310 EIGHTH PACIFIC SCIENCE CONGRESS 



strength is, in the main, determined by one or more critical factors at 

 certain critical points in the life of the brood. Efforts at demonstrating 

 this hypothesis have aroused some controversy. However, it is unques- 

 tionable that the sequence described above is closely dependent upon 

 environmental conditions and that any understanding of it, let alone 

 any measurement of it or formulation of a prediction system, must make 

 demands upon oceanography. 



Natural Mortality:— This term is perhaps even more recalcitrant 

 than the other two. We may argue, a priori, that mortality may be 

 caused by predatism, parasitism, disease, lethal genetic characters, mal- 

 nutrition, and physical factors such as temperature and salinity extremes, 

 but there seems to be little prospect yet of measuring the result of 

 operation of each of these factors separately, or, as an antecedent to that 

 result, the intensity of each factor. Nevertheless, again we must look 

 to oceanography to furnish the fishery biologist with some of the in- 

 formation which he will require in this field. At the present stage 

 natural mortality must be accepted as a sum arrived at after deducting 

 measured fishing mortality from estimated total mortality. It might be 

 possible to correlate variations in natural mortality with variations in 

 environmental factors and thus to approach both analysis of the causes 

 and prediction of intensity. 



Fishing Mortality:— In this field, oceanography has a small contribu- 

 tion to make in respect of the influence of weather and sea conditions 

 on the intensity of fishing operations and their efficiency, but we defer 

 further consideration of this to Section 5 of this paper. 



Fluctuations in Abundance:— A great deal of work has been done in 

 describing the fluctuations in abundance of exploited stocks and analys- 

 ing these fluctuations in terms, chiefly, if variations in strength of brood- 

 classes and in recruitment into fishable stocks. This work can proceed 

 at one or other of various levels: either with simple description of the 

 fluctuations and elementary correlation with various environmental 

 factors— a course which does not promise much reliability in its results; 

 or with varying degrees of penetration of the analysis into the systems, 

 with the conviction that if relationships are truly identified, and rela- 

 tions usefully measured, prediction systems may be evolved with prac- 

 tical and reliable value. 



Fishing Theory:— FerhzLps the ultimate objective of fishery science 

 in respect of fishing operations is to prepare, for each unit fishery, a 

 description of the fish stocks and of the effect on them of the fishing 

 operations in order to permit the design of a plan of management of 

 the fishing operations which will ensure the best exploitation of the 

 stocks. The description of each unit fishery will be along the lines of 



