Postlarval Supply Research Articles

Predicting species richness and abundance of tropical post-larval fish using machine learning

Post-larval prediction is important, as post-larval supply allows us to understand juvenile fish populations. No previous studies have predicted post-larval fish species richness and abundance combining molecular tools, machine learning, and past-days remotely sensed oceanic conditions (RSOCs) obtained in the days just prior to sampling at different scales. Previous studies aimed at modeling species richness and abundance of marine fishes have mainly used environmental variables recorded locally during sampling and have merely focused on juvenile and adult fishes due to the difficulty of obtaining accurate species richness estimates for post-larvae. The present work predicted post-larval species richness (identified using DNA barcoding) and abundance at 2 coastal sites in SW Madagascar using random forest (RF) models. RFs were fitted using combinations of local variables and RSOCs at a small-scale (8 d prior to fish sampling in a 50 × 120 km2 area), meso-scale (16 d prior; 100 × 200 km2), and large-scale (24 d prior; 200 × 300 km2). RF models combining local and small-scale RSOC variables predicted species richness and abundance best, with accuracy around 70 and 60%, respectively. We observed a small variation of RF model performance in predicting species richness and abundance among all sites, highlighting the consistency of the predictive RF model. Moreover, partial dependence plots showed that high species richness and abundance were predicted for sea surface temperatures <27.0°C and chlorophyll a concentrations <0.22 mg m-3. With respect to temporal changes, these thresholds were solely observed from November to December. Our results suggest that, in SW Madagascar, species richness and abundance of post-larval fish may only be predicted prior to the ecological impacts of tropical storms on larval settlement success.

Temporal patterns in the post-larval supply of two crustacean taxa in Rangiroa Atoll, French Polynesia

The post-larval supply of two crustacean taxa (Brachyura and Stomatopoda) was monitored using one crest net over three lunar months at Rangiroa Atoll, French Polynesia. We captured a total of 37,068 brachyuran and 12,697 stomatopod post-larvae during the study. Post-larval supply was higher during the warm season (February–April) than during the cold season (June–July) for both Brachyura (warm season: 85% of total post-larval supply) and Stomatopoda (warm season: 92%). Moreover, the pulse of the brachyuran post-larval supply occurred predominantly around the last quarter, while the pulse of stomatopods occurred predominantly around the new moon. However, for both taxa, the post-larval supply was lowest around the full moon and the first quarter. Overall, our monitoring highlighted that the post-larval supply of Brachyura and Stomatopoda was modulated by seasons and lunar phases at Rangiroa.

Biological and physical coupling in the lee of a small headland: contrasting transport mechanisms for crab larvae in an upwelling region

Delivery mechanisms for crab postlarvae along upwelling coasts have been inferred from weekly sampling, but more frequent sampling is needed to better relate highly variable oceano- graphic conditions to postlarval supply. Settlement of 8 crab taxa was measured in Bodega Bay, California every 2 d from 12 May through 3 July, 2002. Abundance of postlarval and juvenile settlers was cross-correlated with physical variables. Four upwelling and 4 relaxation events were recorded as fluctuations in wind stress and sea temperature during this period. Transitions to and from upwelling conditions in Bodega Bay were brief and temperature changes occurred rapidly. The strongest correlations between crab abundance and physical variables indicative of upwelling and relaxation conditions were observed for Cancer magister, which settled primarily during relax- ation-favorable conditions, and for Cancer antennarius/productus, which settled primarily during upwelling-favorable conditions, suggesting interspecific differences in delivery of postlarvae to adult habitat. Weak correlations with upwelling-favorable conditions were observed for Pugettia producta/ richii and Pagurus spp. Settlement of Hemigrapsus nudus, Petrolisthes cinctipes, and P. eriomerus exhibited significant correlations with changes in tidal height. These results highlight the existence of multiple taxon-specific delivery mechanisms of closely related taxa in one small geographic region.

