Habitat Use by Juvenile Red Snapper in the Northern Gulf of Mexico: Ontogeny, Seasonality, and the Effects of Hypoxia

Red Snapper Lutjanus campechanus in the northern Gulf of Mexico are threatened by the combined influence of fishing pressures, the bycatch of juveniles in the shrimp trawl fishery, and the seasonal formation of hypoxic (<= 2 mg/L O-2) bottom water along the Louisiana-Texas continental shelf. Juveniles are especially at risk from hypoxia because they leave the plankton and transition from pelagic to demersal habitats en masse during summer, when hypoxic conditions peak. We explored the influence of hypoxia on juvenile Red Snapper by examining habitat suitability during years with low (areal extent, <10,000 km(2)), moderate (10,000-19,999 km(2)), and severe (>= 20,000 km(2)) hypoxia through retrospective analyses of groundfish data from the Southeast Area Monitoring and Assessment Program. Significant interannual variability in the strength of juvenile recruitment was evident, and in general the weakest recruitment was observed during years of severe hypoxia. The relative abundance of juveniles in shallow (9-46 m) waters off central Louisiana was significantly reduced during years of severe hypoxia; this reduction was generally accompanied by an increase in relative abundance of Red Snapper longitudinally and in deeper waters. Patterns of habitat use during summer months varied in relation to hypoxia severity, with individuals occupying sites that were deeper, colder, and of higher salinity during years of severe hypoxia. Red Snapper also exhibited ontogenetic habitat shifts into deeper, cooler, and higher-salinity waters as fish size increased. In general, Red Snapper populations appear resilient to the broad-scale effects of severe hypoxia, although more localized effects may be in force but not evident because of the many factors affecting survival.