Growth of planktivorous bay anchovy Anchoa mitchilli, top-down control, and scale-dependence in estuarine mesocosms

Growth of the zooplanktivorous bay anchovy Anchoa mitchilli and its potential to con- trol plankton communities were investigated in estuarine mesocosms. The effects of mesocosm dimensions (size and shape) on anchovy growth and grazing impact were evaluated in 2 experi- ments, of 6 and 15 wk duration, respectively. Estuarine mesocosms of 1 and 10 m 3 volume and 2 shapes were used. The size-shape differences provided mesocosms with 3 volume-to-wall area ratios that determined how 'pelagic' the enclosures were. Anchovy growth rates scaled directly with meso- cosm size and with mesocosm volume:wall area ratio. Growth was slowest in the least 'pelagic' enclo- sures, and fastest in the most 'pelagic' enclosures. The fish had a clear top-down impact on copepod populations. Mesocosms without fish supported copepods that were larger in size and of higher com- munity biomass. Highest anchovy growth rates (and presumed consumption) occurred in mesocosms with lower mean copepod biomasses, further suggesting top-down control. Although not conclusive, trends in trophic relationships were consistent and supportive of the trophic cascade hypothesis. Mesocosms with high mean copepod biomasses tended to have low mean phytoplankton densities, and mesocosms with high anchovy growth rates tended to have high phytoplankton densities. There was no consistent evidence that top-down control by bay anchovy was related to mesocosm sizes or dimensions.