biologists should therefore cover all the stocks 

 which are exploited by the fleets considered by 

 the economist, otherwise there would be a 

 substantial gap in an essential part of the 

 needed information. 



The previous paragraphs were based on the 

 assumption that the pattern of the recruitment 

 to the fish stocks remains unchanged whatever 

 the size of the stock and the level of the fishing 

 effort. In practice, this assumption is certainly 

 not realistic. But the opposite assumption that 

 the level of recruitment is linked solely to the 

 size of the stock is certainly equally erroneous. 



These two remarks oblige us to enter some- 

 what into the intricacies of the computations 

 made by the marine biologists. When these 

 scientists are examining the past catches they 

 proceed along analytical lines which are cor- 

 rected every year according to what has hap- 

 pened. Their analyses are summarized and 

 systematized with the help of mathematical 

 functions. These functions can serve the addi- 

 tional purpose of making forecasts about the 

 effect of a diminishing, sustained, or increased 

 fishing effort in the years to come, ceteris 

 paribus. 



Among these other factors the main one is 

 the pattern of recruitment. When a constant 

 rate of recruitment is assumed, the mathematics 

 lead to a curve tending asymptotically to a 

 minimum yield equal to an exploitation level 

 associated with average yearly recruitment. 

 When recruitment is assumed to be aligned with 

 the size of the stock, mathematics lead to a 

 curve asymptotic to the X axis or to a parabola. 

 In fact, both assumptions are false and known to 

 be false; the real curve for each stock is in 

 between these two different mathematical 

 formulations, but present scientific knowledge 

 in marine biology does not allow us to know 

 when the pattern of recruitment becomes 

 different. 



The resulting margin of error is of course 

 without practical importance when there is a 

 stable fishing effort. When the increase of fishing 

 effort is slow, the impact can be surveyed step 

 by step and the margin of error remains small. 

 But when the increase of fishing effort is fast 

 and furthermore when fishing effort is, as is 

 true in complex fisheries, significantly varying 

 from one year to the other, the margin of error 

 is bound to be as large as the distance between 



the two curves. This precludes an accurate 

 forecast. In any case, it seems that most often 

 the yield curve is relatively flat around the 

 maximum. The Schaefer model tends to exag- 

 gerate the sharpness of the turning point at 

 MSY, whereas the Beverton and Holt model 

 may tend to exaggerate the flatness after MSY. 

 Let us imagine a fish stock exploited as in 

 Figure 4 at a variable level of fishing effort, 

 with fluctuations stabilized at maximum and 

 minimum levels unchanged for a number of 

 years. The calculations of the biologists lead 

 to a derivation of a yield curve as drawn in 

 Figure 4. The margins of error in the calculations 

 are such that, if there were a change in recruit- 

 ment function around the point of average yield, 

 it could not be easily seen; the actual average 

 yield cui-ve could well be drawn by the dotted 

 lines and no one could prove which is the real 

 one. This is not critical if the fishing effort is 

 not increased, but assuming, as it is often the 

 case at present, an increasing demand for 

 protein and improved productivity due to tech- 

 nological change, the only practical problem 

 would be the problem of an increased fishing 

 effort . . . for which, with such data, no forecast 

 at all could be made before a new stabilization 

 of fishing effort for a subsequent number of 

 years. Before such a stabilization, the most 

 detrimental consequences could have materi- 

 alized (cf. the California sardines). The faster 

 the increase in fishing effort, the more difficult 

 are the assessments. 



PARTIAL BIOECONOMIC MODELS 



While initially I attempted to prove that 

 biological models cannot be complete, at least in 

 the most important cases of increasing fishing 

 effort, it is not necessary to stress that com- 

 plete bioeconomic models cannot exist. It is an 

 obvious fact that in bioeconomic models biology 

 comes first; they are fully dependent on the 

 reliability of the basic biological data. This is 

 a very big drawback which would well render 

 the whole exercise of very little practical help 

 in managing fish resources. But it should not be 

 forgotten that, in most cases, the biologists can, 

 with reasonable accuracy, indicate the level of 

 maximum sustainable yields. This limit gives 

 a very important and solid basis for assessment 



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