THE CHALLENGES OF EVALUATING COMPETITION AMONG MARINE FISHES: WHO CARES, WHEN DOES IT MATTER, AND WHAT CAN ONE DO ABOUT IT?

Among species interactions, competition is obviously more nuanced to investigate than predation. Certainly there have been copious, seminal works on competition in marine and freshwater ecosystems, most often on research executed at smaller ecological scales such as rocky intertidal zones, coral reefs, or littoral zones, and often involving experimental manipulation. Such studies have typically emphasized organisms with relatively high site fidelity, and all have focused primarily at the individual level. Here we springboard from those studies to explore competition among marine fishes at broader spatiotemporal scales, scales at which fish populations are distributed, scales at which their associated fisheries operate and are managed, and as observed under ambient (i.e., nonexperimental) system dynamics. Inferring that competition might be occurring among marine fishes requires four conditions: opposite population trajectories, high spatiotemporal overlap, high dietary overlap, and some indication of resource limitation. We used those criteria to examine cases of species pairings from the Northeast United States Large Marine Ecosystem to ascertain if there are broadly applicable rules of thumb to determine when competition might be a significant consideration. We assert that such rules exist, and where competition is strongly suspected, we provide an empirically based method of calculating first-order, model-free interaction terms that can scope the potential magnitude of competitive effects. We do so very much cognizant of several caveats and theoretical considerations; but our main premise is that estimating and evaluating competition for marine fishes from extant, commonly available data is feasible, is highly germane, and has many valuable applications for multispecies and ecosystem models as we move toward ecosystem-based fisheries management.