movements observed between geographic locations shown in Figure 2, and the percentage of movements for the time period (parentheses). Movements from M to M are those between different mounds Reprod. to non During 000- Non.reprod. to Movements observed During reprod. season reprod. season reprod. season rcprod. season R~M (0.7) 26 (15.2) 6 (3.8) 6 (6.6) R ~ S 18 (13.0) 10 (5.8) 10 (6.4) 8 (8.9) R ~ B 5 (3.6) S (8.8) 42 (26.9) 6 (6.6) S ~ M 3 (2.2) 46 (26.9) 5 (3.2) 0(0) S ~ B 18 (13.0) 30(17.5) 36 (23.0) 0(0) S ~ R 14 (10.1) 8 (4.7) 9 (5.8) 0(0) B ~ M (0.7) 6 (3.5) (0.6) 0(0) B ~ S 29 (21.0) 5 (2.9) 4 (2.6) 15 (16.7) B ~ R 8 (5.8) 2 (1.2) 2 (1.3) (1.1) M~M 14 (10, ) 13(7.6) 29 (18.6) 19 (21.1) M ~ S2 22 (15.9) 7 (4.1) 4 (2.6) 28 (31.1) M~B 4 (2.9) 3 (1.8) 5 (3.2) 4 (4.4) M~R 1 (0.7) 0(0) 3 (1.9) 3 (3.3) Totals transition from the non-reproductive season to the reproductive season; 84.2% of tag movements were onto sand (Table 5). No tagged conch moved from area S1 to other regions of the study site (Table 4), and 31.1 % of the movements were from mounds to S1. Movements from the rubble field to Sl were also frequently observed, and intermound movements were particularly frequent during this period. Feeding Ecology. -Active feeding by conch was observed primarily in rubble (Bl) and hard-bottom habitats (Table 6). Conch on sand were occasionally seen feeding on sparsely distributed erect algae, but most were either moving rapidly across the sand, engaged in reproductive behavior, or motionless. Low feeding levels were confirmed by stomach fullness indices (Table 6) in the sand habitat, particularly in the winter, when conch on the sand were few and relatively dormant. The primary item in the stomachs of conch taken from sand habitat was sand. High feeding activity in the hard-ground and rubble habitats was shown by high stomach fullness, particularly in the latter half of the year. Most stomachs of conch in the B area were distended with food, principally algae. Stomach fullness of conch at M 1 was lower in winter than in summer. The contents were dominated heavily by algae in the summer collection. Foraminiferans, bryozoans, and small bivalves and gastropods, probably taken incidentally with other foods, were also found in conch stomachs. Table 5. Summary of changes in substratum type of tagged conch, (See text for definition of the periods.) Values are the number of movements observed between sand (S), rubble (R), and hard ground (H), and the percentage of movements for the time period (parentheses) During reprod. Reprod. to non- During non- Non.reprod. to Movements observed season reprod. season reprod. season reprod. season S~H&R 39 (39.0) 115 (83.9) SS (81.6) 8 (10.5) Between H & R 4 (4.0) 9 (6.6) 4 (2.1) 4 (5.3) H&R~S 57 (57.0) 13 (9.5) 31 (16.3) 64 (84.2) Totals

11 STONER AND SANDT: ECOLOGY OF DEEP-WATER QUEEN CONCH 297 Table 6. Stomach contents of adult queen conch from three different habitats in the deep-water survey off Lee Stocking sland, Bahamas. Fullness is scaled from 0 to 5 (empty to greatly distended) (mean + SO). Values for individual food items are percent of stomach contents in dry weight Habitat Sand Sand Ml Ml Bl Date 2/88 8/88 2/88 8/88 10/88 N Fullness 1.2 ± ± ± ± ± 0.5 Stomach contents Algae Bivalves Bryozoans Detritus Forminiferans Gastropods Ostracods 0.2 Polychaetes 0.1 Sand DSCUSSON n a review of Caribbean resources, Appeldoom et al. (1987) reported that conch are found from intertidal depths to 60 m, with an age-related offshore migration. Quantitative data on conch populations in water deeper than 6-8 m are relatively few. Weil and Laughlin (1984), however, reported that conch density in an unfished area of slas Los Roques, Venezuela, was highest in 4 m depth (41-51 conch, 100 m- 2 ) and decreased with depth to 18 m. This may represent the natural distribution of queen conch in the Caribbean. n the Bahamas, where fishing is limiting to free-diving, adult conch density increases dramatically below 7-8 m (pers. observ.). Densities of adult conch on rubble and hard-ground at the 18-m site near Lee Stocking sland (commonly> 1.0 adult 100 m- 2 and as high as 10 conch/loa m 2 ) are near the 3.0 conch 100 m- 2 mean value reported by Weil and Laughlin (1984) for their 18-m site. Densities at the study site, therefore, may reflect natural levels. n contrast, densities of adult conch in Puerto Rico have been reported to be low «0.2 conch' 100 m- 2 ) from 2 to 30 m depths, with maximum density of 0.7 conch, 100 m- 2 at 30 m (Torres Rosado, 1987). t