SETTLEMENT OF BRACHYURAN MEGALOPAE IN DELAWARE BAY - AN ANALYSIS OF TIME-SERIES DATA

Crab megalopae were collected daily in a secondary estuary near the mouth of Delaware Bay during 4 annual settlement seasons (1989 to 1992). The sampling site consisted of a raft from which 4 replicate collectors were deployed vertically at the water column surface. Each replicate collector was constructed from a segment of 20 cm diameter PVC pipe. The pipe was covered with a removable sheath of loosely interwoven strands of polypropylene upon which the megalopae settled. We assumed that settlement on the collectors was proportional to settlement on natural substrata near the sampling site each day. The 3 families Ocypodidae (Uca spp.), Portunidae (Callinectes sapidus), and Xanthidae comprised the vast majority; of the megalopae collected. Daily settlement events occurred in clusters. Auto-correlation analysis showed decorrelation times ranging from 2 to 3 d for C. sapidus and from 1 to 2 d for xanthids and Uca spp. These define the durations of typical settlement episodes for each taxon. Xanthids and Uca spp, began to settle in mid summer and continued into early autumn. Major settlement of C. sapidus never occurred until late August, and the intensity of settlement usually decreased by late October. Results of G-tests showed no effect of lunar phase on settlement events, regardless of year or taxon. Likewise, binomial goodness-of-fit tests showed no significant effect of the spring/neap cycle on settlement, However, the observed frequency of settlement events for C. sapidus was significantly greater than expected for southward wind events in all 4 yr, while there was no consistent association between winds and settlement of Uca spp. or xanthids. Of the 3 taxa studied, only C. sapidus relies on wind-driven processes for transport to settlement sites. Transport of C. sapidus megalopae back into the estuary requires the coincident occurrence of a southward, alongshore wind event and a nearby patch of megalopae. The stochastic nature of this co-occurrence explains the observed temporal variations in settlement of C. sapidus in the estuary.