Incorporating ecological process and environmental change into spiny lobster population models using a spatially-explicit, individual-based approach

Marine fisheries and the ecosystems that sustain them are increasingly beset by environmental deterioration, yet traditional fishery models used for stock prediction typically handle these dynamics poorly if at all. To do so requires the integration of spatio-temporal change in environmental quality and its subsequent effects on habitat suitability and life history dynamics. Spatially-explicit, individual-based simulation models are particularly well suited to this task and, although they are seeing increased use in fisheries ecology and management, this approach has seen limited application in crustacean fisheries. In 1993, we began development of a spatially-explicit individual-based model (IBM) describing the recruitment of Caribbean spiny lobster ( Panulirus argus) in the Florida Keys, Florida (USA) to investigate the impact of regional changes in environmental quality, habitat structure and postlarval supply on lobster recruitment. The shallow coastal waters of the Florida Keys ecosystem have experienced an unprecedented series of environmental perturbations over the past decade. Seagrass die-offs, cyanobacteria blooms, sponge die-offs and dramatic changes in salinity have occurred and these potentially impact the recruitment of spiny lobsters in the region via both direct and indirect means. Here I provide an overview of the unique approach that we have used to examine these dynamics, an approach that links environmental events that occur on large scales (e.g. changes in habitat structure and salinity) with their population-level consequences for lobsters via impacts that operate on the individual-level. Although not applicable in all situations, spatially-explicit IBMs should see wider use in crustacean fishery applications because of both the ecological insight they yield and their ability to integrate data across hierarchical scales of organization.

Modelling the effect of spatial variation in postlarval supply and habitat structure on recruitment of Caribbean spiny lobster

Many field studies have shown that recruitment of the Caribbean spiny lobster, Panulirus argus, is sensitive to variation in both postlarval supply and local nursery-habitat structure. We used a spatially explicit individual-based model, to investigate the effects of (i) spatio-temporal variation in postlarval supply and (ii) changes in the spatial structure of the nursery habitat on lobster recruitment to the Florida Keys, Florida (USA). By simulating eight different regional scenarios describing postlarval supply, we investigated whether differences in the spatio-temporal delivery of postlarvae to the Florida Keys alters recruitment of subadult lobsters. Our results indicate that random geographical variation in postlarval supply yields the highest predicted recruitment, whereas persistently patchy settlement yields the lowest. Field observations of postlarval supply suggest that the random model is the most realistic. In separate simulations, we determined the sensitivity of the model to changes in the geographic arrangement of nursery habitat and the spatial resolution of habitat structure. The most spatially explicit depictions of habitat structure yielded small, but marginally significant differences in lobster recruitment as compared with more generalized spatial scenarios. These differences may well be magnified when more detailed depictions of postlarval settlement are implemented in the model.

Pre- and post-settlement factors as determinants of juvenile blue crab Callinectes sapidus abundance: results from the north-central Gulf of Mexico

We estimated juvenile abundance and predation potential for young-of-the-year (YOY) blue crabs Callinectes sapidus in upper- and lower-salinity marsh and submerged aquatic vegetation (SAV) 'nursery habitats' of Mobile Bay, Alabama, USA, and surrounding waters, and combined this information with previously published post-larval abundance data from these habitats to evaluate the relative roles of pre- and post-settlement events in determining the abundance of YOY crabs. We found little evidence for a significant relationship between megalopal supply and juvenile abundance, except shortly after a few very large, episodic pulses of postlarvae. However, even after large settlement events, in less than 14 d densities of YOY crabs had declined in a density-dependent manner to prior 'background' levels. These declines in density were probably due to high predation rates that produced as much as 95% mortality on a daily basis. As many previous studies had found, greater predation risk was recorded for crabs on unvegetated substrate than for those in SAV or marsh grass. Overall, losses to predators were high at our study sites compared to those reported in similar studies along the Atlantic coast of the United States. In both years of the study (1990 and 1991), the largest numbers of juvenile crabs were found in poly- and mesohaline SAV and salt marsh habitats. In Year 1, no significant correlations were observed between either marsh stem density or SAV vegetation biomass and YOY crab abundance, although at one location in the second year there was a significant positive correlation between the number of juveniles and mid-bay stem density. No significant differences in crab size (carapace width, CW) were found among locations in either year. In comparison with other, better-studied mid-Atlantic coast estuaries in the US (e.g., the Chesapeake and Delaware bays), very large numbers of megalopae (frequently 1 to 2 orders of magnitude greater) invade the Mobile Bay system, as well as other Gulf Coast estuaries. However, YOY abundance in marsh and SAV 'nursery habitats' was of the same order of magnitude as that reported for mid-Atlantic estuaries. We conclude that although postlarval supply was very large, post-settlement loss to predators was the dominant factor influencing blue crab population dynamics in our north central Gulf of Mexico study area.