was concluded that this depth distribution was a function of heavy fishing pressure that includes the use of scuba. Length-frequency and shell thickness distributions of conch at the study site demonstrate that the population is comprised of mature adult conch, primarily individuals of advanced age. Ninety percent of the population had thicknesses between 20 and 34 mm. Using Appeldoom's (1988) lip thickness-age relationship for queen conch in Puerto Rico, the age of conch at the Lee Stocking sland site could be estimated at 6 to 7 years. Two factors, however, suggest that this age range is conservative. First, Appeldoom (1988) noted that conch growth rates in Puerto Rico are greater than those at higher latitudes and greater depths. Second, the thickness-age curve, which flattens out with increasing age, was developed by Appeldoom using relatively young, uneroded conch. Clearly, growth rates of shell thickness measured at Lee Stocking sland were small relative to those found by Appeldoom, implying that the thick-lipped conch may, in fact, be very old. On the basis of age-weight relationships, Coulston et al. (1988) speculated that some large conch in deep-water habitats off St. Croix, U.S. Virgin slands were as old as 26 years. Although more data will be required to make definitive statements about the use of shell thickness for aging conch,

12 2Q8 BULLETN OF MARNE SCENCE, VOL 51, NO highest growth rates in thin-lipped individuals and obvious erosion in shell length and thickness in the heaviest shells suggest the need for caution, The age-lip thickness relationship for queen conch becomes asymptotic as growth in thickness is offset by bioerosion and dissolution of the outer shell. Because of this agedependent phenomenon, we conclude that lip thickness should not be used as an estimator of age in populations which include many old conch. Old conch would be particularly abundant in unfished populations such as those in deep-water habitats of the Bahamas or other areas where the fishery is closed to scuba. We are currently quantifying long-term growth in adult queen conch in the vicinity of Lee Stocking sland. The low number of young, thin-lipped adults can be interpreted in at least two ways. First, recent year classes may be relatively weak, and the thick-lipped population may be remnant of earlier and stronger year classes. Decline in the strength of recent year classes is widespread in the Caribbean region (Adams, 1970; Appeldoorn et a., 1987; Berg and Olsen, 1989), and individuals who have fished waters near Lee Stocking sland for many years report a decline in shallowwater conch. Second, the deep-water stock appears to receive individuals via adult recruitment (i.e., migration) from adjacent habitats. n effect, deep-water sites may be the final destination in an ontogenetic migration from shallow to deep water whereby only the oldest adults arrive in the habitat. There was no indication of reproductive senescence in even the oldest (i.e., thickest) individuals. Conch of all adult ages were found in the reproductive subhabitat (sand), and the mean lip thickness of conch observed in reproductive activity, including egg-laying, was not different from the overall mean (Stoner et a., 1992). Survey and tag-return data from Lee Stocking sland show clearly that adult conch at the deep-water site make seasonal migrations between the feeding and summer spawning habitats, rather than onshore-offshore migrations documented for shallow-water adults (Hesse, 1979; Weil and Laughlin, 1984). Considering that immigration is the only source of recruitment to the population and that mortality was negligible during the study period (no tagged individuals were found dead), it appears that between May and July the study site received adult conch from the surrounding areas. Given that the study site is predominantly sand bottom (the summer spawning habitat) with mixed hard ground and sand on three sides, it is not surprising that the area received many immigrants during summer months. Stimuli for the movement of adult conch to and from sand-bottom reproductive sites is probably a complex function of interactions between temperature, wave surge, photoperiod, and physiological conditions of the conch. The strong negative correlation between density of conch on hard-ground habitats and day length found in this study and the positive correlation between day length and reproductive behavior (Stoner et a., 1992) both suggest the likelihood that photoperiod has a strong influence on