36


 Having briefly clarified the definition of fishing effort, it

remains to draw a distinction between the equilibrium yield functions

such as those developed by Pella and Thomlinson (1969) and Schaefer

(1957) and non-equilibrium yield functions. Equilibrium yield functions,

as shown above, are derived in such a manner as to produce a relation-

ship between fishing effort and sustainable yield. It is precisely

this relationship between catch and effort that has resulted in the

term equilibrium yield function. The catch resulting from any level of

effort along these functions corresponds to equilibrium (sustainable)

yield.

 The significance of such functions in the analysis of fishery

production is twofold. First, the use of such functions implicitly

ensures that biological equilibrium is achieved. This means that catch

rates are always equated to sustainable yield. A note of caution should

go with such a strong statement, however. In the empirical estimation

of such functions, the degree to which such estimated equilibrium yields

and actual equilibrium yields coincide rests largely on the adherence of

certain underlying assumptions (Pella and Tomlinson, 1969). Thus,

empirically estimated equilibrium yield functions may not incorporate

the biological equilibrium condition of catch equals sustainable yield

to any reasonable degree. Secondly, equilibrium yield functions gen-

erally impose specific functional forms on the observed relationship

between catch and effort. This is significant in that the set of valid

equilibrium yield functions is fairly limited.

 In contrast to the notion of equilibrium yield functions is that of

 non-equilibrium yield functions. In this study, the term non-equilib-

 rium implies that no biological equilibrium condition of catch equal to