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

Very little is known about how periodic hypoxic intrusions into shallow, nearshore habitats influence local migration patterns of mobile species such as the blue crab Callinectes sapidus. These hypoxic events may cause direct and indirect mortality of crabs as well as alter their distribution and abundance patterns. We used biotelemetry techniques with concurrent hydrographic measurements to monitor movement responses of free-ranging crabs to the spatiotemporal dynamics of hypoxic upwelling events. Shallow areas (<1.7 m depth) were rarely hypoxic (<1 % of the time) and should offer refuge for mobile organisms that avoid low dissolved oxygen (DO) concentrations during hypoxic upwelling events within the Neuse River Estuary (NRE), North Carolina, USA. Free-ranging blue crabs generally attempted to avoid hypoxic water (<4 mg l(-1)) by moving inshore towards higher DO concentrations; however, crabs were unsuccessful at moving to normoxic water (>4 mg l(-1)) after exposure to hypoxia during upwelling events. Furthermore, telemetered crabs sometimes remained within severely hypoxic water (<2 mg l(-1)) for several hours. We discuss potential behavioral mechanisms that may explain these surprising results and compare the movement response of our individuals to the population-level movement responses inferred from trawling studies. To better understand the impacts of episodic hypoxic upwelling events on local blue crab populations, future studies need to identify the mechanisms blue crabs use to detect hypoxia and to orient towards more suitable (i.e. normoxic) habitats. Our results suggest that the movement responses of individuals to disturbance events can be used to predict changes in the distribution and abundance patterns of local populations.