PRESETTLEMENT AND POSTSETTLEMENT DETERMINANTS OF ESTUARINE DUNGENESS CRAB RECRUITMENT

Abundance of early juvenile Dungeness crab (Cancer magister) is dramat ically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mec hanisms underlying this habitat-specific pattern in abundance, we conc urrently examined four components of recruitment to intertidal shell a nd mud habitats at two locations within the Grays Harbor estuary (Wash ington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 48 h after settlement substr ates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early ju venile crab in shell and mud habitats over a 4-mo period. We also desc ribe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postl arval supply between two locations, in combination with a predator exc lusion experiment, to define the relative importance of postlarval sup ply vs. post-settlement survival in regulating population size of juve nile crab in certain intertidal habitats. Water column postlarval supp ly (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metr e) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week-long settleme nt pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differences in wind-d riven surface currents, since tidal current speeds in the two location s were similar. Moreover, there was no correlation between current spe ed and flux of megalopae over the bottom. There was generally no diffe rence in postlarval supply between shell and mud habitats. Our experim ental results further indicate that: (1) the abundance of recently set tled crab megalopae in 0.25 m(2) settlement trays was significantly hi gher 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 s ignificant correlation between postlarval supply and density of megalo pae in shell and mud habitats; and (3) there was a positive and signif icant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the numb er of J1 instars at both geographic locations was reduced to similar l evels, equivalent but steadily decreasing densities persisted througho ut the summer growing season. The decoupling of settlement patterns an d density of J1 instars took place within our 48-h sampling interval. Thus, future attempts to examine the correspondence between larval sup ply and post-settlement abundance of marine benthic species with plank tonic larvae should do so at extremely small temporal scales or a crit ical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distribution of juven ile crab, and that predation (including cannibalism) is a key factor r egulating local population size of early juvenile crabs in intertidal habitats where postlarval supply is relatively high.