An agent adjusts its harvest in an intertemporal optimization problem taking other agents' harvests as fixed. With stock size affecting harvest costs, an intertemporal externality is present. Cooperative and competitive solutions are compared given an exogenously fixed number of agents. Then an entry-exit relation is introduced for each regime. Multiple equilbria obtain for each regime. One cannot say that the competitive solution has too many boats and too small a steady state stock of fish relative to the cooperative regime.