| Abstract: | I investigated the questions (i) how much movement of fish between areas within a stock is required before the areas can be managed jointly instead of separately and (ii) how is the trade-off between separate and joint management affected by the spatial behavior of the fishing fleet? I addressed these questions using a simulation model of a fishery on a stock that is divided into two areas (substocks) between which fish can move. Under joint management, fleet spatial behavior is characterized by its “switching level,” or the biomass level in the currently fished area at or below which the fleet will switch to the other area. Catch levels were calculated under both separate and joint management for a range of movement rates and switching levels. I also studied the effect of differences in natural mortality rates between the two areas. When the natural mortality rates were the same for the two areas, (i) separate management resulted in higher catch than joint management, (ii) joint management only approached the catch of separate management when movement rate of fish between the two areas was very high, (iii) the difference between separate and joint management was greatest when (a) the switching level of the joint fleet was low (i.e., inertia was high) and (b) the joint fleet had a preference for one area. When natural mortality rate was different in the two areas, and (i) the joint fleet did not prefer one area, (a) separate management produced higher catches at low fish movement rates while joint management produced higher catches at high movement rates and (b) switching level had no effect on catch, and (ii) when the fleet had a preference for the area with the higher natural mortality rate, separate management resulted in higher catches than joint management, and the difference increased with increasing fish movement rate. These simulations suggest that the relative merits of separate and joint management of two areas depends on the assumptions one makes about the spatial behavior of the fishing fleet. This behavior is as important as movement of fish between the areas, which is normally assumed to be the overriding determinant of the relative merits of separate and joint management. |
