a^ is constant, and there are 9 cells with carrying 

 capacities 10 times that of the rest. These 9 cells 

 are arranged in a square; the distribution of fish is 

 almost constant. Compare this to Figure lb, where 

 a^ is proportional to the degree of saturation of the 

 local capacity. Varying the diffusivity in this way 

 allows the fish to aggregate substantially. This 



FISHERY BULLETIN: VOL. 87, NO. 2, 1989 



heterogeneity may be maintained even if the cells 

 with higher capacities are not contiguous, as in 

 Figure 2a. The proportion of the biomass in each 

 cell that originated in one of the two anomalous 

 cells is shown in Figure 2b. The data for Figure 2b 

 were prepared by keeping separate account of 

 biomass produced in the two anomalous cells. This 



RBUNDflNCE 

 NO FISHING 



ABUNDANCE 

 SOME FISHING 



Figure 2.— Dependent diffusivity: distribution of fish when there is 

 no fishing (a), and the proportion of the local biomass which origin- 

 ated within the two 'hot spots' (b). Fishing affects both the overall 



356 



