-5- 



Constant catch occurs in fisheries where there is a constant market and 

 prices adjust to changes in demand and supply. If a fished biomass becomes 

 lower, usually the catch per unit effort decreases (if the species is not a 

 shoaling pelagic species and not caught exclusively during spawning), and the 

 price usually increases, stimulating a higher effort of fishing. However, there 

 is a lower limit for catch per unit effort (and/or an upper limit for price) 

 and when these limits are reached, fishing might cease. 



Fishing simulation with constant and density dependent portions presents 

 most realistically what is happening in many fisheries; for example, there is a 

 certain amount of stock (density) independent catch, as modern search methods are 

 effective in locating fishable shoals even when stocks are at a low level of 

 abundance, and also stock density dependent catch, either targeted or mostly 

 bycatch. However, in some fisheries (e.g., trawl fishing for flatfishes) the 

 catches are mostly stock biomass density dependent. 



Fishing mortality operates on the exploitable biomass. In simulations of the 

 effect of fishing, the fishing mortality coefficient must be adapted to either 

 number based computations or biomass based computations. In the latter case, the 

 fishing mortality coefficient is calculated relative to either exploitable or 

 total biomass. The latter is used when no age (size) class separation is made 

 in the model. The quantitative relations between these coefficients were described 

 by Laevastu (I983). In this study the fishing mortality coefficient was calculated 

 relative to the whole biomass. 



5. FORMULAE AND INITIAL VALUES USED IN THIS STUDY 

 The monthly biomass (B ) was computed with Formula 1 from the previous month 

 biomass (B , ) : 



B, = B^_,e9---^ (,) 



