10


 Biological Theory


 The need to consider the biological characteristics of fish popula-

tions in economic analyses of fisheries becomes immediately evident when

the nature of the resource (fish population) is considered. Fish popu-

lations can be placed in the category of use-dependent flow resources

with a critical zone (Schaefer, 1957). A critical zone as used here is

defined to be a rate of decrease in flow which cannot be reversed

economically or technologically. Thus, in terms of fish population, a

critical zone would correspond to that level of population which has an

insufficient reproductive potential to remain viable.


Sustainable Yield


 The main aspect of biological theory that is relevant to production

analysis of a fishery is that of population dynamics. More precisely,

the notion of sustainable yield (SY) is one of the cornerstones of bio-

economic theory.

 The size of a non-exploited fish population (biomass) can, in

general, be assumed to be a function of three factors: growth, recruit-

ment and natural mortality (Gulland, 1965). Each of these variables, in

turn, is a function of the biomass. Individual growth is generally

assumed to be at a maximum at low levels of population and to decrease

as the population size increases. Natural mortality acts in the oppo-

site manner, being low at low levels of population. As, population size

increases, natural mortality increases due to increased competition for

food and other such factors. Recruitment, the rate at which individuals

enter into the fishable population, is generally assumed to be low at