FISHERY BULLETIN: VOL. 85, NO. 4 



capture is independent of age, Jolly-Seber sur- 

 vival estimates are not greatly affected by age- 

 dependent mortality (Seber 1982). 



There are methods available for determining 

 age-dependent mortality rates from Jolly-Seber 

 data (Pollock 1981; Pollock and Mann 1983). 

 Queen conchs, however, are difficult to age (Ap- 

 peldoorn 1987). Length groups could be used, but 

 these would have to be defined narrowly so that 

 all individuals could grow into the next largest 

 size group within the 3 months between sampling 

 periods, thereby reducing sample sizes within 

 length groups to impractical levels. Data pooled 

 over time would have forced confounding of natu- 

 ral and fishing mortality effects, which was 

 deemed undesirable. 



Tags were held securely in place because spines 

 on the queen conch shell spire prevented tie lines 

 from slipping off, and they became more secure 

 over time due to fouling. Numbers remained 

 readable throughout the 2-yr period. In only those 

 individuals with poor spine development could 

 the tag be lost, and then only within the short 

 time prior to fouling. These included some very 

 small juveniles (<12 cm) and a few very old queen 

 conchs where spines had been eroded. The total 

 number of such potential cases was exceedingly 

 small and is, therefore considered negligible. 



Estimation of Emigration and 

 Natural Mortality 



Permanent emigration of queen conch was 

 known to occur. Reports from fishermen placed 

 some individuals as much as 9 km away from the 

 study area, although the time this took is un- 

 known. The degree of emigration needs to be ac- 

 counted for if a more accurate estimate of natural 

 mortality is to be obtained. Emigration can occur 

 by either of two dispersion processes: random dif- 

 fusion, and directed migration or drift. Seasonal 

 migrations are expected in late fall and early 

 spring based on other studies (Hesse 1979; Appel- 

 doorn 1985), although the expected distance trav- 

 elled is unknown. However, no evidence of such 

 migration is apparent in the data for either mor- 

 tality or recruitment. Possible reasons for this are 

 1) the mortality component is confounded by fish- 

 ing effects thereby masking seasonal trends, 2) 

 Jolly-Seber estimates of B are typically imprecise 

 (Seber 1982), and 3) sampling periods were too 

 few and inopportunely placed. Since the data do 

 not support the occurrence of significant drift, at 

 least to the point where it can be partitioned from 



diffusion, the degree of emigration was estimated 

 by analyzing diffusion only. 



Skellam (1951) presented a two-dimensional 

 diffusion model which can predict the proportion 

 of a population (P^) outside the area of radius (p^) 

 in a given time it) if the average distance trav- 

 elled (e) per unit time (M ) is known. Assuming no 

 mortality or birth, the equation is 



Pt = exp[- (p,2)/(^  e2/An] 



To estimate emigration an average value of e is 

 needed. Dispersal ability in Strombus is related 

 to size (Hesse 1979; Miller 1972; Appeldoorn and 

 Ballantine 1983). Specifically, if total movement 

 is expressed solely as diffusion, the data of Hesse 

 (1979) from a 1.5-mo period encompassing 

 episodes of both diffusion and drift indicated that 

 adults travelled twice as fast as "maturing" 

 queen conchs (adults with thin lips or very large 

 juveniles) and three times as fast as juveniles. 

 Hesse (1979) recorded adults to move commonly 

 50-100 m/day, but no average figure was given, 

 and it is assumed that rates less than this were 

 also common. Clifton et al. (1970) tracked one 

 group of adult queen conchs at 45-55 m/day over 

 several days. Given these rates, a value of 50 m/ 

 day seems a reasonable estimate of e for adults, 

 averaging higher values during migration with 

 lower values at other times. The La Parguera 

 population consists of both juveniles and adults, 

 so this value needs to be adjusted downward ac- 

 cordingly. Since the majority of queen conchs 

 were old juveniles or young adults, an average 

 stage of "maturing" can be assumeed, and a value 

 of e, one half that for adults, would be most appro- 

 priate, i.e., e = 25 m/day. If it is assumed that the 

 0.4 km^ study area is a circle of radius ca. 350 m, 

 then in one sampling period (90 days) 11.2% of the 

 population would be expected to emigrate. This 

 results in an instantaneous rate of annual emi- 

 gration equal to 0.481. Subtracting this from the 

 estimate of natural mortality plus emigration 

 (1.533) yields a corrected estimate of natural mor- 

 tality rate at 1.05. 



This value of natural mortality rate is lower, 

 but consistent with values reported by Alcolado 

 (1976), Berg (1976), and Baisre and Paez (1981), 

 which might be expected since their estimates 

 were limited to juveniles. However, it is still 

 much greater than those reported by Wood and 

 Olsen (1983). Hoenig (1983) presented empirical 

 equations predicting mortality rate on the basis 

 of oldest known age, which can be used for com- 



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