The recent introduction of the European green crab Carcinus maenas and Asian shore crab Hemigrapsus sanguineus to the west and east coasts of North America, respectively, presents a unique opportunity for investigation into competitive dynamics among intertidal crabs. Juvenile C. maenas utilize rocks, shell, and other cover, and their arrival on the west coast places them in potential competition for these resources with an abundant native grapsid, H. oregonensis. Similarly, H. sanguineus use intertidal shelter on the east coast, thus placing C. maenas in possible competition with grapsids both as an invader on the west coast and as 'resident' on the east coast, having been established there for more than 150 yr. Field sampling and laboratory experiments testing competition for space between C. maenas and Hemigrapsus spp. of equal carapace width were conducted on both coasts, as were videotaped feeding trials to examine and quantify agonistic interactions between species. When competing for food (a single damaged, anchored bivalve), C. maenas dominated over H. oregonensis, while H. sanguineus were overwhelmingly dominant over C. maenas. Within-quadrat, stratified sampling of rocks and sand revealed striking differences in habitat utilization by C. maenas living in the presence or absence of Hemigrapsus. Only ~20% of juvenile C. maenas occurred under rocks in areas occupied by either Hemigrapsus species, while north of the present distribution of H. sanguineus (in Maine) > 97% of the C. maenas were found beneath rocks. This pattern was reflected in laboratory trials as well, where both species of Hemigrapsus consistently dominated in contests for shelter. Given the importance of intertidal cover for small crabs, such competitive interactions will likely affect patterns of habitat use by C. maenas on the east coast and may have important implications for the ultimate distribution and impact of this species in the northeastern Pacific.

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Tidal migrations are inherent in the life histories of numerous demersal predators, and both the indirect and direct effects of these forays are important to the structure and function of littoral communities. In coastal estuaries of the Northeastern Pacific, Dungeness crabs Cancer magister are abundant and compose a significant portion of estuarine biomass. The nursery role of complex littoral habitats for young-of-the-year (0+) C. magister is well documented, yet the ecology of subsequent age classes within coastal estuarine systems, and within littoral areas in particular, remains unclear. The goal of our study was to elucidate habitat use and migratory patterns of subadult C. magister (40 to 130 mm carapace width; 1+ and >1+ yr classes) in littoral eelgrass Zostera marina, oyster Crassostrea gigas, and unstructured littoral habitats (ULH). We employed 3 sampling techniques (trapping, acoustic telemetry, and underwater video) designed to examine various aspects of migratory behavior within Willapa Bay, Washington, a representative coastal estuary. Baited trap surveys revealed that relative catches of subadult C. magister are 30 to 50% higher on ULH than eelgrass or oyster beds, and are negatively correlated with catches of another large cancrid crab, C. productus. Ultrasonic telemetry observations suggest that subadult C. magister making nighttime foraging incursions prefer ULH to other littoral habitats, and underwater video observations show that migrations are influenced by tidal rhythms since movements are correlated with the direction and velocity of current flow in adjacent channels. Our results dramatically alter the perception of C. magister as a predominantly sublittoral predator, and underscore the significance of littoral habitats as important foraging areas.

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The European green crab (Carcinus maenas) is invasive on the U.S. West Coast. This study uses a high-resolution circulation model to determine the likelihood that green crab larvae spawned in Willapa Bay, Washington could be retained by circulation and behavior long enough to reach maturity and re-settle within the bay. A particle-tracking method (the "diffusive Lagrangian return map") is presented that makes it possible to track the dispersion of hundreds of thousands of model larvae-each subject to three-dimensional advection, vertical turbulent diffusion, and imposed vertical migration behavior-over their full 30-50 d development time with modest computational resources. Larvae spawned in summer show significant retention (5-40%) in the southern and western portions of the bay, including the Stackpole shoals near the mouth, the area most likely to be colonized by late-stage megalopae arriving from the coastal ocean. Larvae spawned in spring show much less retention throughout the bay, because of 1) increased flushing caused by increased river input relative to summer conditions and 2) longer development times caused by lower water temperatures. The role of larval swimming behavior is secondary to hydrodynamics in setting these spatial and seasonal patterns of retention.

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Predicting the effects of aquaculture development for coastal ecosystems remains challenging, particularly for data-limited systems, and tools that account for complex ecological interactions are needed to support ecosystem approaches to aquaculture. Here, we used qualitative network models (QNMs) to examine the potential community effects of increasing bivalve aquaculture in South Puget Sound, a large estuarine system in Washington, United States. QNMs are formalized conceptual models that require only a qualitative understanding of how variables composing a system interact (that is, the sign of interactions: +, –, and 0) and are therefore well-suited to data-limited systems. Specifically, we examined community-wide responses to scenarios in which bivalve cultivation effort increased for three different bivalve species (Manila clam Venerupis philippinarum, Pacific oyster Crassostrea gigas, and geoduck Panopea generosa). Further, we evaluated community-wide responses to the removal of benthic bivalve predators, a future increase in nutrient loadings, and combinations of these scenarios acting simultaneously. The scenarios enabled identification of potential trade-offs between increased aquaculture and shifts in the abundance of community members and assessment of the possible effects of different management actions. We also analysed the QNM to identify key interactions that influence the sign outcome of community responses to press perturbations, highlighting potential points for management intervention and linkages deserving of more focused quantitative study. QNMs are mathematically robust and highly flexible, but remain underutilized. We suggest that they may serve as valuable tools for supporting ecosystem approaches to aquaculture.