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

We estimated juvenile abundance arid predation potential for young-of-the-y ear (YOY) blue crabs Callinectes sapidus in upper- and lower-salinity marsh and submerged aquatic vegetation (SAV) 'nursery habitats' of Mobile Bay, A labama, USA, and surrounding waters, and combined this information with pre viously published post-larval abundance data from these habitats to evaluat e the relative roles of pre- and post-settlement events in determining the abundance of YOY crabs. We found little evidence for a significant relation ship 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 th ose in SAV or marsh grass. Overall, losses to predators were high at our st udy 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 a nd salt marsh habitats. In Year 1, no significant correlations were observe d between either marsh stem density or SAV vegetation biomass and YOY crab abundance, although at one location in the second year there was a signific ant positive correlation between the number of juveniles and mid-bay stem d ensity. No significant differences in crab size (carapace width, CW) were f ound among locations in either year. In comparison with other, better-studi ed mid-Atlantic coast estuaries in the US (e.g., the Chesapeake and Delawar e bays), very large numbers of megalopae (frequently I to 2 orders of magni tude greater) invade the Mobile Bay system, as well as other Gulf Coast est uaries. However, YOY abundance in marsh and SAV 'nursery habitats' was of t he same order of magnitude as that reported for mid-Atlantic estuaries. We conclude that although postlarval supply was very large, post-settlement lo ss to predators was the dominant factor influencing blue crab population dy namics in our north central Gulf of Mexico study area.