FISHERY BULLETIN: VOL. 73, NO. 4 



C = rC 



max 



i 



(Coupling to lov/tr trophic level) 



a = a. 



(P) 



tc.r\ 



Q = <x^ 



I 



I' 



Q 



r- 



if 



kC - Q > S , 



max ' 



S = 5 



max 



— - G - kC - Q - Sj^ax 

 at 



If 



< kC - Q < Sn^axr 



S - kC - Q 



dW 



— = G = 



dt 



If 



kC - Q < 0, 



S = 

 dW 



dt 



= G = kC - Q 



V 



E = NS = NuVJ 



R = 1 



a + b 

 E 



dt 



Dnat = -MN 



I 



°PRED "_EPliP 

 W 



(coupling to higher 

 trophic level) 



FISH 



= -FN 



^STARV °n 



^PRED ^ ^STARV 



+ D 



FISH 



Figure 1. -Relationships between principal component equations describing a single species. 



fishery works by the simple decaying exponential 

 function 



/)nat = - MN, 



(14) 



where M is a numerical parameter, the "coefficient 

 of instantaneous natural mortality." The function 

 has been applied very widely, whatever the 



predation rate may be (even where it is zero) and 

 in situations where starvation probably does not 

 occur. Lacking data to support another assump- 

 tion, it seems reasonable to use Equation (14) tO' 

 express the more limited category of natural mor- 

 tality of the present model also. In the model, then, 

 natural mortality is all mortality not due to 

 predation or starvation (the trophic controls) or 



700 