Advection and settlement rates in a benthic invertebrate: recruitment to first benthic stage in Homarus americanus

Neustonic post-larvae of the American lobster (Homarus americanus) undertake exploratory forays to search for and select bottom habitat for settlement. Assessments at the end of the settlement season at multiple sites have shown that there may be about a 10-fold increase in concentration from the dispersed pelagic phase to the aggregated early benthic phase in lobsters. Here we examine recruitment at a single site over short time intervals (5–14 d) to evaluate the role of advection, estimate the proportion of potential settlers that recruited to the bottom, and evaluate whether time- or density-dependent effects influenced recruitment. We found that (1) tidal advection easily accounts for the average increased concentration from pelagic to benthic stages; (2) recruitment ranged from 0.1% to 1.8% of the potential settlers advected over the site; (3) the proportion of post-larvae that recruited was similar in high and low recruitment years and had a tendency to increase late in the settlement season; and (4) recruitment was positively correlated with post-larval supply. Densitydependent effects may have existed at the highest levels of recruitment. 2000 International Council for the Exploration of the Sea

Estuarine fronts as conduits for larval transport:hydrodynamics and spatial distribution of Dungeness crab postlarvae

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 164:73-82 (1998) - doi:10.3354/meps164073 Estuarine fronts as conduits for larval transport: hydrodynamics and spatial distribution of Dungeness crab postlarvae David B. Eggleston1,*, David A. Armstrong2, Ward E. Elis1, William S. Patton2 1Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8202, USA 2University of Washington, School of Fisheries, Box 357980, Seattle, Washington 98195, USA *E-mail: eggleston@ncsu.edu Frontal zones are common hydrographic features in estuaries throughout the world, yet there is no consensus on the importance of fronts to larval transport processes. Frontal circulation may act as a 'barrier' to larval transport since it constrains cross-frontal flows, leading to strong along-frontal flows, which in turn may serve as a type of 'larval conduit' by funneling larvae collected at the front to settlement locations. We examined the larval conduit hypothesis by characterizing hydrodynamic features of apparent frontal structure in Grays Harbor estuary (Washington, USA) and quantifying the effects of frontal structure on the distribution and transport potential of Dungeness crab Cancer magister megalopae during spring-time recruitment pulses. CTD casts made perpendicular to fronts indicated that frontal structure was characteristic of buoyant plume and axial convergent fronts at the bay mouth and at a subtidal channel site within the estuary, respectively. Surface drifters deployed seaward of a front at the bay mouth during early flood tide were advected into the front, demonstrating convergent circulation. Surface drifters remained within buoyant plume fronts, which moved up-estuary during flood tide at 0.4 m s-1, and were then entrained in axial convergent fronts. Drifters moved an average of 10 km up-estuary during a single flood tide; the direction that drifters tracked corresponded to prevailing winds. The mean concentration of megalopae, as measured with neuston nets, was significantly higher in fronts than in adjacent water, 20 to 30 m upstream of the front, irrespective of year or location within the estuary. Paired plankton nets deployed at the bay mouth during flood tide indicated that the mean concentration and flux of megalopae was higher at the surface than at 3 to 4 m depth; this trend was significant for megalopal flux. Variable wind stress during the recruitment period of Dungeness crab, combined with the suggested effect of fronts serving as larval conduits whose direction of transport is influenced by prevailing winds, could significantly influence spatial variation in postlarval supply and subsequent settlement within the estuary. Cancer magister · Drifters · Dungeness crab · Estuarine hydrodynamics · Fronts · Larval transport · Recruitment Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 164. Publication date: April 09, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Spatiotemporal variation in postlarval recruitment of the Caribbean spiny lobster in the central Bahamas:lunar and seasonal periodicity, spatial coherence, and wind forcing

