Summary of session: Ecology of early benthic juveniles

The unifying theme of the presentations in the session on the ecology of early benthic juveniles reflected a general research thrust in marine ecology and a major emphasis in blue crab ecology, specifically that the roles of dispersal and mortality in determining habitat-specific distribution patterns of young juveniles remain largely unknown. As a consequence, presentations on the ecology of postlarvae and early benthic juveniles focused on four major processes—recruitment, predation, movements, and habitat relationships, with most of the presentations integrating two or more of these processes. Two of the presentations examined recruitment and habitat relationships of young juveniles (Spitzer et al., 2001; Rakocinski et al., 2001), two dealt with dispersal of postlarvae and young juveniles in relation to habitat (Blackmon and Eggleston, 2001; Stockhausen and Lipcius, 2001), one examined predation upon young juveniles (van Montfrans et al., 2001), and one discussed mortality and movements of young juveniles (Etherington et al., 2001). The last four presentations emphasized processes in seagrass beds, whereas the others related work in systems with and without seagrass as a major habitat

Relationship of habitat and spatial scale with physiological state and settlement of blue crab postlarvae in Chesapeake Bay

Physiological state (molt stage) of planktonic and benthic blue crab (Callinectes sapidus) postlarvae (megalopae) was quantified within and outside Chesapeake Bay at various spatial scales. Physiological state of planktonic megalopae advanced significantly from the continental shelf off the Chesapeake Bay mouth, through the bay mainstem, and into upriver stations in the York River, a tributary of Chesapeake Bay. These results imply that settlement and metamorphosis of blue crab megalopae is dependent on location relative to the shelf. The physiological evidence supports the export-reinvasion theory of blue crab recruitment and is inconsistent with a larval retention hypothesis. In the tributary, benthic megalopae were significantly more advanced in molt stage state than planktonic megalopae. Temporal variation in molt stage was observed over days and months. In addition, time to metamorphosis was significantly and positively correlated with molt stage. Advancement in physiological state of megalopae during reinvasion of the estuary may serve as an indicator of likelihood of settlement

Design of a Recreational Fishing Survey and Mark-Recapture Study for the Blue Crab, Callinectes sapidus, in Chesapeake Bay

The development of bay wide estimates of recreational harvest has been identified as a high priority by the Chesapeake Bay Scientific Advisory Committee (CBSAC) and by the Chesapeake Bay Program as reflected in the Chesapeake Bay Blue Crab Fishery Management Plan (Chesapeake Bay Program 1996). In addition, the BiState Blue Crab Commission (BBCAC), formed in 1996 by mandate from the legislatures of Maryland and Virginia to advise on crab management, has also recognized the importance of estimating the levels and trends in catches in the recreational fishery. Recently, the BBCAC has adopted limit and target biological reference points. These analyses have been predicated on assumptions regarding the relative magnitude of the recreational and commercial catch. The reference points depend on determination of the total number of crabs removed from the population. In essence, the number removed by the various fishery sectors, represents a minimum estimate of the population size. If a major fishery sector is not represented, the total population will be accordingly underestimated. If the relative contribution of the unrepresented sector is constant over time and harvests the same components of the population as the other sectors, it may be argued that the population estimate derived from the other sectors is biased but still adequately represents trends in population size over time. If either of the two constraints mentioned above is not met, the validity of relative trends over time is suspect. With the recent increases in the human population in the Chesapeake Bay watershed, there is reason to be concerned that the recreational catch may not have been a constant proportion of the total harvest over time. It is important to assess the catch characteristics and the magnitude of the recreational fishery to evaluate this potential bias. (PDF contains 70 pages

Habitat fragmentation in a seagrass landscape: Patch size and complexity control blue crab survival

Habitat fragmentation is increasingly common on land and in the sea, leading to small, isolated habitat patches in which ecological processes may differ substantially from those in larger, continuous habitats. Seagrass is a productive but fragmented subtidal habitat that serves as a refuge from predation for many animals because its structural complexity limits the detection and capture of resident prey. The singular influence of seagrass habitat fragmentation (e.g., patch size) on faunal survival is largely unknown and has been difficult to quantify because seagrass habitat complexity (e.g., shoot density) and patch size are often confounded and vary seasonally. In early summer 1998 we quantified the effect of seagrass habitat fragmentation on juvenile blue crab (Callinectes sapidus) survival in the absence of covarying complexity by exposing tethered crabs to predators in density-controlled, artificial eelgrass (Zostera marina) plots embedded within natural seagrass patches of four broad size classes (m(2) to \u3e 30000 m(2)). We repeated this experiment in late summer 1998 with three different shoot densities, after predictable environmental events (defoliation and bioturbation) had increased seagrass habitat fragmentation and decreased shoot density. In early summer, crab survival was inversely correlated with seagrass patch area; survival of juvenile blue crabs increased as patch size decreased. in contrast to patterns typically observed in terrestrial and marine systems. This pattern appears to have been due to low abundance of adult blue crabs, the chief predator of juvenile conspecifics, in small patches. In late summer, blue crab survival was greater than in early summer, and survival increased with artificial seagrass shoot density but did not vary with patch size. The breakdown of the relationship between crab survival and patch size in late summer may have resulted from influx of cownose rays, which fragmented large, continuous patches of seagrass into smaller patches in midsummer, potentially equalizing fragmentation across the seagrass meadow. These results show that (1) fragmented seagrass landscapes hold significant refuge value for juvenile blue crabs, (2) fragmentation and crab survival vary temporally. and (3) crab survival increases with habitat complexity (shoot density) regardless of patch size. The findings indicate that habitat patch size and complexity jointly drive organismal survival, and that their influence differs temporally in this dynamic landscape. Thus, ecological processes are sensitive to landscape structure, and studies of habitat structure should incorporate multiple scales of space and time, as well as potentially confounding structural variables

