In some earlier fisheries population studies it has been assumed that natural 

 (predation) mortality can be considered (and computed) as consisting of two parts: 

 one part being independent of population density ("constant fraction"), and the 

 other part changing in proportion to the change in stock size. We follow the 

 same assumptions here. The "constant fraction" of predation mortality was about 

 half of the total predation mortality when the biomass was at the equilibrium. 

 The biomass density dependent part of predation mortality was made to change 

 linearily with the size of the actual biomass. Thus, the effects of fishing on 

 predation mortality (including the effects of cannibalism) are simulated 

 indirectly via this density dependent predation. 



Proportioning of constant and density dependent predation varies from species 

 to species and is, in general, difficult to ascertain in a single species 

 consideration, but can be computed in a holistic ecosystem simulation, such as 

 DYNUMES. In the present study, an approximately half-half relation was selected 

 and the total predation value was tuned at biomass equilibrium (which is the 

 initial biomass input value). 



^. SIMULATION OF CONSTANT AND DENSITY DEPENDENT FISHING 



The effects of fishing were simulated with three different approaches: 

 1) a constant catch, independent from biomass density, except it vanished entirely 

 at low biomasses (i.e., when the biomass is about one fourth of the equilibrium 

 biomass); 2) fishing consisting of a constant catch plus a biomass density 

 dependent catch; and 3) a biomass density dependent catch only. The effects of 

 these three different fishing mortalities at different levels were studied with 

 the model . 



