Behavioral responses of free-ranging blue crabs to episodic hypoxia. I. Movement

Abstract

Episodic hypoxic events in estuaries can alter the trophic dynamics of important benthic predators. During hypoxic upwelling events mobile predators may reduce their feeding activity as they migrate to relatively shallower, oxygenated water, and may reinvade deep-water habitats dur- ing relaxation of hypoxia to exploit vulnerable infaunal prey (e.g. clams and polychaete worms) that have reduced their burial depth in response to hypoxia. We used biotelemetry techniques with con- current measurements of dissolved oxygen (DO) to monitor the feeding and movement responses of free-ranging blue crabs Callinectes sapidus to episodic hypoxic upwelling and subsequent relaxation events within the Neuse River Estuary (NRE), North Carolina, USA. Although telemetered crabs fed in hypoxic water with DO concentrations as low as 1.01 mg l -1 , percent feeding occurrence declined slightly when crabs were exposed to mild (DO = 2 - 4 mg l -1 ) and severe hypoxia (DO 4 mg l -1 ). Crabs reduced the proportion of time spent feed- ing during hypoxic upwelling conditions except for the most severe events when DO dropped rapidly from normoxia to severe hypoxia. The proportion of time crabs spent feeding did not increase and crabs did not reinvade deeper water habitats during relaxation events, as was hypothesized. These results are somewhat inconsistent with previous studies and we suggest that crabs may have fed on prey other than benthic infauna, or that upwelling events may not have lasted long enough to cause infauna to migrate close enough to the sediment surface to be vulnerable to predation by blue crabs. Our study highlights the importance of examining the complex interaction between the hydrody- namics of episodic events and various behaviors (e.g. feeding and movement) when trying to under- stand the impact of these events on estuarine trophic dynamics.