Pre- and post-settlement factors as determinants of juvenile blue crab Callinectes sapidus abundance: results from the north-central Gulf of Mexico

Abstract

We estimated juvenile abundance and predation potential for young-of-the-year (YOY) blue crabs Callinectes sapidus in upper- and lower-salinity marsh and submerged aquatic vegetation (SAV) 'nursery habitats' of Mobile Bay, Alabama, USA, and surrounding waters, and combined this information with previously published post-larval abundance data from these habitats to evaluate the relative roles of pre- and post-settlement events in determining the abundance of YOY crabs. We found little evidence for a significant relationship between megalopal supply and juvenile abundance, except shortly after a few very large, episodic pulses of postlarvae. However, even after large settlement events, in less than 14 d densities of YOY crabs had declined in a density-dependent manner to prior 'background' levels. These declines in density were probably due to high predation rates that produced as much as 95% mortality on a daily basis. As many previous studies had found, greater predation risk was recorded for crabs on unvegetated substrate than for those in SAV or marsh grass. Overall, losses to predators were high at our study sites compared to those reported in similar studies along the Atlantic coast of the United States. In both years of the study (1990 and 1991), the largest numbers of juvenile crabs were found in poly- and mesohaline SAV and salt marsh habitats. In Year 1, no significant correlations were observed between either marsh stem density or SAV vegetation biomass and YOY crab abundance, although at one location in the second year there was a significant positive correlation between the number of juveniles and mid-bay stem density. No significant differences in crab size (carapace width, CW) were found among locations in either year. In comparison with other, better-studied mid-Atlantic coast estuaries in the US (e.g., the Chesapeake and Delaware bays), very large numbers of megalopae (frequently 1 to 2 orders of magnitude greater) invade the Mobile Bay system, as well as other Gulf Coast estuaries. However, YOY abundance in marsh and SAV 'nursery habitats' was of the same order of magnitude as that reported for mid-Atlantic estuaries. We conclude that although postlarval supply was very large, post-settlement loss to predators was the dominant factor influencing blue crab population dynamics in our north central Gulf of Mexico study area.