BEHAVIORAL-RESPONSE OF FISH LARVAE TO LOW DISSOLVED-OXYGEN CONCENTRATIONS IN A STRATIFIED WATER COLUMN

Density stratification and respiration lead to vertical gradients in d issolved oxygen in many aquatic habitats. The behavioral responses of fish larvae to low dissolved oxygen in a stratified water column were examined during 1990-1991 with the goal of understanding how vertical gradients in dissolved oxygen may directly affect the distribution and survival of fish larvae in Chesapeake Bay, USA. In addition, the effe cts of low oxygen on 24-h survival rates were tested so that results o f behavior experiments could be interpreted in the context of risk to the larvae. Naked goby [Gobiosoma bose (Lacepede)] and bay anchovy [An choa mitchilli (Valenciennes)] larvae strongly avoided dissolved oxyge n concentrations < 1 mg l(-1), were lethal within 24 h at 25 to 27 deg rees C. In addition, naked goby larvae, whose behavior was tested at a wider range of dissolved oxygen concentrations, also showed a reduced preference for an oxygen concentration of 2 mg l(-1), which leads to reduced survival during long-term exposures and to reduced feeding rat es. There were no major differences in behavior or survival between th e two species, or between the two age classes of naked gobies tested. Results suggest that behavioral responses to oxygen gradients will pla y a large role in producing marked vertical changes in abundance of fe eding-stage larvae in Chesapeake Bay; mortality from direct exposure t o low oxygen will likely be much less important in producing vertical patterns of larval abundance.