Practical Problems of Constructing Bioeconomic 

 Models for Fishery Management 



Paul Adam' 



ABSTRACT 



In many practical cases it is impossible to construct a complete bioeconomic model 

 of a given fish stock, such as when one or several fleets move irregularly from one stock 

 to another, or when fishing effort increases so rapidly that it is not possible to 

 accurately specify a reliable yield/effort relationship. A continuing bioeconomic model 

 is proposed here which will allow inclusion of these dimensions while allowing both 

 for year-to-year fluctuations in managed effort and also for gradual adjustment of labor 

 and capital to those levels designated as optimal within the broad ranges of this 

 continuing model. Year-to-year re-evaluation offish stocks and capital-labor requirements 

 is stressed. 



INTRODUCTION 



This paper is devoted to the problem of mixed 

 fisheries. Few fish stocks are exploited by one 

 fishing fleet only and few fishing fleets are 

 dependent upon only one fish stock. In the rare 

 cases of isolated fisheries (one main species, 

 one fleet, one market) there are often incidental 

 catches which, although they may be relatively 

 small, are important for the overall profitability 

 of the fleet. It can be said that in most fisheries 

 the rule is to switch from one type of fishing to 

 another or from one stock to another, according 

 to the seasons or to the variable fish abundance 

 in the different stocks. These continuous adjust- 

 ments, occurring irregularly, make the problem 

 of fishery management a most complex one. 



Furthermore, it must be added that in the 

 last 10-15 years the techniques used in some of 

 the most important world fisheries have been 

 considerably improved. These developments 

 include: long distance stern trawling associated 

 with freezing at sea, purse seining for pelagic 

 species in the North Atlantic, purse seining for 

 tuna species in the Central Pacific and Atlantic, 

 double beam trawling in the North Sea, etc. As 

 a consequence of these recent developments, it 

 is more difficult to study those fisheries which 



' Head of the Fisheries Division, Organization for 

 Economic Cooperation and Development. The author is 

 solely responsible for the ideas and information presented 

 in this paper. 



are the most advanced and consequently the 

 most interesting. 



The study made in this paper will obviously 

 be economic, but no serious or complete eco- 

 nomic study of any fishery can be undertaken 

 without consideration of the available resources. 

 In other words, the work of the economist in this 

 context cannot begin or would have no solid 

 basis without starting with the findings of 

 marine biologists. It is therefore indispensable 

 to examine the nature, the scope and especially 

 the shortcomings of the biological findings inas- 

 much as they have to be used by the fishery 

 economists. 



SHORTCOMINGS OF THE 

 BIOLOGICAL MODELS 



The whole process of the fishing operations 

 is expressed in Figure 1. The arrows indicate 

 the basic components of an operating fishery. 

 It makes it apparent that any research which 

 would isolate either biologj' or economics would 

 be cut off from the feedback occurring in reality. 

 Any model used to describe reality will be false 

 if it is divided into two isolated parts. 



The traditional catch curve derived from the 

 biological findings on one fish stock cannot be 

 directly u.sed by the economists. In fact, this 

 curve, which is an average catch curve, should 

 be supplemented with two curves indicating the 

 maximum and minimum yields according to the 



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