ASSESSMENT OF MORTALITY IN AN OFFSHORE POPULATION OF QUEEN 

 CONCH, STROMBUS GIGAS L., IN SOUTHWEST PUERTO RICO 



Richard S. Appeldoorn' 



ABSTRACT 



A Jolly-Seber multiple tag-recapture experiment was conducted for 2 years on a queen conch popula- 

 tion ofTshore of La Parguera, Puerto Rico in order to estimate mortality rates. Over 2,000 individuals 

 were tagged in 9 sampling periods spaced at 3-month intervals from August 1983 to August 1985. The 

 occurrence of fishing in half the intervals allowed estimates to be made of both fishing and natural 

 mortality rates. Fishing mortality averaged 1.14 over the study period. An upper limit of natural 

 mortality including effects of emigration, was estimated to be 1.53. Assuming random diffusion, 

 emigration was estimated and subtracted from the above yielding a corrected value of natural 

 mortality of 1.05. 



The queen conch, Strombus gigas, is one of the 

 most valuable fishery resources of the Caribbean. 

 Under heavy fishing pressure, stocks throughout 

 the region have declined (Brownell and Stevely 

 1981), and the need for management has become 

 increasingly obvious. Ideally, management deci- 

 sions should be based on a firm understanding of 

 queen conch biology, stock dynamics, and rates of 

 exploitation. Estimates of natural and fishing 

 mortality rates are fundamental to understand- 

 ing the dynamics of exploited populations and are 

 prerequisites of many stock assessment tech- 

 niques, e.g., yield-per-recruit analysis (Beverton 

 and Holt 1957). 



All previous reports of natural mortality rate in 

 queen conch populations have been made using 

 only juveniles (Alcolado 1976; Baisre and Paez 

 1981; Wood and Olsen 1983; Berg 1976). There 

 are two primary reasons for this: 1) populations 

 where adult queen conch are not fished have not 

 been studied or assessed, thus mortality esti- 

 mates would include losses due to fishing as well 

 as from natural causes; 2) estimates derived from 

 length-frequency analysis cannot be made for 

 adults because growth in length ceases at the 

 onset of sexual maturity. These past estimates of 

 natural mortality rate have all been high, rang- 

 ing between 1.0 and 4.0, depending upon the age- 

 group studied, except those of Wood and Olsen 

 (1983) who reported values of 0.19 and 0.04 be- 

 tween ages 1 and 2 and ages 2 and 3, respectively. 

 Thus, there is a dichotomy among reported val- 



iDepartment of Marine Sciences, University of Puerto Rico, 

 Mayaguez, Puerto Rico 00709. 



Manuscript accepted July 1987. 



FISHERY BULLETIN: VOL. 85, NO. 4, 1987. 



ues. Intuitively, it is difficult to predict whether 

 high or low values of natural mortality rate 

 should be expected. A low value of natural mor- 

 tality rate, comparable to that of Wood and Olsen, 

 might be expected for S. gigas because, in gen- 

 eral, natural mortality is inversely related to size 

 (Ursin 1967; Peterson and Wroblewski 1984; 

 Blueweiss et al. 1978; Pauly 1980) and other 

 large mollusks, from temperate areas, have typi- 

 cally low values of natural mortality. However, 

 natural mortality should also be significantly re- 

 lated to temperature, both directly and through 

 growth-rate mediated effects (Pauly 1979, 1980). 

 Thus, higher values should be expected in tropi- 

 cal species. A casual literature review revealed 

 only one estimate of natural mortality (M) in 

 tropical mollusks for comparative purposes: 

 M = 3.66 for the aplysiid gastropod Dolabella au- 

 ricularia from the Philippines (Pauly and 

 Calumpong 1984). 



In the present study, rate of mortality was esti- 

 mated for an offshore queen conch population 

 spanning a wide size range and including both 

 juveniles and adults. The Jolly-Seber multiple 

 tag-recapture method was used. Separate esti- 

 mates were made during periods of fishing and 

 nonfishing, thus allowing partitioning of mortal- 

 ity into its fishing and natural components. 



METHODS 



The study area was located in southwest Puerto 

 Rico, 7 km south of La Parguera (Fig. 1). The area 

 has a uniform depth of 17 m and consists of a 

 broad, patchy sand and macroalgal plain with oc- 



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