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Résumé :
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Complex intertidal habitats characteristic of northeastern Pacific coastal estuaries provide critical nursery environments for young-of-the-year Dungeness crab, Cancer magister, yet their role in supporting subsequent year classes remains unclear. Subadult C. magister (40-130 mm; 1+ and >1+ year classes), which reach densities as high as 4,300 crabs ha(-1) in subtidal channels during low tides, migrate during flood tides from subtidal refuges into intertidal habitats to forage. As with other brachyuran species that undertake extensive tidally-driven migrations, intertidal foraging may contribute significantly to the energy budget of subadult C. magister. In order to explore the energetic incentive for intertidal migrations by subadult crabs, we developed an ontogenetically-based bioenergetics model for crabs within Willapa Bay, Washington. The model showed that energetic demand varied spatially across the bay, with the highest average energetic demand of a population of subadult crabs (2.13 x 10(6) kJ ha(-1)) occurring in a habitat stratum termed lower side channel (LSC) and characterized by relatively little subtidal area and extensive intertidal flats. Comparison of model results with subtidal prey production revealed that the latter could not satisfy subadult C. magister energetic demands, especially in LSC where modeled crab predation depleted subtidal prey biomass within 17 simulation days. We estimate that 1 ha of subtidal crabs from LSC would minimally require an additional 1.6 ha of intertidal area to satisfy energetic demands without depleting prey biomass. Our model results support the assertion that C. magister make regular migrations to forage on productive intertidal flats, and suggest that intertidal foraging may contribute significantly to the diet of subadult crabs in coastal estuaries.
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