POPULATION REGULATION OF BLUE CRABS CALLINECTES-SAPIDUS IN THE NORTHERN GULF-OF-MEXICO - POSTLARVAL SUPPLY

Larval supply and early postsettlement mortality are crucial to the ma intenance of most marine populations. Larval supply may be especially important in regulating populations of blue crabs Callinectes sapidus because many larvae could be lost during long migrations between estua ries and offshore waters. We examined the relative contributions of di fferential transport and habitat preferences of megalopae (postlarvae) to 5 potential nursery habitats in Mobile Bay and Mississippi Sound, Alabama, USA. Settlement in each habitat was determined daily during s ummer and fall for 2 yr and was related to sea surface temperature, wi nd stress, tides, current velocity and the lunar and tidal amplitude c ycles. In 1990, most megalopae settled when winds blew megalopae onsho re and tidal amplitudes were minimal. In 1991, onshore winds were ligh t and infrequent, and megalopae settled even more densely during minim um amplitude tides. Semidaily collections of megalopae at dusk and daw n revealed that most megalopae recruit to estuaries during nocturnal f lood tides, which only occur during summer and fall in this diurnal ti dal regime. Thus, onshore winds facilitated transport of megalopae int o estuaries episodically, but megalopae recruited regularly and abunda ntly during nocturnal minimum amplitude flood tides even when onshore winds were light or absent. Most megalopae were collected in the lower bay although some megalopae settled at the head of the estuary 50 km away from the baymouth. Megalopae required about 2 d to travel this di stance as they became increasingly competent to settle from the plankt on. Physiological tolerances may explain why most larvae settled in hi gh salinity waters, but differences in current regimes may best explai n differences in settlement among sites in the lower bay. Several type s of substrate were transplanted from nursery habitats to a sandy area at the baymouth to determine whether megalopae prefer to settle in se agrass beds, marshes or unvegetated substrate once they arrive at a si te. Megalopae strongly preferred to settle on vegetation, but it remai ns unclear whether or not they discriminate among plant types. Thus, p assive delivery and habitat preferences of megalopae both may determin e the initial distributions of juvenile blue crabs.