IVERSEN ET AL.: BIOLOGICAL DATA ON QUEEN CONCHS 



Survival-Mortality 



Estimates of monthly and annual survival of un- 

 penned and penned queen conchs in the Berry 

 Islands were derived from tagging studies. The 

 estimates assume that tags are not overlooked, that 

 tags do not fall off or affect survival, that the tagged 

 population is similar to the untagged population in 

 all other respects and that no emigration occurs dur- 

 ing the experiments. 



Monthly survival rate of unpenned queen conchs 

 ranged from 57 to 80%, depending on location (Table 

 8). The estimates for Little Cockroach, Vigilant, and 

 Cat Cays are the most reliable, because more conchs 

 were tagged over a longer period of time at these 

 three locations than at others. Annual survival was 

 low for these areas, ranging from 2 to 9%. These 

 proportions result in estimates of total instantane- 

 ous mortality rate, Z, from 2.41 to 3.91, considerably 

 higher than those reported by Alcolado (1976) for 

 queen conch in Cuba (annual survival 15-35%, Z 

 from 1.06 to 1.90) or by Wood and Olsen (1983) for 

 recruited queen conch in St. Thomas, U.S. Virgin 

 Islands (Z from 0.22 to 1.80). 



Appeldoorn (1985) found mortality of small juven- 

 ile queen conchs (<6.4 cm) in Puerto Rico higher at 

 Z = 8.62, or an annual survival of 0.02%. In a re- 

 cent study of large juveniles and adults, Appeldoorn 

 (in press) estimated annual Z = 2.67, with M plus 

 emigration = 1.53 and F = 1.14. Our survival 

 estimates are probably low because of problems in- 

 herent to tagging studies mentioned above. 



Survival of penned queen conchs in 4 of our pens 

 was much higher than survival of unpenned animals 

 (Table 8). Monthly survival ranged from 90 to 97% 

 for penned animals, and annual survival ranged 

 from 28 to 73%. However, all conchs died in the 2 

 deeper water pens. We attribute much of the in- 

 creased survival rate of queen conch in our best pens 

 to reduced predation, although an undetermined 



portion of the increase is due to eliminating emigra- 

 tion and the increased probability of finding tagged 

 animals in an enclosure. However, the positive in- 

 fluence of increased survival must be balanced with 

 the slow growth rates of all but the smaller (4.5-8 

 cm) sizes in pens. 



Seasonality of burying, as demonstrated in our 

 penned conch data and by Hesse (1979), using un- 

 penned queen conch, can affect the estimated sur- 

 vival rate because some of the animals cannot be 

 found. Based on our results in pens, the possible 

 error in survival estimate due to burying would 

 probably be relatively small because over about a 

 1-yr period, a total of 25 out of 200 queen conchs 

 (about 12%) were buried; however, on any single 

 monitoring trip only 1 or 2 individuals were buried. 



Causes of Mortality 



It is well documented that predation plays a sig- 

 nificant role in the survival of queen conchs (Jory 

 1982; Jory and Iversen 1983; Iversen et al. 1986) 

 and will not be detailed here. After settlement, at 

 all sizes, even flared lipped, thick-shelled adults are 

 subject to predation by large turtles and fishes; 

 however, the rate of predation is significantly higher 

 on the small, thin-shelled juveniles (2-5 cm) and 

 decreases as the animals grow. 



Based on our observations, those of our colleagues 

 and reports in the literature, predation, rather than 

 abiotic factors of the environment, or parasites and 

 disease, seem to be the most important causes of 

 queen conch mortality. Hence, stock size after settle- 

 ment appears to be predator-controlled. This is not 

 an unusual finding when the wide range of species 

 of queen conch predators feeding on all sizes of 

 conch is considered (Randall 1964; Jory and Iversen 

 1983; Iversen et al. 1986), together with the impor- 

 tant role that predation plays in the mortality of 

 many other mollusks (Jory et al. 1984). 



'Only 11 months involved. 



^Monthly survival estimates made using Jackson's formula. 



^Annual survival estimates made using Heincke's formula. 



307 



