520 



Fishery Bulletin 91(3). 1993 



used hatchery-reared conch (39-53 mm SL) as subjects 

 for this experiment. 



Nine 190-L aquaria containing 5 cm of coarse sand 

 from Neighbor Cay (zone B) were used in the experi- 

 ments. Water temperature was maintained at approxi- 

 mately 28°C, and aeration was provided by under- 

 gravel filters. Ten juvenile conch were placed in each 

 of the aquaria and subjected to a schedule of 12 hours 

 light: 12 hours darkness. After 4 days, when the bur- 

 rowing cycles of the animals became apparent, the ani- 

 mals were subjected to three different light regimes 

 during the subsequent 4 days: three aquaria were kept 

 under natural cycle (12:12), three were placed under 

 continuous darkness and three under continuous light. 



Observations on burrowing behavior were made twice 

 during the day and twice during the night (0400, 1100, 

 1500, 0000 hours). Food, in the form of seagrass detri- 

 tus, was placed on the surface of the sediment. 



subtidal, in the surf berm, and at the bottom of the 

 slope, respectively). All others were found in the veg- 

 etated zone E (Fig. 3). No early juvenile conch were 

 found in the most offshore zones F and G. Mean den- 

 sity of early juveniles (1.25 conch/m 2 ) was greatest in 

 zone C, in association with the coarser grain sizes and 

 low sediment cohesiveness (Table 1). In contrast to the 

 narrow, high beach distribution of early juveniles, later 

 stage juveniles were found in every zone, and highest 

 densities in the seagrass zones E, F, and G (Fig. 3). 



Most of the early juvenile conch were distributed 

 between transects 3 and 9 (Fig. 2) over an area of 

 approximately 250 m 2 . Based on a mean density of 1.25 

 conch/m 2 in zone C and 4 conch/m 2 observed in some 

 areas of zone C, the early juvenile population was be- 

 tween 275 and 1000 conch. The densities observed prob- 

 ably accounted for most of the total population since 

 500 conch were tagged during night observations. 



Results 



Habitat characteristics 



Transects from the intertidal beach to the shallow 

 subtidal seagrass bed were characterized by increas- 

 ing water depth ranging from -0.2m (at MLW) in the 

 center of intertidal zone A to 1.9 m in zone G. 

 Sediment grain-size decreased with increasing depth 

 (Table 1). In zones A through E sediments were coarse 

 sands (984-521 urn), while medium sands (496-490 urn) 

 were found in zones F and G. Organic content of the 

 sediments varied little across the different zones, rang- 

 ing from 2.59 to 2.74% of dry weight (Table 1). Sedi- 

 ment cohesiveness was relatively constant across the 

 offshore zones (40-48 mm), except in the surf berm 

 (zone C), where cohesiveness was low (i.e., penetrabil- 

 ity and porosity were high) (Table 1). 



Seagrasses, primarily Thalassia testudinum, were 

 present in relatively low biomass in zones E, F, and G 

 (Table 1). Dry weights of macrodetritus were also rela- 

 tively low, with the highest mean value (4.8 g dry wt/ 

 m 2 ) observed in zone E. 



Conch distribution 



In the January survey, all conch found were less than 

 60 mm or greater than 80 mm SL. Given the summer 

 spawning season of queen conch in the Exuma Cays 

 (Stoner et al., 1992) and estimated growth, it was as- 

 sumed that the former represented the young-of-the- 

 year class (0+), while those larger than 80 mm were 1- 

 and 2-year old conch. Eighty-six percent of the early 

 juvenile conch (35-54 mm SL) were found in 

 unvegetated zones B, C, and D (i.e., in the immediate 



Enclosure experiment 



No mortality occurred during the first two periods of 

 the transplant experiment; therefore, no replacements 

 were necessary and growth rates were calculated for 

 the original conch (Table 2). At the end of the experi- 

 ment, two conch were found dead, one on sand and 

 one on seagrass. Five conch were unaccounted for, two 

 in the sand treatment, two in the rocky zone, and orte 

 in the seagrass habitat. 



Growth rate in the sand habitat was much lower 

 (0.012 mm/day) than the rates in seagrass (0.112mm/ 

 day) and rock habitats (0.094 mm/day) (Table 2). The 

 differences were significant (ANOVA, F = 769.35, 



