MEPS 133:73-88 (1996)  -  doi:10.3354/meps133073

Larval supply and early postsettlement mortality are crucial to the maintenance 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 estuaries and offshore waters. We examined the relative contributions of differential 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 summer and fall for 2 yr and was related to sea surface temperature, wind stress, tides, current velocity and the lunar and tidal amplitude cycles. In 1990, most megalopae settled when winds blew megalopae onshore and tidal amplitudes were minimal. In 1991, onshore winds were light and infrequent, and megalopae settled even more densely during minimum amplitude tides. Semidaily collections of megalopae at dusk and dawn revealed that most megalopae recruit to estuaries during nocturnal flood tides, which only occur during summer and fall in this diurnal tidal regime. Thus, onshore winds facilitated transport of megalopae into estuaries episodically, but megalopae recruited regularly and abundantly 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 distance as they became increasingly competent to settle from the plankton. Physiological tolerances may explain why most larvae settled in high salinity waters, but differences in current regimes may best explain differences in settlement among sites in the lower bay. Several types of substrate were transplanted from nursery habitats to a sandy area at the baymouth to determine whether megalopae prefer to settle in seagrass beds, marshes or unvegetated substrate once they arrive at a site. Megalopae strongly preferred to settle on vegetation, but it remains unclear whether or not they discriminate among plant types. Thus, passive delivery and habitat preferences of megalopae both may determine the initial distributions of juvenile blue crabs.

Larval settlement . Larval transport . Habitat preferences . Nursery areas . Estuaries