FISHERY BULLETIN: VOL. 71, NO. 2 



build up the spawning stock, since this will 

 almost certainly increase the probability of 

 future year classes being of average strength or 

 better. However, there is no guarantee that, 

 even if in the extreme case the catches are cut 

 to zero, there will be any increase in recruit- 

 ment compared with what would have occurred 

 with unrestricted fishing. 



This means that, since there is no good 

 knowledge of the stock/recruit relation, there 

 is no catch uniquely definable on scientific 

 grounds as the best catch, though the optimum 

 policy, for a stock at a low level, must lie between 

 catching nothing (for the most rapid rebuilding 

 of the adult stocks) and fishing at the rate 

 which makes the best use of those fish which 

 have, in fact, been recruited. 



A catch that can be objectively defined, which 

 will often lie in this range and which may also 

 be a reasonable catch to take for the benefit of 

 the fishery, is the replacement catch. This can 

 be defined, in exactly the same way as before, 

 as that catch which will ensure that the stock 

 at the end of the year is the same as that at the 

 beginning of the year. The stock in question 

 could be the spawning stock, or the total fishable 

 stock, and its magnitude could be expressed 

 either as weight or numbers. (The use of num- 

 bers makes calculations easier and clearer.) 



For example, recent reports of ICNAF's 

 Assessment Sub-Committee have set out the 

 changes in the numbers in the Georges Bank 

 haddock stock, separating additions through 

 recruitment, and removals by fishing and 

 natural mortality. Thus during 1970 there were 

 16 million recruits, and some 3 million fish 

 died through nonfishing causes, i.e., a net 

 natural increase of 13 million fish, which was 

 about twice the catch in numbers (ca. 5.3 

 million). However, because the deaths (through 

 both fishing and natural cases) were mainly of 

 large fish (average age in U.S. landings were 

 6.6 years of 2.4 kg weight), the deaths (in 

 weight) were much larger than the weight of 

 recruits, though this was almost balanced by 

 the growth of the survivors. Thus the catch of 

 12,000 tons was about equal to the net natural 

 additions, i.e., the catch was equal to the replace- 

 ment catch, in weight, though less than the 

 replacement catch in numbers. 



The replacement catch varies very greatly. 

 Thus between 1968 and 1969 when the natural 

 deaths in the Georges Bank were 5 million fish 

 and the recruits only 1 million, the stock, in 

 numbers, could be maintained only by introduc- 

 ing 4 million fish onto the grounds, i.e., a re- 

 placement catch of minus 4 million fish. Con- 

 versely, when a very strong year class is 

 recruited to the fishery, the replacement catch 

 would be large and require a fishing effort well 

 in excess of that giving the optimum mortality. 

 The variation in the replacement catch, defined 

 in terms of weight, will be less severe. Even so, 

 it is clear that the replacement catch cannot be 

 used blindly as the determinant for the catch 

 to be taken in any particular season. It will, 

 however, provide some sort of guide as to 

 whether the proposed action will improve things 

 (proposed catch is less than the replacement 

 catch), or allow them to get worse. Unfortu- 

 nately it is not a perfect guide. For example the 

 average condition of the stock, over a period, 

 will be maintained only if, when a strong year 

 class is entering the fishery, the opportunity 

 is taken to build up the stock to balance the 

 occasions when poor year classes occur, i.e., 

 less than the replacement catch should be taken 

 when strong year classes are being recruited. 

 Also, the practical application depends on 

 obtaining good and early estimates of the 

 strength of the incoming recruitment. 



Regulations, such as catch quotas, based on 

 a catch defined in terms of a particular harvest- 

 ing rate would seem to form a better guide. 

 Though the optimum mortality cannot be 

 determined unless the form of the stock/recruit 

 relation is known, an optimum rate (for any 

 given economic or social policy) can be cal- 

 culated on the assumption of constant recruit- 

 ment. If the recruitment is affected by the 

 abundance of the adult stock and the stock is 

 at a low level, the optimum rate must be some- 

 what less. Therefore an upper bound can be 

 set on the desirable level of catch. Further, 

 various assumptions can be made concerning 

 the form of the stock/recruit curve, and the 

 corresponding relation between fishing mor- 

 tality and total yield calculated. The scientific 

 advice could then be presented in four columns: 

 the first would list the possible objectives; the 



334 