Variable functional responses of a marine predator in dissimilar homogeneous microhabitats

Adult soft-shelled clams (Mya arenaria) persist at low densities in Chesapeake Bay sandy habitats despite Intense predation by blue crabs (Callinectes sapidus). Clam persistence may be a consequence of variation in blue crab foraging rates as a function of clam density and sediment composition. In laboratory aquaria, we measured the functional responses (prey consumption per predator as a function of prey density) of large blue crabs to six densities of adult soft-shelled clams buried at natural depths in two sediment types (mud and sand). Functional responses in sand and mud were differentiated statistically by analyses of(1) residuals and residual sums of squares of discrete and continuous-time models, and (2) the exponent {3 of a general functional response model.

Simulated effects of seagrass loss and restoration on settlement and recruitment of blue crab postlarvae and juveniles in the York River, Chesapeake Bay

Seagrass meadows provide important settlement habitat, food and refuge for postlarvae and young juveniles of the blue crab, Callinectes sapidus. In the York River, Chesapeake Bay, area] cover and distribution of seagrass beds has declined historically. Beds which existed 12-25 km upriver from the mouth disappeared and have not recovered. A model for planktonic postlarval behavior, coupled with a 3-dimensional hydrodynamic transport model for the York River, was used to investigate potential effects of the decline in seagrass abundance, and hypothetical restoration, on blue crab settlement and recruitment to the benthos, both in seagrass and to unvegetated bottom. Effects of habitat loss were investigated in model simulations using two historic patterns of seagrass cover (ca. 1965 and ca. 1996), five patterns with intermediate loss of cover, and three settlement rates. Declines of crab settlement in seagrass

Shelter Selection By Spiny Lobster Under Variable Predation Risk, Social Conditions, And Shelter Size

Shelter use patterns of den dwelling Caribbean spiny lobster, Panulirus argus, appear to be regulated by predation risk. The risk of predation may be modified by (1) social structure, which alters the effectiveness of communal defense, and (2) the scaling between lobster size and shelter size, which enhances the protective capacity of the den. These hypotheses were tested with field enclosure experiments using artificial lobster shelters, which examined the effects of predation risk (i.e., presence or absence of a major predator, the nurse shark Ginglyostoma cirratum), spiny lobster size, social condition (i.e., presence or absence of conspecifics), and shelter size upon den choice by juvenile and adult P. argus. To corroborate the findings of the enclosure experiments we also quantified seasonal, size specific abundance patterns of P. argus in the field by deploying artificial lobster shelters (casitas) of different sizes in two habitats that differed primarily in the potential for gregarious interactions: an inner bay, sand seagrass flat with high lobster densities, and an outer bay, seagrass bed adjacent to coral reefs with sparsely distributed lobsters. The experimental and observational field results were strikingly similar-social condition and the scaling of lobster size to shelter size jointly regulated den choice patterns of adult and juvenile Panulirus argus, particularly under high predation risk. When conspecific density and predation risk were low, lobsters resided primarily in shelters whose dimensions were scaled to their own; when conspecific density was high and predation risk was low, lobsters resided predominantly in large shelters offering the highest potential for gregariousness; when conspecific density and predation risk were high, lobsters shifted to gregarious habitation in smaller, scaled shelters; and, when predation risk was high and conspecific density was low, lobsters occupied smaller shelters. The frequency of gregariousness in the field was much higher at the inner bay site, where lobsters were dense, than at the outer bay site, where lobsters were sparse, even accounting for the difference in lobster density between sites. This study indicates that the density of conspecifics in a given habitat can enhance gregariousness in spiny lobsters, which in turn influences the relative impact of lobster size, shelter size, and predation risk upon den choice. In defining the critical determinants of den choice for P. argus, we also provide an empirical and conceptual framework for identifying how variation in the availability of resources, such as conspecifics and appropriately scaled refuges, influence the distribution and abundance of social, shelter dwelling species

Mechanisms governing ontogenetic habitat shifts: role of trade-offs, predation, and cannibalism for the blue crab

Nursery habitats play a major role in the population dynamics of marine and estuarine species, with the blue crab Callinectes sapidus serving as a model invertebrate. The current paradigm of blue crab habitat use postulates that juvenile survival decreases with size in submerged aquatic vegetation (SAV) due to a reduction in suitably scaled refuge, triggering an ontogenetic shift from SAV to unvegetated habitats. However, alternative mechanisms for this habitat shift have not been examined. We evaluated the paradigm of blue crab habitat use by conducting field tethering experiments in York River (Virginia, USA) nursery habitats using a broad range of juvenile size and SAV cover. Cameras were deployed to identify key predators of juvenile blue crabs and to assess the relative importance of predation and cannibalism as sources of juvenile mortality. Probability of survival increased significantly and additively with crab size and SAV cover. The absence of an interaction between crab size and SAV cover is inconsistent with the current paradigm. Rather, the ontogenetic habitat shift by juvenile blue crabs is likely driven by a density-dependent trade-off between predation risk and foraging efficiency. In images of predation events, adult blue crabs, northern puffers Sphoeroides maculatus, striped burrfish Chilomycterus schoepfi, and oyster toadfish Opsanus tau were identified as predators of juveniles in seagrass beds and sand flats. The high frequency of successful predation events by adult blue crabs suggests that cannibalism is an important source of juvenile mortality and may be as, or more influential, to blue crab population dynamics than finfish predation