MULLEN: FISH THROUGH VARIABLE DIFFUSIVITY 



biomass diffused and died as the rest, in propor- 

 tion to its abundance; production was determined 

 by the total biomass but ascribed according to 

 location. 



Fishing has an effect upon the distribution of fish; 

 the effect is at first surprising (Fig. 2c). A relative- 

 ly high cost per unit of effort constrains effort; a 



profit is possible only in those cells with the highest 

 concentrations of fish. In such cells the effort in- 

 creases unto a profit is no longer realized. The abun- 

 dance has then been reduced to a level that can sus- 

 tain existing fishing operations, but the catch rate 

 offers no incentive for further investment. From the 

 equation for effort (Equation (4)), the equilibrium 



PROPORTION FROM ' HOT SPOTS' 

 NO FISHING 



flBUNDflNCE 

 nUCH FISHINi, 



Figure 2.— Ccmtin««rf— abundance and its distribution (c). Halving the cost 

 of effort halves the level to which the fishery can reduce local populations, 

 and the area of fishing expands to most of the region (d). 



357 



