Stoner and Waite Habitat associations of Strombus gigas within seagrass meadows 



583 



large individuals suggests that there is strong selec- 

 tive pressure for habitat choice in small conch, probably 

 via size-specific mortality. Susceptibilty of juveniles to 

 predation is known to decrease with increasing conch 

 size (Appeldoorn 1984). 



Whether the observed distributional patterns are 

 ultimately a response to foods and/or predators can- 

 not be determined from data presented here; however, 

 new experiments using artificial structures and pred- 

 ator manipulations would be useful. In any case, strong 

 habitat preferences suggest that evolved behavioral 

 mechanisms are influential, and the evolution of habitat 

 choice in conch is probably related to both procurring 

 sufficient foods and avoiding predators. 



Significant effects of date in the density and biomass 

 of conch in the seagrass beds was related to the normal 

 seasonality of reproduction and recruitment in conch 

 of the Exuma Cays. The 1 -i- year-class (~40-100 mm), 

 spawned during the previous summer (1987), was most 

 abundant in July. In the Exuma Cays, spawning occurs 

 between April and October (Stoner et al. In review), 

 with greatest recruitment to the benthos probably oc- 

 curring between August and October. Juvenile conch 

 spend most of their first year buried in the sediment; 

 therefore, only the 1987 and older cohorts were 

 surveyed in July 1988 and Febnaary 1989. In February, 

 the 1988 cohort had not yet emerged from the sediment 

 and, therefore, the 1 + year-class was not found in that 

 survey. 



Information on the settlement and distribution of 

 juvenile conch in their first year of life will be par- 

 ticularly useful in elucidating mechanisms of distribu- 

 tion and abundance. Conch less than 40 mm have been 

 found in very shallow, unvegetated subtidal sediments, 

 moving to deeper waters or seagrass beds with age 

 (Weil and Laughlin 1984; Stoner and Sandt, unpubl. 

 data). Highest abundance of 1-year-old conch in sparse 

 seagrass close to sand shoals, therefore, could be a 

 result of ontogenetic migration. 



The relationship between juvenile conch and easily 

 measured characteristics of the macrophyte community 

 should prove useful in predicting conch distributions 

 and in planning stock-enhancement programs involv- 

 ing the seeding of hatchery-reared conch. The lack of 

 statistical interaction among the factors site, station, 

 and date suggests that the strong relationships be- 

 tween conch and seagrass shoot density (and other 

 macrophyte characteristics) are relatively universal. It 

 must be pointed out, however, that the surveys were 

 conducted within areas known to be long-term conch 

 nursery sites, and there are extensive areas of sea- 

 grasses on the Exuma Bank near Lee Stocking Island 

 which appear to have similar habitat characteristics but 

 no resident conch populations. Recent transplant ex- 

 periments have shown that many of the habitats with 



appropriate seagrass, sediment, and detritus char- 

 acteristics do not provide good growth or survivorship 

 in juvenile conch (Stoner and Sandt, In press). Clear- 

 ly, there are unmeasured variables in the seagrass 

 meadows which are important aspects of habitat qual- 

 ity for conch. Habitats without natural conch popula- 

 tions, which produced good growth and survival rates, 

 may be recruitment limited. Such limitation may be a 

 function of larval supply or ontogenetic movements in 

 the species. This study demonstrates that there is a 

 strong relationship between conch density and biomass 

 and the amount of seagrass structure in the habitat; 

 however, general seagrass characteristics cannot be 

 used as predictors of conch distribution independent 

 of other variables such as food quality, presence of 

 predators, hydrographic considerations, and recruit- 

 ment processes. 



Acknowledgments 



This research was supported by a grant from the 

 National Undersea Research Program, NOAA, U.S. 

 Department of Commerce. We thank P. Bergman, 

 L. Cox, B. 011a, K. McCarthy, V. Sandt, C. Tanner, 

 R. Wicklund, and E. Wishinski for assistance in the 

 field and for discussion of the field data. R. Appel- 

 doorn, L. Marshall, L. Riggs, C. Ryer, V. Sandt, and 

 R. Wicklund provided thoughtful criticism of the 

 manuscript. 



Citations 



Adams, J.E. 



1970 Conch fishing industry of Union Island, Grenadines. West 

 Indies. J. Trop. Sci. 12:279-288. 

 Appeldoorn. R.S. 



1984 The effect of size on mortality of small, juvenile conchs 

 (Strombus (ligag Linne and S. costatus Gmelin). J. Shellfish 

 Res. 4:37-4;i 

 Bell. J.D.. and M. Westoby 



1986 Abundance of macrofauna in dense seagrass is due to 

 habitat preference, not predation. Oecologia (Berl.) 68: 

 205-209. 

 Berg, C.J. Jr. 



1976 Growth of the queen conch, Strombus (figaa. with a discus- 

 sion of the practicality of its mariculture. Mar. Biol. (Berl.) 

 34:191-199. 



Bertram, G.C.L.. and C.K.R. Bertram 



1968 Bionomics of dugongs and manatees. Nature (Lond.) 

 218:423-426. 

 Brownell, W.N.. and J.M. Stevely 



1981 The biology, fisheries, and management of the queen 

 conch. Strombus yigas. Mar. Fish. Rev. 43(7): 1-12. 

 Brownell, W.N., C.J. Berg Jr., and K.C. Haines 



1977 Fisheries and aquaculture of the conch Strombus gigas 

 in the Caribbean. FAO Fish. Rep, 206:59-69. 



