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this fact will become obvious when costs and revenues are incorporated

into the theoretical models. More precisely, it is this property of the

sustainable yield function which leads to many conclusions rendered by

economists with respect to the workings of perfect competition.

Secondly, any given sustainable yield function is defined for a given

set of environmental and ecological parameters. Any change in these

parameters will bring about a shift in the sustainable yield function

(Schaefer, 1957).


Stock Production Models

 The mathematical models describing the sustainable yield concepts

have mainly been in the form of biological stock production models. Two

of the more prominent models in fisheries theory are the Schaefer model

(Schaefer, 1957) and the Generalized Stock Production Model (Pella and

Tomlinson, 1969). These models are, in general, empirically oriented.

This orientation has resulted primarily from the lack of time series

data on population sizes. Thus, these models generally invoke several

assumptions which make it possible to express sustainable yields as a

function of fishing effort. While the term fishing effort is more

specifically dealt with in the following sections, it should suffice

here to define effort simply as some measure of fishing activity

directed toward the resource stock.

 The Schaefer model is actually a special case of the Generalized

 Stock Production Model. The following discussion will deal with the

 most general model while in the process pointing out its relation to the

 Schaefer formulation.