seasonal migrations of adult queen conch. Further investigation will be required to elucidate mechanisms of seasonality in migration and reproduction. The importance offeeding on algae-covered hard substrata (mounds and rubble) is suggested by high stomach fullness dominated by algal materials and low sand content relative to stomachs of conch collected on sand. Movement to the foodpoor sand habitat may be necessary for egg laying (Robertson, 1959; Davis et a., 1984); however, movements of tagged conch to and from hard substrata during the reproductive season suggest that foraging trips may occur during the relatively long April to August reproductive season. Close proximity of feeding and repro-

13 STONER AND SANDT: ECOLOGY OF DEEP-WATER QUEEN CONCH 299 ductive habitats, therefore, may be important characteristics of an optimal spawning site. High turnover rate for the conch population, demonstrated by tag-recovery data, is probably related to high levels of motility in the conch and the fact that the study site is located within an extensive area of the shelf where conch are abundant and move continuously. Stock assessment conducted in 1991 yielded an estimate of 75,000 adult conch in a 12-km long section of the 1.5-km wide island shelf adjacent to Lee Stocking sland (Stoner and Schwarte).1 Underwater observations at many locations along the western edge of Exuma Sound show that the 18-m-deep platform of interspersed sand and hard bottom examined in this study is typical of geomorphology windward from the Exuma Cays. Given the abundance of adult conch on this platform, the relative rarity of sexually mature conch in shallow waters, and the abundance of conch larvae in the passes between the Exuma Cays (Stoner et a., 1992), it is likely that the deepwater population off the island chain is the primary reproductive stock sustaining the species on the heavily fished Exuma Bank. For the past 4 years, the deep-water conch population at the Lee Stocking sland site appears to have been relatively stable; however, given that recruitment overfishing has occurred in many conch populations, the free-diving policy for conch fishing in the Bahamas may be an effective means of protecting reproductive stocks. The potential importance of deep-water reproductive stocks was mentioned by Wicklund et ai. (1988) and Berg and Olsen (1989). The effectiveness of policy protecting deep-water spawners will depend upon the source of offshore stocks. f the source is shallow bank areas, protection of deep-water populations only delays the effects of overfishing. t is critical that the relationship between reproductive stocks and local recruitment of queen conch be understood. ACKNOWLEDGMENTS This research was supported by a grant from the National Undersea Research Program of NOAA (U.S. Department of Commerce) to the Caribbean Marine Research Center. A. Bardales, P. Bergman, N. Christie, J. Colley, K. French, R. Gomez, K. McCarthy, O. Monterossa, K. Schwarte, J. Waite, and E. Wishinski assisted in the field work. P. Bergman conducted the stomach analysis and assisted with data analysis. R.. Wicklund provided seawater temperature data. We are grateful to R. Appeldoom, L. Colin, M. Davis, D. Eggleston, and R. Wicklund for reviews of the manuscript. LTERATURE CTED Adams, J. E Conch fishing industry of Union sland, Grenadines, West ndies. 1. Trop. Sci. 12: Alcolado, P. M Crecimiento, variaciones morfologicas de la concha y algunas datos biologicos del cobo Strombus gigas L. (Mollusca, Mesogastropoda). Acad. Ciec. Cuba, nst. Oceanologica 34: Appeldoorn, R. S Age determination, growth, mortality and age of first reproduction in adult queen conch, Strombus gigas L., off Puerto Rico. Fish. Res. 6: , G. D. Dennis and O. Monterrosa-Lopez Review of shared demersal resources of Puerto Rico and the lesser Antilles region. Pages in R. Mahon, ed. Report and proceedings of the expert consultation on shared fishery resources of the lesser Antilles region. FAO Fish. Rept Berg, C. J., Jr Growth of the queen conch Strombus gigas, with a discussion of the practicality of its mariculture. Mar. Bio. 34: and D. A. Olsen Conservation and management of queen conch (Strombus gigas) fisheries in the Caribbean. Pages in J. F. Caddy, ed. Marine invertebrate fisheries: their assessment and management. Wiley, New York. Brownell, W. N Reproduction, laboratory culture and growth of Strombus gigas. S. costatus. and S. pugi/is in Los Roques, Venezuela. Bull. Mar. Sci. 27:

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