APPELDOORN: MORTALITY IN QUEEN CONCHS 



ability equal for all individuals, that rates of sur- 

 vival are equal for all individuals, and that tag 

 loss is negligible. No field study can hope to ma- 

 nipulate the environment to such a degree that 

 all assumptions are perfectly met. Thus, potential 

 deviations from these assumptions must be con- 

 sidered and their significance and resulting im- 

 plications assessed (Begon 1983). Presented 

 below is a review of each assumption. Based on 

 known or suspected aspects of the biology and 

 ecology of queen conch, potential problems with 

 respect to each assumption are raised. Each po- 

 tential problem is then considered in light of the 

 specifics of the present study, and an attempt is 

 made to assess its significance, if any. Finally, 

 consideration is given to the robustness of Jolly- 

 Seber estimates. In general, it will be shown that 

 deviations from Jolly-Seber assumptions, if 

 present, would be small and have little impact, 

 and in addition, the Jolly-Seber method is robust 

 to such deviations. 



In the present study, sampling was conducted 

 on an areal basis. Although sampling was not 

 truly random (i.e., with areas predetermined in a 

 random fashion), dive sites were spread through- 

 out the study area and chosen without prior refer- 

 ence to, or knowledge of, the specific nature of the 

 habitat or conch density in the immediate area. 

 The two stage sampling design employed allowed 

 every section of the study area to have an equal 

 probability of being covered. The assumption of 

 equal catchability is the more important property 

 here, and there were several points where poten- 

 tial departures from this assumption could have 

 arisen. Queen conchs usually showed a distinct 

 clustered distribution. Within a cluster all indi- 

 viduals were felt to have an equal probability of 

 capture, regardless of size. However, large queen 

 conchs were easier to see at a distance. This would 

 affect capture probabilities if distinct size groups 

 characterized clusters; thus, clusters of large 

 queen conchs would be noticed more frequently. 

 While it was thought that there existed areas dif- 

 ferentially characterized by the abundance of ju- 

 veniles and adults, these areas generally occurred 

 on a larger scale than typically covered on a dive. 

 The random allocation of dive sites, then, should 

 have minimized any effect of this heterogeneity. 

 The tendency of very small queen conchs to re- 

 main buried is a second factor potentially affect- 

 ing catchability. Thus, a small tagged queen 

 conch (<13 cm) might have a reduced probability 

 of recapture until it grows large enough to be 

 exposed most of the time. However, only a very 



small fraction of queen conchs sampled would 

 have been affected by this. By the time the ani- 

 mals are large enough to be sampled in signifi- 

 cant numbers, they will, by the time of next sam- 

 pling, have grown to a size where burial is not a 

 problem; thus recapture would not be affected. In 

 sum, for the size range sampled, it is thought that 

 departures from the assumption of equal catch- 

 ability are small, if present. Pollock and Mann 

 (1983) and Carothers (1973) have both found 

 Jolly-Seber estimates of survival to be robust 

 against heterogeneity of capture probabilities. 



Mortality of queen conch could have been de- 

 pendent upon mark status if tags attracted preda- 

 tors. There is no direct evidence (e.g., observa- 

 tions or signs of predators or predation during 

 tagging periods) to indicate that this problem oc- 

 curred, implying that it did not. Tags quickly 

 fouled with macroalgae and were subsequently 

 impossible for divers to see at any distance. Thus, 

 any tag attraction effect could only occur immedi- 

 ately after initial tagging. As such, the problem is 

 thought to be negligible. Departures from equal 

 probability of survival may have occurred owing 

 to size related processes, but not to a serious de- 

 gree. Natural mortality is thought to decrease 

 with age, with the effect being predominant 

 among small juveniles (Appeldoorn in press a, b). 

 However, the majority of queen conchs sampled 

 here were large juveniles and adults, which 

 should have had similar survival probabilities. 

 Although fishermen usually take all sizes of 

 queen conchs found, large conchs should have suf- 

 fered a greater mortality because generally fish- 

 ermen will not dive in an area unless conchs are 

 visible (i.e., large) from the surface or on shallow 

 test dives. Thus, areas predominantly character- 

 ized by small queen conchs should have been 

 undersampled by fishermen. Again, the potential 

 effect of this is offset because the majority of 

 conchs were large. Also, only a portion of the 

 small queen conchs were from areas particularly 

 characterized by such individuals. Lastly, rate of 

 dispersion is also a function of size (Hesse 1979; 

 Appeldoorn and Ballantine 1983), so the emigra- 

 tion component of survival could have been 

 higher for larger individuals. This, to some de- 

 gree, would counter the size-mortality trend, with 

 the effect being predominant among small juve- 

 niles. As such, the effect here should also have 

 been minor. In sum, there is no reason to suspect 

 substantial departures from the assumption of 

 equal probability of survival, and if survival is 

 independent of mark status and probability of 



801 



