Unfortunately the Beverton and Holt model 

 is not so easy to handle. In its simplest expres- 

 sion, it reads: 



(2) Z = M + F = 



log,, number of fish at beginning of year 

 number of fish at end of year 



where 



Z = total mortality 



M = natural mortality 



F = fishing mortality. 



If calculated on a weight basis instead of a 

 number basis, account should also be taken of 

 the rate of growth of live fish. 



With such a model converting the figures of 

 the biologists into units which can be utilized 

 by the economists is most often impossible. 

 No mathematical barrier exists as long as it is 

 understood that the natural logarithm of a 

 ratio between the catches or the stocks of two 

 years is, in fact, a percentage. However, an 

 important part of the data utilized by the 

 biologists, when it is all published, is scattered 

 in many different publications. It is not suf- 

 ficient to know the ratio of abundance derived 

 from fishing effort (F) and the ratio of natural 

 mortality (M); the ratio of the growth of the 

 fish and the assumed recruitments are also 

 indispensable but not easily available. Fui'ther- 

 more, the relationship between ratios and actual 

 figures are too often summarized to an extent 

 which forbids reconstruction of the details of 

 the computations and of the results. 



While the present paper is mainly directed 

 toward an improvement of the cooperation 

 between biologists and economists, it should 

 be stressed that a prerequisite is to have access 

 to the results of the computations of the other 

 discipline. Cooperation does not require working 

 at the same desk, but it would ask for this 

 minimum of understanding. 



Unfortunately, the facility with which the 

 Schaefer model can be used by the economists 

 does not always mean that there is a perfect 

 and total understanding between fishery biolo- 

 gists and economists. More important perhaps 

 than the unit of measurement are a few basic 

 concepts which are commonly used with different 

 meanings. The fishing effort concept is by far 

 the most important one. 



Fishing effort is in fact usually expressed in 



many different ways: either by its physical 

 characteristics or by its returns in weights of 

 different fish species or in money values (either 

 returns or costs, or profits). The usage of these 

 different units should be systematized, other- 

 wise the concept of fishing effort would be 

 misleading as is too often the case when so 

 many researchers use it with different and 

 implied assumptions on the way it should be 

 expressed. In fact, there could not be one single 

 way of expressing fishing effort; fishing effort 

 considered in its full and general meaning is a 

 combination of the different units by which it 

 could be expressed. 



Physical Characteristics of Vessel and Gear 



This could include any kind of measure 

 describing the characteristics of the vessel: 

 GRT, power, length . . . also taking into account 

 items like the number of berths (which might 

 be significant for pole-and-line techniques), or 

 the sonar (for purse seining), or the number of 

 pots (for crab or lobster, etc.). Obviously, for 

 each specific case the most important character- 

 istic(s) to be used as a measure of the impact of 

 the fishing on the stock or as a measure of the 

 fishing power in relation to a given fish stock 

 will vary. Therefore, a multipurpose vessel has 

 a different fishing power according to the gear it 

 is utilizing; it might even be that the fishing 

 power has to be different when the same vessel 

 with the same gear is exploiting different stocks. 

 As a result the fishing effort of the same boat 

 would have to be expressed differently for each 

 type of exploitation, each season, each year, 

 each stock, etc. 



Cost of Fishing Effort 



Building costs and operating costs which 

 could be combined by using operating costs 

 including depreciation plus overhead are a more 

 permanent type of unit. First, the costs of a 

 given boat are not so much changed when it 

 changes gear. Secondly, many boats have been 

 built for a definite type of usage. The costs of 

 a boat will be easily defined by so much per 

 day at sea. 



101 