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 174:33-49 (1998) - doi:10.3354/meps174033 Spatiotemporal variation in postlarval recruitment of the Caribbean spiny lobster in the central Bahamas: lunar and seasonal periodicity, spatial coherence, and wind forcing David B. Eggleston1,*, Romuald N. Lipcius2, Livingston S. Marshall Jr3, Stephen G. Ratchford1 1North Carolina State University, Department of Marine, Earth and Atmospheric Sciences, Raleigh, North Carolina 27695-8208, USA 2The College of William and Mary, School of Marine Science, Gloucester Point, Virginia 23062, USA 3Morgan State University, Biology Department, Baltimore, Maryland 21239, USA *E-mail: eggleston@ncsu.edu ABSTRACT: A large-scale recruitment study of the Caribbean spiny lobster Panulirus argus in the central Bahamas identified (1) strong spatial coherence in settlement to inshore nursery habitats, (2) temporal variability due in part to stochastic wind forcing, and (3) lunar and seasonal periodicity in settlement. First, we quantified lunar variation in settlement on standardized artificial substrates to determine whether or not intra- and inter-annual variability in recruitment could be adequately described by measuring influx of postlarvae during the first quarter of each new moon, as suggested by previous studies. Next, we compared settlement data obtained from artificial surface substrates to concentrations of postlarvae in the water column and benthic settlement. Long-term (6 yr) spatiotemporal variation in postlarval settlement was then quantified at 8 sites spanning a longitudinal and onshore distance of 43 km and 11 km, respectively. Lastly, we examined the relationship between postlarval settlement and environmental variables with time-series analyses. The key findings were that: (1) significant lunar (first quarter of lunar phase) and seasonal (fall peak) periodicities in settlement were consistent across sites; (2) postlarval settlement varied more across sites (~600%) than between years at a single site (~50%), and inter-site differences were consistent across time (i.e. some sites always had higher settlement than others); (3) settlement was correlated with along-shore winds blowing towards the southeast, but not with cross-shelf winds or water temperature; (4) floating, 'Witham-type' artificial settlement substrates yielded an accurate index of postlarval concentration and flux past a given point; and (5) floating substrates were a relatively poor indicator of benthic settlement, though the relationship between surface and benthic settlement may depend upon benthic habitat availability and postlarval supply. These results indicate that artificial settlement substrates provide a reliable index of postlarval supply to inshore nursery habitats, and that a combination of stochastic and deterministic forces influence recruitment over various scales of time and space. KEY WORDS: Environmental forcing · Panulirus argus · Larval supply · Recruitment · Settlement · Spiny lobster Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 174. Publication date: November 26, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Pre- and post-settlement processes in recruitment of the American lobster

In censuses conducted between 1989 and 1993 the cause of consistent local differences in benthic recruitment of the American lobster ( Homarus americanus Milne-Edwards) to coastal sites in Maine was unclear. Field experiments were conducted to assess the role of pre- and post-settlement processes in causing high and low extremes in recruitment on opposite sides of an outer coastal island in our study area. The west side of the island has some of the highest population densities measured in New England. Our results indicate that postlarval supply determines these differences in recruitment between the two sites. Standardized replicate cobble plots deployed on each side of the island ruled out habitat differences as an explanation for these differences, because they exhibited the same east-west difference in recruitment as the natural habitat. We also ruled out differing rates of post-settlement loss because we recovered previously marked and released settlers in equal numbers in similar plots from both sites. The distribution of neustonic postlarvae and hydrographic evidence indicate that wind-driven surface transport produces an asymmetric postlarval supply to the two sides of the island during the settlement season. Differences in the degree of asymmetry from year to year correspond to differences in the magnitude of eastward transport. We also conducted experiments at the site receiving high recruitment to assess whether new recruits or older year classes were near the saturation of cobble habitat for these animals. The combination of saturation seeding trials, using hatchery-reared lobsters, and weekly counts of natural recruits and immigrants suggests that lobsters may become increasingly subject to crowding as they grow. Movements away from the initial settlement site, probably in part caused by crowding, tend to smooth the inequality in population density which is set initially by postlarval supply.

