Stoner and Schwarte: Distribution of Strombus gigas 



173 



Survey methods 



The shelf region was surveyed in each of seven 

 depth zones between 2.5 and 30 m (described above) 

 along nine transects (perpendicular from the Cays 

 into Exuma Sound) placed at approximately 1.0-km 

 intervals. At each of the 63 shelf stations, divers 

 swam parallel to the isobaths for a distance mea- 

 sured with a calibrated General Oceanics flow meter 

 equipped with a large propeller for low velocity 

 flows. Calibration was performed by towing the 

 meter repeatedly (n>6) through calm water at the 

 side of a small boat over a pre-measured distance 

 of 100 m. Precision was ±2%. Current velocity on the 

 shelf adjacent to Lee Stocking Island is generally 

 low (<3 cm/sec) and to the northwest, parallel to the 

 isobaths (Smith, 1992 1 ). Recognizing the potential 

 effect of current on the calculated distance, each dive 

 included two legs, one up-current and one down- 

 current in parallel lines of equal length separated 

 by approximately 20 m. 



Two dives were made at most stations for density 

 determinations and shell measurements (described 

 below). For density, all queen conch were counted 

 in an 8-m wide path defined by a line held between 

 two divers. The average swim distance was 380 m, 

 resulting in coverage of just over 3000 m 2 . Conch 

 density was calculated by using only those conch in 

 the 8-m band. Shell measurements were made for 

 animals outside the 8-m band in areas with low 

 conch densities. Underwater visibility was usually 

 high and the area of bottom searched was actually 

 much larger than the swim path alone. Conse- 

 quently, all conch within approximately 30 m could 

 be collected for measurement. In areas where conch 

 densities were high, one dive was made to collect 

 density data and another to collect only measure- 

 ment data. An attempt was made to measure at 

 least 100 adults from each depth zone, but this was 

 not possible in the 0-5, 5-10, and 25-30 m zones 

 because of low densities in these zones. Statistical 

 differences in density among the survey zones were 

 evaluated with the non-parametric Kruskal-Wallis 

 test (Sokal and Rohlf, 1969) with stations used as 

 replicates (n-9). 



The shallowest depth zone (0-2.5 m) was limited 

 primarily to sandy coves on the major islands of the 

 survey area. Adult queen conch were few in these 

 areas, and juveniles were distributed unevenly; 

 therefore, the important seagrass areas of the shal- 

 low coves were thoroughly searched. Density mea- 

 sures were not made but all conch encountered were 

 measured (as described below). 



1 N. P. Smith, Harbor Branch Oceanography Inst., Fort Pierce. 

 FL, pers. commun. 1992. 



Sparse distribution of adult conch and the large 

 surface area of the Great Bahama Bank required the 

 use of different survey methods from those applied 

 on the shelf. Because the bank waters are shallow 

 and conch were easily seen, large areas were sur- 

 veyed by towing a diver at the surface in continu- 

 ous lines. The bank region was divided into 95 — 1 

 x 1 km squares oriented along lines of latitude. 

 Then, in a systematic grid of lines running diago- 

 nally through the squares, every square was crossed 

 at least once during the survey. Additional tows were 

 made in areas already known to have concentrations 

 of adults, i.e., near nurseries previously mapped 

 (Fig. 1; Stoner et al., in press.). Divers were towed 

 a total distance of 126 km. 



Although water clarity on the bank was not as 

 high as that on the island shelf, the towed diver 

 could usually see at least 2.5 m on either side of the 

 transect line. Surveys were not conducted on a few 

 days when visibility was restricted. While being 

 towed at approximately 50 cm/sec, the diver signaled 

 numbers of adult queen conch to the boat operator, 

 who recorded position. Positions for the ends of all 

 straight line transects were determined with GPS, 

 tow distance was estimated by chart, and conch 

 density was calculated on the basis of the 5-m wide 

 path examined. During the bank survey, 472 adults 

 were gathered and measured. Presence of juveniles 

 on the bank was noted but not quantified in this 

 study. For comparison with shelf sites, a random 

 collection of 322 juvenile conch was made from a 

 nursery west of Lee Stocking Island during August 

 1991. These conch were measured for shell length. 



The total number of adult queen conch was esti- 

 mated crudely for each bank and shelf area by ex- 

 trapolating the average density of conch for an in- 

 dividual zone over the total surface area for the 

 same zone. Because variances in the density data 

 were large, confidence intervals for the extrapolated 

 numbers of conch were not calculated. 



Shell measurements 



Queen conch reach sexual maturity between 3.5 and 

 4 years of age, a few months after the shell edge has 

 formed a broadly flared lip (Appeldoorn, 1988). Af- 

 ter the lip flares, queen conch stop growing in length 

 but continue to deposit shell material on the inside 

 of the lip (Egan, 1985; Appeldoorn, 1988). Therefore, 

 with certain limitations, thickness of the shell lip is 

 an indicator of approximate age (Stoner and Sandt, 

 1992). In this study, shell-lip thickness was mea- 

 sured with calipers in the area of greatest thickness, 

 about two-thirds of the distance posterior from the 



