HERRNKIND ET AL : RECRUITMENT OF SPINY LOBSTERS 



lack of postlarval influx, which was higher than 

 that at the unsilted Burnt Point site, but instead 

 to low rates of postlarval settlement. Results from 

 our habitat selection experiments support this 

 hypothesis because settlement was significantly 

 lower in silted algal clumps than in unsilted 

 clumps. Previous studies showed that postlarvae 

 selectively settled in highly architectured materi- 

 als, like algal clumps (Herrnkind and Butler 

 1986). Thus, heavy silt covering an otherwise pre- 

 ferred habitat either masks the stimuli triggering 

 settlement or contains stimuli that elicit rejection 

 by pueruli. This question remains for further 

 study. We cannot conclusively ascertain from our 

 laboratory experiments the mechanisms govern- 

 ing habitat choice in the field where silted and 

 unsilted habitats may not be adjacent, as they 

 were in our aquaria. Yet for many species with 

 planktonic larvae, these kinds of experiments, 

 coupled with field observations of more general 

 patterns of behavior and abundance, provide 

 valuable insights into natural processes (Sulkin 

 1986). 



Twenty percent of the pueruli we tested in lab- 

 oratory tanks settled in silted clumps despite the 

 general rejection of this habitat. Pueruli settling 

 in silted algae probably metamorphose normally 

 into the first benthic instar, as indicated by the 

 equivalent time-to-metamorphosis and early post- 

 settlement survival in both silt levels tested. 

 However, subsequent residency by juveniles pre- 

 sumably is limited, given their great mobility and 

 preference for unsilted algae with high food levels 

 (Marx and Herrnkind 1985b; Herrnkind and But- 

 ler 1986). We found that prey abundances were 

 significantly lower in silted algae. Thus, even if 

 pueruli settle in the silted habitat the subsequent 

 juveniles may leave to obtain adequate food. Fre- 

 quent interclump movement by juvenile spiny 

 lobsters, searching either for food or unsilted 

 habitat, would predictably result in increased 

 predatory mortality. Susceptibility to predation 

 is much greater for juveniles in the open, than it 

 is for individuals amidst algal clumps or dense 

 seagrass (Herrnkind and Butler 1986). Thus, ju- 

 venile residency patterns and susceptibility to 

 predation may, in addition to locally low settle- 

 ment, contribute to the paucity of lobsters in the 

 silted habitat. 



The algal-bound silt load at No Name Key was 

 roughly twice that at Burnt Point where spiny 

 lobster recruitment was considerable. Our cur- 

 sory surveys from Key Largo to Boca Chica Key 

 indicate that similar silt levels are common. 



though geographically variable in Florida Bay. 

 Benthic algae, including La//re/icm spp., serve as 

 sediment traps (Scoffin 1970) and demonstrate a 

 remarkable resistance to siltation, growing pro- 

 fusely even in heavily silted areas. Silt in these 

 areas is primarily calcareous, most of it probably 

 a byproduct of sediment processing by deposit 

 feeding shrimp (particularly Callianassa), an- 

 nelids, and sea cucumbers. 



We did not evaluate the geographic extent of 

 siltation relative to spiny lobster settlement in 

 Florida Bay. However, the demonstrated aver- 

 sion to settling in naturally silted algae, charac- 

 teristic of the region around No Name Key, 

 strongly suggests that low postlarval recruitment 

 and juvenile abundances would occur in similar 

 conditions elsewhere. The sparse juvenile popula- 

 tion at our silted site, one-tenth that of the un- 

 silted site, suggests deleterious impact of high 

 chronic silt levels in areas of potential recruit- 

 ment. Human activities also cause siltation (Mor- 

 ton 1977; Allen and Hardy 1980). We noted that 

 algal stands adjacent to heavily trafficked boat 

 channels typically were more heavily silted than 

 adjacent areas. We suspect the effect of manmade 

 siltation to be similar to that from natural causes. 

 Although it is now generally accepted that Flor- 

 ida Bay shallows serve as the main nursery 

 grounds for the south Florida spiny lobster popu- 

 lation (Marx 1986), the regional distribution of 

 settlement and early juvenile habitation remains 

 to be mapped. Future wide-area surveys by con- 

 cerned researchers and agencies should include 

 sampling of new spiny lobster recruits as well as 

 silt levels. Meanwhile, sizable human activities 

 such as channel construction, dredging, spoil 

 dumping, coastal development, and mineral min- 

 ing must be viewed as potentially deleterious to 

 spiny lobster recruitment. 



ACKNOWLEDGMENTS 



We thank the Sea World Center for Marine Sci- 

 ence and Conservation on Long Key, FL, for logis- 

 tical support and the use of their facilities. John 

 Hunt and Jim Marx of the Florida Department of 

 Natural Resources Marine Research Laboratory 

 in Marathon provided field assistance and helpful 

 advice throughout the project. Comments by 

 P. Greenwood, D. Wilber, and two anonymous re- 

 viewers substantially improved the manuscript. 

 This research was supported by a Sea Grant 

 award (R/LR-B-16) to W. F. Herrnkind. Addi- 

 tional support was provided M. J. Butler via 



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