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Fishery Bulletin 94(3). 1996 



on mechanistic issues such as reproductive output, 

 recruitment dynamics, and larval import and export. 

 Our data on adult shell size and age, juveniles, and 

 larval size and abundance make this analysis unique 

 and allow considerable insight into distribution 

 mechanisms in the Exuma Cays. An understanding 

 of such mechanisms is critical for making effective 

 management policies. 



Site-specific differences in shell size and shape 

 described for queen conch (Stoner and Schwarte, 

 1994; Martin-Mora et al., 1995) are useful in explain- 

 ing adult recruitment patterns at WW and LSI. Ju- 

 veniles in the shelf environment of Exuma Sound 

 grow rapidly, produce shells that are long and slen- 

 der, and reach sexual maturity (hence terminal 

 length) at a larger size than conch developing on the 

 Great Bahama Bank. On the basis of shell size and 

 shape, low numbers of adults on the bank and near 

 inlets, and high fishing pressure, Stoner and 

 Schwarte (1994) concluded that little migration oc- 

 curred between bank and sound at LSI and that small 

 numbers of juveniles in shallow regions of the sound 

 were the primary source of deep-water reproductive 

 stock. Adults on the bank were mostly young (i.e. 

 thin-lipped), and most were removed by fishing be- 

 fore they could migrate offshore to the primary re- 

 productive grounds in the deeper shelf environment. 



Adults on the bank in the MFR near WW had a 

 length-frequency distribution virtually identical to 

 those on the bank at LSI (i.e. small adults). How- 

 ever, unlike the LSI shelf where large adults domi- 

 nated the population, adult length-frequency in the 

 shelf environment of WW was bimodal. The smaller 

 WW adults originated on the bank and were able to 

 migrate to deeper shelf waters in the absence of fish- 

 ing. Therefore, although there was only one primary 

 source of adults for the sound population at LSI, there 

 were two sources at WW, and migration from the 

 unfished bank provides at least a partial explana- 

 tion for higher densities and the bimodal distribu- 

 tion observed on the adjacent Exuma Sound shelf. 

 Although, in some areas, fishing may result in re- 

 moval of large individuals and in artificial selection 

 for small conch (Appeldoorn, 1994b), this does not 

 appear to be a problem near LSI, where adults were 

 relatively large. 



On the basis of shell lip thickness, it appears that 

 conch were removed from the bank near LSI at an 

 early age, whereas older, thick-lipped conch accumu- 

 lated on the bank in the MFR near WW. The reverse 

 was evident in deep shelf waters where the average 

 age of conch was higher at LSI than at WW. It is 

 possible that harvesting of young adults from shal- 

 low bank and shelf waters near LSI has reduced re- 

 cruitment to the deep-water spawning population. 



It has been assumed that, by allowing fishermen to 

 collect conch only by free-diving, a deep-water ref- 

 uge is secured for conch spawning stock. Although 

 this is true in general, virtually all juveniles and first- 

 year adults in the Exuma Cays live in shallow (<15 

 m) water and all are available to free-diving fisher- 

 men until after reaching adulthood. Protection of 

 habitat used as a migratory pathway is critical in 

 the management of conch stocks. 



Patterns of abundance for queen conch juveniles 

 and larvae suggest that differences in recruitment 

 processes, in addition to fishing pressures, may af- 

 fect differences in the numbers of adult conch be- 

 tween the two study sites. In 1991, juvenile aggre- 

 gations in the MFR were an order of magnitude larger 

 than those in the fished area, where the aggrega- 

 tions were relatively small in all six annual surveys. 

 The large juvenile aggregations in the MFR can be 

 explained by relatively high densities of competent 

 larvae arriving at the site. In 1993, the mean con- 

 centration of late-stage larvae at the offshore shelf 

 stations at the MFR was 17 times higher than the 

 concentration in the fished area. In 1994, the differ- 

 ence was approximately fourfold. The supply of lar- 

 vae to the MFR nurseries appears to be much higher 

 than the supply to nurseries in the fished area. 



Differences in larval supply to a site are affected 

 by three primary processes — production, survival, 

 and transport. Virtually nothing is known about the 

 predators of queen conch larvae or about the sur- 

 vival of queen conch in the plankton. There is little 

 doubt, however, that site differences in larval pro- 

 duction, demonstrated by the density of early-stage 

 larvae, reflect local spawning stock size and density. 

 In terms of transport, near-surface flow along the 

 Exuma Cays shelf is to the northwest at LSI, at WW, 

 and at a station midway between the two sites. From 

 June through September 1993, the current on the 

 shelf at WW flowed to the northwest at 1.6-3.4 cm/s 

 (1.4-2.9 km/day (Hickey 2 ). Given a larval life of 16- 

 28 d (Davis et al., 1993), late-stage queen conch lar- 

 vae arriving in the MFR near WW must originate 

 well to the south, and their high densities most likely 

 reflect a natural accumulation of larvae produced by 

 spawning stocks all along the Exuma Cays. This 

 abundance of recruits must also contribute to high 

 densities of juveniles and adults observed in the 

 MFR. Larvae of a closely related, but unfished 

 strombid, Strom bus costatus, were also highly con- 

 centrated in the MFR in 1994 (Stoner, unpubl. data). 

 We conclude, therefore, that the high density of adult 

 conch at WW results not only from reduced fishing 



-Hickey, B. A. 1995. School of Oceanography, Univ. Washing- 

 ton, Seattle, WA 98195. Unpubl. data. 



