COMPARATIVE POPULATION STUDIES IN FISHERY BIOLOGY 57 



applies (see Gulland (1959) for discussion of this problem in connection with 

 Sebastes investigations in the North Atlantic). 



Holt (19 57^) also showed that a simple substitution can be made in order 

 to express the catch in terms only of the ratios of mortality to growth rates 

 and of the sizes offish at recruitment, at first liabihty to capture by the gear 

 and at the end of their fishable life-span, all relative to the asymptotic sizes 



-'-'00 » ''00' 



Thus we find 



— i z^(- in- in- (fe^r 1 



For particular types of catch prediction other simpHfying substitutions are 

 possible. Thus in a note on the assessment of proposals for mesh regulation. 

 Holt (1958^) showed that the expected long-term benefit to be obtained by 

 increasing the age at first liabihty to capture from ^t^ to ^tc is, in terms of the 

 pre-regulation catches, given by 



where Y = original average total catch weight, of which a part ^Y consists 

 offish younger than age 2^c ^^^ 2^ consists offish older than 

 that, 

 Y* =z the expected post-regulation catch, 



and F = fishing mortality coefficient for recruited fish between the ages 



2^0 and i^e- 



Making the substitution for age in terms of fish length, as before, we find 



Uoo - lU 



where ^l^ and 1?^ are the lengths offish at ages g^c ^^^ i^c respectively. 



Recently a practical improvement has been made to this method (see 

 Gulland, 1961). 



THE REQUIREMENTS FOR PARAMETER ESTIMATION 



The various rearrangements of a yield equation demonstrated in the section 

 above show that it may not always be necessary to estimate separately all the 

 parameters of reproduction, growth and mortality; or at least that some 

 reasonable attempt at catch prediction might be made before this is possible. 

 They also demonstrate that in such situations it is not possible to lay down a 

 single schedule of data or of estimates which must be obtained for purposes 

 of fish stock assessment and yield predictions. 



The methods of estimating growth parameters when the ages of fish in 



