Stoner and Davis: Outplanting queen conch, Strombus gigas 



407 



than C2 is that juvenile conch realize density-depen- 

 dent protection from predation. Strombus species of- 

 ten live in aggregations (Catterall and Poiner, 1983; 

 Stoner et al., 1993), and recent experiments have 

 shown that juvenile queen conch actually increase 

 their survivorship by living in densities sufficiently 

 high to compromise growth rate in certain cases 

 (Stoner and Ray, 1993; Ray and Stoner, 1994). This 

 aggregation or "herding" behavior probably reduces 

 predation by providing a "probability refuge" 

 (Bertram, 1978; Pulliam and Caraco, 1984); that is, 

 the effects of predators are diluted by the presence 

 of large numbers of alternative prey. Gregariousness 

 provides an explanation for repeated movements of 

 tagged conch from release site C2 toward CI, which 

 was centered in a known juvenile aggregation. Stoner 

 and Ray ( 1993 ) observed a similar response in queen 

 conch translocated at another site near Lee Stock- 

 ing Island. High growth rates were found outside an 

 aggregation, but mortality was high, and movements 

 were always toward the center of the aggregation. Re- 

 gardless of the exact mechanisms involved, it is clear 

 that both density and absolute numbers of conch re- 

 leased may be critical factors in an equation for suc- 

 cess in stock enhancement. Typical nursery aggrega- 

 tions in the Exuma Cays range in size from less than 

 100,000 to several million juvenile conch (Wicklund et 

 al.,1991; Stoner, unpubl. data). We speculate that re- 

 leases at individual sites will need to be made with 

 tens of thousands of conch; however, new research 

 should be designed to determine the optimal number 



that can be released in an area, and how releases at 

 several sites instead of just one or two might improve 

 the probability of stock enhancement. 



In this study there was no significant effect of den- 

 sity on growth rate over the ranges tested (0.5 and 

 2.5 conch-m~ 2 ); however, inverse density-dependent 

 growth was observed in another study at the same 

 site (CD with conch densities ranging from 1.0 to 

 4.0 conch-m -2 (Stoner, 1989b). It is not surprising 

 therefore, that juvenile aggregations in the Exuma 

 Cays are normally found with less than 2.0 conch-m -2 

 (Stoner et al., 1993). Such values are probably site 

 specific. For example, at least one long-term nurs- 

 ery site near Lee Stocking Island rarely has more 

 than 0.2 conch -m -2 (Stoner, unpubl. data). Stocking 

 density will need to be evaluated carefully. 



Different kinds of testing 



General patterns of growth and mortality between 

 the two stock types and the two outplant sites were 

 relatively consistent among three kinds of experi- 

 ments (free-ranging conch, enclosures, and tethers) 

 (Table 11 ); however, the actual values measured were 

 undoubtedly influenced by different manipulations. 

 For example, recovery of free-ranging conch during 

 the first two months of the experiment ranged from 

 only 19 to 52%, whereas during the same period 70- 

 95% survival was observed in enclosures. The cover- 

 less enclosures apparently excluded some important 

 predators (such as the tulip snail, Fasciolaria tulipa ), 



