Growth and survival of bay anchovy Anchoa mitchilli larvae in mesocosm enclosures

Abstract

Factors that affect sunrival and growth of bay anchovy Anchoa mitchilli larvae in the Chesapeake Bay, USA, were studied in situ in 3.2 m3 mesocosm enclosures. Experiments tested the effects of variable prey level and anchovy egg and larvae densities on growth and survival of larvae to at least 16 d post-hatching. Eggs were stocked at 100 m-3 with microzooplankton (< 150 ~m) prey added at nominal levels of 50, 500, 1500, or 5000 1-l. In a second series of experiments, eggs were stocked at 10, 100, or 2 1000 m-3 and prey level was maintained near 500 1-l. Larvae growth was rapid (0.39 to 0.63 mm d-l), even at the lowest prey levels and highest initial egg stocking densities. Growth rates tended to decrease as egg density increased. Instantaneous mortality rates were high (0.08 to 0.23 d-l) but lower than those reported for bay anchovy larvae from estuaries with gelatinous zooplankton and fish predators present. Highest mortality unexpectedly occurred in mesocosms with the highest prey level and lowest mortality occurred in enclosures with the highest initial egg stocking densities. Relative production decreased as prey level increased but Gw:Z ratios were above unity for all except the highest prey level experiment. In the stocking density experiments, highest production occurred in enclosures with the lowest initial egg densities. Some unexpected results may have been induced by effects of the mesocosm enclosures.