SETTLEMENT OF BLUE-CRAB POSTLARVAE IN WESTERN NORTH-ATLANTIC ESTUARIES

We quantified variability in daily settlement of blue crab postlarvae (megalopae) on identical artificial settlement substrates at up to 6 s ites concurrently over a broad geographic expanse (similar to 1300 km) of the western North Atlantic (Delaware-South Carolina, USA). The 4-y ear study encompassed the blue crab recruitment season (generally July -November) from 1989-1992. Regional settlement was characterized by: ( 1) constant low levels of daily settlement punctuated by significantly non-random, episodic peaks of variable duration and intensity with pe aks collectively accounting for at least half the total annual settlem ent at a site; (2) spatial and temporal variability leading to a gener al lack of coherence between sites in a given year and across years wi thin a site; (3) occasional coherence in patterns between sites during a given year, suggesting linkages in regional processes affecting set tlement; and, (4) significant semilunar patterns of episodic settlemen t pulses at the York River and Charleston-Harbor sites over a 4-year p eriod. Thus, regional settlement patterns exhibit both consistent (i.e ., semilunar periodicity, episodic pulses) and variable (i.e., tempora l and spatial variation) elements, which are likely due to a combinati on of stochastic and deterministic processes. Such patterns may impart an ecological advantage to crabs settling en masse (i.e., reduced enc ounter rate with predators through predator swamping) or at continuous low levels (i.e., below a density-dependent threshold) during the rec ruitment season. An identical study illustrated that settlement in Gul f of Mexico estuaries exhibited similarly episodic and highly variable patterns. Daily mean and total annual settlement were up to a hundred -fold greater for gulf than Atlantic Coast estuaries implying populati on limitation by post-settlement processes in the gulf and greater rec ruitment limitation in the Atlantic. These studies emphasize the merit of conducting research over a broad geographic range using standardiz ed techniques to attempt meaningful ecological comparisons.