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 d u n n g 4 annual settlement seasons (1989 to 1992) The sampllng site consisted of a raft from w h ~c h 4 rephcate collectors were deployed vert~cally at the water column surface Each replicate collector was constructed from a segment of 20 cm diameter PVC plpe The pipe was covered with a removable sheath of loosely interwoven strands of polypropylene upon whlch the megalopae settled We assumed that settlement on the collectors was proportional to settlement on natural substrata near the sampllng site each day The 3 famllles Ocypodldae (Uca spp ) Portunldae (Call~nectes sapidus), and Xanthidae comprised the vast majonty of the megalopae collected Dally settlement events occurred In clusters Auto-correlation analysls showed decorrelatlon times ranging from 2 to 3 d for C sapldus and from l to 2 d for xanthids and Uca spp These deflne the duratlons of typical settlement episodes for each taxon Xanthlds and Uca spp began to settle In mld summer and c o n t ~n u e d into early autumn Malor settlement of C s a p ~d u s nevel 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 s a p ~d u s was slgnlficantly greater than expected for southward wlnd events In all 4 yl while there was no consistent associatlon between winds and settlement of Uca spp or xanthlds Of the 3 taxa studied only C s a p ~d u s relles on wind-dnven processes for transport to settlement sltes Transport of C s a p ~d u s megalopae back into the estuary requlres the coincident occurrence of a southward, alongshore w n d event and a nearby patch of megalopae The stochast~c nature of thls CO-occurrence explains the observed temporal vanatlons in settlement of C s a p ~d u sIn the estuary KEY WORDS Recruitment.Larval transport.Wind-driven .