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Fishery Bulletin 88|2), 1990 



Rico. Jory and Iversen (1983) found in their studies in 

 the Berry Islands that the highest levels of predation 

 occurred during the summer and the lowest during the 

 winter. They also suggested that reduced mortality in 

 the winter may be due to reduced predator activity and 

 a decrease in conch activity. 



Small conch do not grow in length during the winter 

 period of burying (Appeldoorn 1985, Iversen et al. 

 1986). Therefore, when they emerge in the spring to 

 begin active foraging on the substrate they would have 

 gained no obvious "size" survival advantage as a result 

 of their "hibernation." Johnson et al. (1964) demon- 

 strated that the shell thickness of queen conch in- 

 creases with activity, and because buried conch would 

 not be expected to be very active during the winter, 

 shell thickness would probably not increase substan- 

 tially over this time. 



Conch survival estimates show trends common to 

 many mollusks: low survival for small animals, increas- 

 ing as the size of the mollusk increases (Jory et al. 1984; 

 Appeldoorn 1984, 1988; Iversen etal. 1986). Recoveries 

 from 259 tagged wild conchs (SLR 5.0-21.0 cm) at 

 Little Whale Cay, Bahamas, over a 6-month period 

 showed a significant positive correlation between 

 length at tagging and percent recovered (r = 0.95; 

 jD<0.05). This correlation suggests that within this size 

 range mortality decreased with increased conch size. 

 Using floating cages to provide protection from pred- 

 ators and increase survival, we demonstrated that as 

 high as 96% survival was achieved for conch in the size 

 range 5.0-8.0 cm over a 1-year period, while survival 

 of tagged non-caged small conch of similar size after 

 6 months was only between and 10%. Similar evi- 

 dence on differential survival by conch size was 

 gathered in Puerto Rico by Appeldoorn (1984) who, by 

 using short-term (~8 weeks) mark-recapture data for 

 both queen conch and milk conch (S. costatus), deter- 

 mined that larger individuals suffered less mortality 

 than smaller ones. Jory and Iversen (1988) provided 

 direct evidence of a lack of increase in shell thickness 

 in small juveniles by examining the relationship be- 

 tween shell breaking strength and shell size of queen 

 conch. They found little increase in breaking strength 

 up to a size of ~5.5 cm, suggesting that queen conch 

 shells are relatively thin below this length. Above ~5.5 

 cm, pressure to crush shells increased rapidly with in- 

 creasing shell length. 



As queen conch increase in size, their spines increase 

 in length and become more robust. Spines may reduce 

 predation on mollusks in various ways (Palmer 1979). 

 They increase the overall size of the shell, thereby 

 limiting predation to larger predators. Conch spines 

 also distribute crushing pressure over a greater area 

 of the shell, thus increasing the pressure required to 

 crush the shell (Jory and Iversen 1988). Spines also 



serve as additional area for attachment of epibionts 

 which may help conceal mollusks from predators 

 (Feifarek 1987). 



The amount of attached organisms on the shells of 

 conch increases with the size of the conch, suggesting 

 a decrease in long-term burying activity with increase 

 in conch size (Iversen et al. 1986). All of the smallest 

 conch we collected had very clean shells with no growth 

 of attached organisms, in contrast to the rapid and 

 heavy growth of algae we observed on the shells of 

 similar-sized conch in hatchery tanks at the University 

 of Miami laboratory and in floating cages at Little 

 Whale Cay, Bahamas. These results further support 

 the hypothesis that young conch spend most of their 

 early life (up to ~1 year of age) below the surface of 

 the substrate, a strategy that may provide haven from 

 some predators (Hesse 1979, Appeldoorn 1985, Iversen 

 et al. 1986). 



The relationships between shell size and strength and 

 between survival and burying have practical applica- 

 tion in suggesting place and time of year to release 

 hatchery-reared conchs, in order to both minimize 

 predation and increase the efficiency of planting pro- 

 grams using hatchery-reared juveniles of queen conch 

 and several other mollusk species. Placement of young 

 hatchery-reared conch in relatively safe, natural areas 

 optimal for burying and with certain physical char- 

 acteristics such as strong currents, fine-to-coarse sand, 

 and abundant intact and broken mollusc shells, where 

 red and green algae and sponges occur (as described 

 in Iversen et al. 1986) would obviate the need for over- 

 wintering in an expensive hatchery facility. Further, 

 as suggested by Jory and Iversen (1983), fall planting 

 of small hatchery-reared conch would provide time for 

 them to acclimate to their environment before the 

 spring or early summer "emergence." This is par- 

 ticularly relevant in view of evidence available for other 

 mollusk species, where hatchery-reared juveniles are 

 more vulnerable to predators when released in nature 

 than wild juveniles (Tegner and Butler 1985). Develop- 

 ment of an inexpensive enclosure for holding large 

 numbers of young-of-the year under the surface of the 

 substrate could further increase survival of hatchery- 

 reared conch juveniles. 



Acknowledgments 



This research was funded jointly through the Univer- 

 sity of Miami by the Wallace Groves Aquaculture Foun- 

 dation and the Kirby Foundation. This support is deep- 

 ly appreciated as is the field assistance provided by 

 many individuals. Employees on Little Whale Cay gave 

 generously of their time and knowledge of the area. 



