FISHERY BULLKTIN: VOL. 86, NO. 2 



pod recruitment. Our research focuses on the im- 

 pact of siltation on spiny lobster postlarvae and 

 early benthic juveniles, stages that are morpho- 

 logically and behaviorally distinct from adults. 



Late stage P. argus phyllosome larvae drift in 

 the oceanic plankton for 6-9 months after hatch- 

 ing and metamorphose offshore into nonfeeding 

 pueruli (postlarvae) that swim inshore and settle 

 in benthic vegetation (Marx 1986). Nev^ly settled 

 pueruli metamorphose into cryptically colored 

 benthic juvenile instars after about one week. 

 Pueruli preferentially settle in highly architec- 

 tured benthic algal assemblages where subse- 

 quent survival and growth depend upon available 

 prey and physical refuge from predators 

 (Herrnkind and Butler 1986). Ubiquitous, widely 

 distributed stands of bushy red algae, Laurencia 

 spp., provide these essential conditions and prob- 

 ably serve as the most important regional settle- 

 ment and nursery habitat for juvenile spiny lob- 

 sters (Marx and Herrnkind 1985a, b; Herrnkind 

 and Butler 1986; Marx 1986). The early instars 

 remain within the algae for several months until 

 attaining about 20 mm carapace length (CL) 

 (Andree 1981; Marx and Herrnkind 1985a) when 

 they begin to occupy crevices in rubble or under 

 sponges, coral, and exposed seagrass rhizome 

 mats. In Florida, postlarval settlement is year- 

 round with vernal, autumnal, newmoon, and oc- 

 casionally aperiodic peaks (Little 1977; Little and 

 Milano 1980; Marx 1986). The spatial pattern of 

 settlement is poorly known although new recruits 

 are widely dispersed within algal habitats; diver 

 surveys have yielded estimates of one juvenile per 

 36 m^ of profuse algal growth (Marx and Herrn- 

 kind 1985a). Yet because postlarvae settle con- 

 tinuously and juveniles grow rapidly, a single 

 hectare of the above habitat is estimated to nur- 

 ture about 1,000 spiny lobsters annually (Marx 

 and Herrnkind 1985a; Marx 1986). There is no 

 compelling evidence suggesting that benthic 

 stage lobsters immigrate into Florida waters from 

 other Caribbean areas, although their planktonic 

 larvae presumably do so (Lyons 1980; Marx 

 1986). Recruitment is thus primarily limited to 

 postlarval influx. Therefore, precise knowledge of 

 the factors influencing postlarval recruitment 

 and recruit mortality is essential to managing the 

 intensive Florida spiny lobster fishery. 



Here we report on studies undertaken to inves- 

 tigate the impact of the observed algal siltation 

 on spiny lobster recruitment. We compared field 

 abundances of both pueruli and early juveniles in 

 a representative silted and unsilted area, exam- 



ined the relationship between siltation and avail- 

 able epifaunal prey, determined the impact of silt 

 load on puerulus survival from settlement 

 through metamorphosis, and tested the prefer- 

 ence of settling pueruli and algal-dwelling juve- 

 niles for silted and unsilted algae. 



METHODS 



Postlarval-Juvenile Abundance in 

 Silted and Unsilted Habitats 



During June through August 1985, we com- 

 pared the natural abundance of newly settled ju- 

 venile spiny lobsters (6-20 mm CL) in previously 

 sampled, chronically silted and unsilted areas. 

 The silted site (No Name Key) was located ap- 

 proximately 30 m off the western shore of No 

 Name Key (Monroe County, FL, U.S.A.) and the 

 unsilted site (Burnt Point) 30 m off the northwest 

 shore of Grassy Key at Burnt Point (Fig. 1). The 

 benthic habitat at both sites was similar and 

 characterized by nearly contiguous stands of 

 algae (Laurencia spp.) at depths of 1.5-3.0 m. In- 

 tensive visual search in algal clumps by divers 

 was used to estimate the relative number of ben- 

 thic juveniles. Because newly settled spiny lob- 

 sters are almost exclusively found associated with 

 algal clumps (Marx and Herrnkind 1985a), catch 

 per unit effort (CPUE) as search time within 

 algae, gives a more suitable estimate for our pur- 

 poses of comparing abundance than density/area 

 per se. Modified Witham-type postlarval collec- 

 tors (Witham et al. 1964, 1968; Little and Milano 

 1980; Marx and Herrnkind 1985a) were used to 

 compare postlarval abundance among sites. 

 Twelve collectors were initially deployed at both 

 sites and visited approximately every 2 weeks for 

 3 months. Collector results are reported in CPUE 

 to standardize catch records biased by the loss of 

 collectors and different sampling durations. 



Algal Silt Content and Prey Content 



To determine the amount of silt and macro- 

 fauna contained in algal clumps at the silted and 

 unsilted sites, we bagged individual clumps (ap- 

 proximately 25 cm diameter) of Laurencia in the 

 field for subsequent laboratory processing. Care 

 was taken to ensure that loose silt present on the 

 surface of the algae was not disturbed during col- 

 lection. Ten clumps, ranging in displacement vol- 

 ume from 55 to 300 mL, were collected at each 

 site. Algal samples were rinsed through a series 



332 



