Stoner and Schwarte: Distribution of Strombus gigas 



177 



w 



c 



-XL 



Q. 



50 



40 

 30 

 20 

 10 

 



° Bonk 

 • Shelf 



- ^i t • • 



v Jn 



♦. 400. 



10 15 20 25 



Shell Length (cm) 



30 



Figure 5 



Scatterplot of shell lip-thickness vs. shell length for 

 adult queen conch, Strombus gigas, collected from the 

 Great Bahama Bank and from the Lee Stocking Island 

 shelf. Two-hundred and fifty randomly chosen points 

 were plotted for each site. 



(5—6 spines/whorl vs. 7-9 spines/whorl in shelf ju- 

 veniles). Bank conch had a maximum shell diameter 

 between 80 and 90% of shell length at 100 mm 

 length, whereas juveniles from the shelf had diam- 

 eters between 50 and 60% of shell length. These 

 characteristics persisted to adult stages with bank 

 conch having longer spines. The outer whorls of 

 shelf adults, even young individuals, were often 

 nearly smooth. 



Growth rates 



Juvenile queen conch on the island shelf at Charlie's 

 Beach grew in length at a rate approximately 2.4 

 times the rate observed at the Children's Bay Cay 

 site. Conch recovered at Charlie's Beach grew 0.139 

 mm/day (SD=0.025, n=135). At Children's Bay Cay, 

 mean growth rate was 0.058 mm/day (SD=0.021, 

 n=48). The differences in growth rate between bank 

 and shelf juveniles were highly significant (Mann- 

 Whitney [/-test, P<0.001). 



Discussion 



The rapid increase in adult queen conch density at 

 depths greater than 10 m is probably a direct func- 

 tion of fishing, which is limited to free-diving on the 

 bank and shallow nearshore shelf areas around Lee 

 Stocking Island. This conclusion is substantiated by 

 observations of conch depth distribution in other 

 localities. In unfished areas of Islas Los Roques, 

 Venezuela, Weil and Laughlin (1984) found that 



60 B0 100 120 140 160 180 200 220 240 260 280 



Shell Length (mm) 



Figure 6 



Length-frequency distribution for juve- 

 nile queen conch, Strombus gigas, from 

 the Great Bahama Bank and from two 

 depth zones on the island shelf near Lee 

 Stocking Island, Bahamas. 



density of queen conch was highest in 4.0 m of wa- 

 ter and density decreased with depth to 18 m. This 

 may represent the natural distribution of queen 

 conch. In comparable 4-m deep habitats not pro- 

 tected from fishing, densities were 5 times less than 

 those in the protected area. Similarly, in the Exuma 

 Land and Sea Park, a 500-km 2 fishery reserve 90 

 km north of Lee Stocking Island, there are large 

 numbers (unquantified) of adult conch at 2-4 m 

 depth, and many of these shallow-water conch have 

 been observed laying eggs (Stoner, pers. observ.); 

 whereas adults are uncommon in shallow water 

 near Lee Stocking Island and spawning has never 

 been observed at less than 5 m depth. Similar to the 

 pattern reported in this study for Lee Stocking Is- 

 land, Torres-Rosado ( 1987) found maximum density 

 of adult queen conch between 10 and 20 m in Puerto 

 Rico, where fishing is heavy in shallower waters. 



It is recognized that queen conch move to greater 

 depths with age and size (Randall, 1964; Weil and 

 Laughlin, 1984); this has been confirmed in the Lee 

 Stocking Island area by the recovery of individuals 

 that were tagged as juveniles at Charlie's Beach and 

 subsequently found in deeper offshore waters 



