FISHERY BULLETIN; VOL. 85, NO. 2 



Management Implications: 

 Yield per Recruit 



Yield-per-recruit analysis, based on our estimate 

 of the von Bertalanffy growth equation and total 

 mortality from tagging, was conducted for the 

 shallow-water queen conch populations in the south- 

 ern Berry Islands. Yield per recruit was computed 

 from the model given by Beverton and Holt (1957) 

 which assumes that growth rate, susceptibility to 

 capture, and natural mortality remain constant after 

 age of recruitment. 



We believe that the combined data from unpenned 

 queen conch in all areas (excluding Little Cockroach 

 Cay) gave us accurate estimates of growth and mor- 

 tality. We estimated maximum meat weight for 

 Berry Island conchs to be 463 g, based on the shell 

 length to whole animal weight regression from the 

 Frazer's Hog Cay-Chub Cay area, which was the 

 largest sample. Age at recruitment was assumed 

 to be 3 years (corresponding to a length of 15.4 

 cm). Maximum age of queen conch, based on L^ = 

 30.3 cm and our von Bertalanffy equation, was 11 

 years. 



Using an overall mean annual survival of 7%, Z 

 is 2.66. Estimates of yield per recruit were obtained 

 for a range of values of M between 0.50 and 2.6. F 

 varied between 0.16 and 2.66, by increments of 0.50, 

 and ^0 varied between 1 and 5 years by increments 

 of 1.10 for each level of M. This analysis probably 

 encompassed any value of M and F that actually ex- 

 isted during the study. 



AtM = 0.50, age liable to capture that maximizes 

 yield in weight per recruit is 3 years, over a range 

 of F from 0.16 to 2.66. Thus, if fishermen take queen 

 conch of approximately 15 cm and larger, they 

 would maximize the yield available from the popula- 

 tion. At all values of M above 0.50, results indicate 

 a stage of underfishing because yield in weight per 

 recruit increases over the range of F and decreases 

 with increasing age liable to capture. 



The results of yield-per-recruit analyses are 

 limited for several reasons. First, larger, faster 

 growing queen conchs from Little Cockroach Cay 

 were excluded from the analyses; the von Bertal- 

 anffy equation did not describe well the growth of 

 conchs from the full data set. Second, the range of 

 sizes in the tagging studies did not accurately reflect 

 the range of sizes in the total queen conch popula- 

 tion in the southern Berry Islands. Most data were 

 collected on immature conchs (before lip formation) 

 that were living on shallow flats near islands. While 

 the purpose of analyzing prospects for mariculture 

 were adequately fulfilled by these data, they should 



not be used for fisheries management because they, 

 for the most part, do not include the larger adults 

 found in deeper channels between cays. 



While these data are preliminary, they indicate 

 an important management principle that was also 

 determined for queen conch in the U.S. Virgin 

 Islands by Wood and Olsen (1983); namely, that 

 maximum yield per recruit is obtained at age of first 

 harvest, which is just at onset of lip formation. In 

 the Virgin Islands they found the maximum yield 

 could be obtained by harvest between 3 and 5 years, 

 at an average length of 15.78-19.1 cm. Maximum 

 yield per recruit may occur below onset of matur- 

 ity, however, since there is some evidence that 

 queen conch may not be reproductively active until 

 some time after lip formation (Wilkins et al., in 

 press). 



Mariculture Potential 



Queen conch mariculture potential, one objective 

 of this study, was investigated as a possible means 

 to increase conch production in the Bahamas. A 

 hatchery was established at the University of Miami. 

 Techniques were developed for mass-rearing queen 

 conch from egg masses collected in the wild through 

 the larval stages (Siddall 1983). However, because 

 of the high hatchery costs and high mortality 

 associated with planting young mollusks in the wild 

 (Iversen et al 1986; Jory et al. 1986), supplementing 

 natural conch stocks by extensive mariculture does 

 not appear to be economically feasible at this time. 

 We placed juvenile conchs in pens at densities slight- 

 ly higher than those in nature and found very slow 

 growth, meanwhile experiencing considerable dif- 

 ficulty in physically maintaining the pens. Further, 

 complete mortalities occurred in some pens, which 

 we cannot explain. 



Our results suggest that, for the numbers of queen 

 conch required for supplementing natural stocks, the 

 techniques available could probably only be success- 

 ful in certain well-protected areas. In Bonaire, a 

 degree of success has been reported by Hensen 

 (1983). For intensive mariculture, unless a special 

 area is found with good water exchange, natural 

 food availability, where most predators can be ex- 

 cluded, and large juvenile conchs released, this tech- 

 nique of attempting to enhance production does not 

 appear to have much potential at this time. With 

 additional research, particularly on developing 

 dependable hatchery techniques and cost-efficient 

 means of predator protection, intensive mariculture 

 may some day play a useful role in increasing 

 production. 



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