FISHERY BULLETIN: VOL. 71, NO. 2 



in the nearly unexploited fishery — the tangent 

 to the catch curve at the origin — and a catch per 

 unit effort of 10% of this value. The limiting 

 point beyond which any increase in fishing 

 would certainly not be worthwhile — assuming 

 a marginal yield of 10% of the initial catch per 

 unit effort is not worthwhile — is where the 

 tangent to curve is parallel to this 10% line. The 

 selection of 10% is arbitrary but once the 10% 

 figure is accepted the corresponding catch can 

 be calculated objectively. Thus it can be used 

 to provide a Commission or other management 

 body objective guidance based on scientific 

 grounds. 



Unless there are marked density -dependent 

 changes in mortality or growth this target will 

 be achieved by exerting some fixed rate of fishing 

 mortality. It may be noted that the optimum 

 fishing mortality, exerted at a constant rate 

 during the whole of its life, above the age at 

 first capture, is the same for any strength of 

 year class, but slightly larger catches would in 

 principle be obtained by fishing less hard when 

 the fish are young, and harder when they are 

 old, i.e., concentrating catches more at the age 

 when the total biomass of the year class is at 

 its maximum. The fishing mortality in a par- 

 ticular year which leads to the greatest catch 

 over a period will be the mean of the best 

 mortality for each year class present, weighted 

 according to their strengths. If the year classes 

 are equal, this will be equal to the optimum 

 constant rate for a single year class during its 

 entire life. If there is a big variation in year- 

 class strength there would be some theoretical 

 advantage, other things being equal, in fishing 

 slightly harder when the strongest year class 

 present is middle-aged (since they will not grow 

 much more but will suffer losses by natural 

 mortality), and less hard when the strong year 

 classes have just recruited. However, the 

 theoretical increases in yield are not likely to be 

 great, and it is simpler to keep, as the objective, 

 the same constant fishing mortality independent 

 of year-class change. From an economic stand- 

 point the optimum level of fishing mortality will 

 increase slightly when strong year classes are 

 present, because then it becomes more worth- 

 while to squeeze out an extra 1 or 2% of the 

 maximum yield. For those few fisheries for 



which the investigators are fortunate enough 

 to have immediately available a measure of total 

 effort which provides a measure of fishing 

 mortality consistent from year to year, the 

 optimum level of fishing can be defined at once 

 in terms of total fishing effort, without the need 

 for year to year adjustments. 



Usually difficulties of standardization in a 

 multinational or multigear fishery, or changes 

 in the effectiveness of a nominal unit of effort, 

 will mean that the amount of fishing in each 

 year will have to be controlled in terms of total 

 catch. The scientists can, in principle, given 

 adequate information, calculate what the 

 magnitude of this catch should be, taking into 

 account the strength of the year classes already 

 present in the fishery, and those that will be 

 recruited during the year in question. This 

 catch might be defined as the catch for optimum 

 harvesting rate. 



Often a precise optimum level of fishing mor- 

 tality cannot be defined or cannot be agreed 

 upon. It is still possible to estimate the catches 

 in each year which would be required to attain 

 any prescribed value of fishing mortality. These 

 mortalities may be those which occurred at 

 some previous time when it was believed that 

 the fishery was in better condition (in some 

 general, unspecified sense) than the present, or 

 some convenient figure which the scientists 

 believe approximates to the optimum condition. 

 In this way the scientists, without preempting 

 the administrators' duty to decide on the objec- 

 tive of management, can provide some figures 

 derived in a reasonably objective way, on which 

 it may be possible for agreement to be reached. 

 An example of such calculations are those made 

 by the Assessments Sub-Committee of ICNAF 

 for the cod stock at West Greenland. This stock 

 undergoes moderately strong year-class fluctua- 

 tions, and estimates have been made of the 

 catches required to attain fishing mortalities of 

 0.8 and 0.6, as set out in Table 2 (from ICNAF, 

 1970). 



It may be noted that for virtually all patterns 

 of mortality, there is a drop in predicted catch 

 from 1969 to 1970 and again from 1970 to 1971, 

 due to the entry of weak year classes into the 

 fishery. 



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