Stoner and Schwarte: Distribution of Strombus gigas 



175 



in the inner section of the Great Bahama Bank with 

 only 0.19 conch/ha (SD=0.15, n=28). Density of adult 

 conch in the outer (seaward) section of the bank was 

 close to the value for the 2.5-5 m depth zone on the 

 shelf. Although not quantified, numbers of adults 

 in the nearshore (0-2.5 m) zone of the shelf were 

 negligible. 



Few juvenile conch were observed on the shelf 

 between 2.5-and 15-m depth (Fig. 2). A total of 372 

 juveniles were found in densely aggregated patches 

 on seagrass beds off the eastern beaches of Lee 

 Stocking Island. On the Great Bahama Bank, most 

 juveniles were aggregated in specific nursery loca- 

 tions documented previously (Stoner et al., in 

 press.). None was found deeper than 15 m. 



The area between 10 and 20 m depth on the is- 

 land shelf was a particularly important habitat for 

 adult queen conch (Table 1). Approximately 74% of 

 all conch in the 12-km long survey area reside in 

 this narrow depth zone. It is also clear that large 

 expanses of shallow bank habitat support a rela- 

 tively small proportion (15.2%) of the adult popula- 

 tion. Mating conch and demersal egg masses were 

 very abundant during summer months at shelf 

 sites deeper than 10 m, but none were observed on 

 the bank. 



Shell morphology 



The shelf sites were characterized by large adult 

 queen conch, primarily between 200 and 260 mm 

 (mean=227, SD=23, n=572), whereas most adult 



conch on Great Bahama Bank 

 were between 170 and 210 mm 

 shell length (mean=187, 

 SD=16, n=472). Pooling all 

 adults measured, there was a 

 highly significant difference in 

 the length-frequency distribu- 

 tion of conch on the shelf and 

 on the bank (Kolmogorov- 

 Smirnov test, P<0.001). The 

 distributions (Fig. 3) show 

 clearly the separation in size of 

 adults between bank and shelf 

 sites, particularly when com- 

 paring nearshore (0-5 m) shelf 

 zones with those from the 

 bank. The distributions show a 

 decrease in shell length be- 

 tween the nearshore shelf and 

 deeper zones, while those be- 

 tween 5 and 25 m are obviously 

 similar. 



Bank conch had thin shell 

 lips (mean=10, SD=6); conch 

 from nearshore (2.5-5 m) regions of the shelf were 

 intermediate in lip thickness (mean=18, SD=5); and 

 deep-shelf (5-25 m) conch had the thickest shell lips 

 (mean=30, SD=7)(Fig. 4). All three of these groups 

 were significantly different from one another in 

 terms of lip thickness distribution (Kolmogorov- 

 Smirnov tests, P<0.01). There was obvious similar- 

 ity in the thickness distributions of shells in depth 

 zones between 5 and 25 m; therefore, these four 

 depth categories were pooled. 



Distinctness of the morphs collected on the bank 

 and shelf is further suggested by a plot of shell 

 length and lip thickness for 250 randomly chosen 

 individuals from each of the two regions (Fig. 5). 

 Also, when length and lip thickness data for all 

 1,029 conch measured in the survey were used in 

 canonical discriminant function analysis, a highly 

 significant separation was found between conch col- 

 lected in the two different regions (Hotelling-Lawley 

 Trace, F= 1,854, P<0.001). Less than 5% of the conch 

 in the survey were not collected in the region pre- 

 dicted by the multivariate equation. Bank conch 

 were small and had thin shell lips, whereas conch 

 from the island shelf were large and had thick shell 

 lips. Results of the analysis, however, do not rule out 

 the possibility that the smallest adult conch from 

 the shelf region, particularly apparent in the 5-10 

 m depth zone, could be older animals from the bank. 

 Length-frequency distributions of juvenile queen 

 conch were different on the Great Bahama Bank and 

 island shelf (Fig. 6). Both the bank and nearshore 