Temporal patterns and transport processes in recruitment of spiny lobster ( Panulirus argus ) postlarvae to south Florida

We used time-series analysis to identify weekly and annual patterns in the supply of spiny lobster, Panulirus argus (Latreille, 1804), postlarvae to the Florida Keys, USA, over an 8 yr period. We also investigated the relationship between postlarval influx and wind forcing as a transport mechanism using the complex vector-scalar correlation analysis. Postlarval supply had a lunar phase periodicity at 4.5 wk intervals, with postlarval abundance peaking between the new moon and first-quarter lunar phases. A distinct annual cycle of postlarval supply with two peak periods was also apparent. Cross-correlation analysis between relative postlarval abundance and a 12 mo cycle showed that the annual peak occurs in spring, centered around March. With the 12 mo periodicity removed, a smaller peak at 5 mo intervals was also well defined. Wind-forcing for 7 d prior to the time of postlarval collection was marginally correlated with postlarval abundance through the entire time-series; the association was strongest during the late fall to early spring months. The analysis indicated that postlarval supply was correlated with winds from the northeast (ca. 45°), which are associated with winter atmospheric fronts. In contrast to results reported for other spiny lobster populations, these patterns suggest that recruitment of lobster postlarvae to south Florida is predictable only at a gross level and is presumably affected by the temporally inconsistent structure of regional oceanic gyres and variability in the timing of lobster spawning in the Caribbean.

Hydrodynamic decoupling of recruitment, habitat quality and adult abundance in the Caribbean spiny lobster: source - sink dynamics?

Marine species possess dispersive stages that interconnect subpopulations, which may inhabit ‘source’ and ‘sink’ habitats, where reproduction and emigration either exceed or fall short of mortality and immigration, respectively. Postlarval supply, juvenile density and adult abundance of the Caribbean spiny lobster, Panulirus argus, were measured at four widely separated sites spanning >100 km in Exuma Sound, Bahamas. Adult abundance was lowest at a site with the highest postlarval supply and little nursery habitat; hence, it was tentatively classified as a sink. Circulation in Exuma Sound is dominated by large-scale gyres which apparently concentrate and advect postlarvae toward the nominal sink. The remaining three sites, including one marine reserve, had higher adult abundances despite lower postlarval supply, and are therefore tentatively classified as sources. Postlarval supply is probably decoupled from adult abundance by physical transport. Adult abundance is likely decoupled from postlarval supply by the effects of varying habitat quality upon postlarval and juvenile survival, as indicated by non-significant differences among sites in juvenile density. It appears that some sites with suitable settlement and nursery habitat are sources of spawning stock for Panulirus argus, whereas others with poor habitat are sinks despite sufficient postlarval influx.

Role of physical refugia: implications from a mass sponge die-off in a lobster nursery in Florida

In 1991 and 1992, cyanobacterial blooms depleted sponges, the primary refuge of juvenile Caribbean spiny lobsters, in ~20% of the nursery in the Florida Keys, USA. Long-term data from the affected middle Keys were used to study the impact of sponge loss, juvenile abundance, recruitment and shelter use. A manipulative experiment (1991–93) involved artificial shelters on 27 ~0.5-h sites. Conditions on 19 sites over the affected ~ 500 km 2 area were compared before and after the blooms. The entire nursery (~10,000 km2) was surveyed to estimate the impact of the disturbance on total juvenile recruitment. Refuge and lobster abundances declined and the pattern of shelter use changed on previously sponge- rich sites without alternative shelter (solution holes, coral heads, cement blocks, etc.). Although sponge loss often locally reduced juvenile abundance, the juvenile lobster population overall declined by ~5%. The availability of alternative, previously underused shelter (solution holes, coral heads, etc.) in the affected region, continued production in the larger unaffected nursery region, and high postlarval supply that fortuitously coincided with sponge loss all offset a stronger effect. However, postlarval supply is unlikely to remain high until the sponges repopulate the middle Keys (10+ years), so a major factor ameliorating the effect of sponge loss on lobster recruitment may disappear.

Quantitative relationships between postlarval production and benthic recruitment in lobsters, Homarus americanus

Relationships between lobster postlarval supply and benthic recruitment were evaluated within and between oceanographically distinct segments of the range of the American lobster. Postlarvae (PL) were sampled by neuston nets in western Rhode Island Sound and the western Gulf of Maine, USA, from June to September 1989–95. Benthic lobsters were sampled in sublittoral cobble habitat by using a diver-operated airlift at the end of the settlement season. Average annual recruitment densities of young-of-year (YOY) lobsters ranged from 0.3 to 1.7 m-2. YOY recruitment was positively correlated between areas. Integrated seasonal abundance of postlarvae was often much greater in Rhode Island than Maine, but production estimates (PL 1000 m-2 season-1), calculated from moult cycle stages and temperature-dependent growth rates, differed by a factor of <0.5. PL production was positively correlated between areas and explained ≥81% of the annual variation in recruitment in each area and 90% for the two areas combined. In Maine, among-site differences in YOY recruitment persisted for a year after settlement and then began to lessen, at least in part because larger individuals moved into areas of initially lower recruitment

Population regulation of blue crabs Callinectes sapidus in the northern Gulf of Mexico:postlarval supply

Author(s): Morgan, SG; Zimmer-Faust, RK; Heck, KL; Coen, LD | Abstract: Larval supply and early postsettlement mortality are crucial to the maintenance of most marine populations. Larval supply may be especially important in regulating populations of blue crabs Callinectes sapidus because many larvae could be lost during long migrations between estuaries and offshore waters. We examined the relative contributions of differential transport and habitat preferences of megalopae (postlarvae) to 5 potential nursery habitats in Mobile Bay and Mississippi Sound, Alabama, USA. Settlement in each habitat was determined daily during summer and fall for 2 yr and was related to sea surface temperature, wind stress, tides, current velocity and the lunar and tidal amplitude cycles. In 1990, most megalopae settled when winds blew megalopae onshore and tidal amplitudes were minimal. In 1991, onshore winds were light and infrequent, and megalopae settled even more densely during minimum amplitude tides. Semidaily collections of megalopae at dusk and dawn revealed that most megalopae recruit to estuaries during nocturnal flood tides, which only occur during summer and fall in this diurnal tidal regime. Thus, onshore winds facilitated transport of megalopae into estuaries episodically, but megalopae recruited regularly and abundantly during nocturnal minimum amplitude flood tides even when onshore winds were light or absent. Most megalopae were collected in the lower bay although some megalopae settled at the head of the estuary 50 km away from the baymouth. Megalopae required about 2 d to travel this distance as they became increasingly competent to settle from the plankton. Physiological tolerances may explain why most larvae settled in high salinity waters, but differences in current regimes may best explain differences in settlement among sites in the lower bay. Several types of substrate were transplanted from nursery habitats to a sandy area at the baymouth to determine whether megalopae prefer to settle in seagrass beds, marshes or unvegetated substrate once they arrive at a site. Megalopae strongly preferred to settle on vegetation, but it remains unclear whether or not they discriminate among plant types. Thus, passive delivery and habitat preferences of megalopae both may determine the initial distributions of juvenile blue crabs.

Pre‐ And Post‐Settlement Determinants of Estuarine Dungeness Crab Recruitment

Abundance of early juvenile Dungeness crab (Cancer magister) is dramatically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mechanisms underlying this habitat—specific pattern in abundance, we concurrently examined four components of recruitment to intertidal shell and mud habitats at two locations within the Grays Harbor estuary (Washington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 8 h after settlement substrates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early juvenile crab in shell and mud habitats over a 4—mo period. We also describe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postlarval supply between two locations, in combination with a predator exclusion experiment, to define the relative importance of postlarval supply vs. post—settlement survival in regulating population size of juvenile crab in certain intertidal habitats. Water column postlarval supply (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metre) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week—long settlement pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differneces in wind—driven surface currents, since tidal current speeds in the two locations were similar. More—over, there was no correlation between current speed and flux of megalopae over the bottom. There was generally no difference in postlarval supply between shell and mud habitats. Our experimental results further indicate that: (1) the abundance of recently settled crab megalopae in 0.25 m2 settlement trays was significantly higher in shell than in mud habitats, irrespective of whether the trays were placed in 3—5 ha of shell vs. mud; (2) there was a positive and significant correlation between postlarval supply and density of megalopae in shell and mud habitats; and (3) there was a positive and significant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the number of J1 instars at both geographical locations was reduced to similar levels, equivalent but steadily decreasing densities persisted throughout the summer growing season. The decoupling of settlement patterns and density of J1 instars took place within our 48—h sampling interval. Thus, future attempts to examine the correspondence between larval supply and post—settlement abundance of marine benthic species with planktonic larvae should do so at extremely small temporal scales or a critical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distribution of juvenile crab, and that predation (including cannibalism) is a key factor regulating local population size of early juvenile crabs in intertidal habitats where postlarval supply is relatively high.