GULLAND and BOEREMA: SCIENTIFIC ADVICE ON CATCH LEVELS 



Table 2. — Catches (thousands of tons) of cod from the West 

 Greenland stock in 1970 and 1971 corresponding to different 

 values of fishing mortality (in parentheses). 



For most fish stocks there is a reasonable 

 likelihood that changes in the abundance of the 

 adult stock brought about by fishing will have 

 some effect on the average level of recruitment, 

 though the extent of such effects may not be 

 known. When they are known, they can be 

 incorporated into the calculations to obtain the 

 catch rates corresponding to different objectives. 



The management of salmon stocks generally 

 corresponds to this situation. The relation 

 between spawning stock (escapement) and sub- 

 sequent recruitment is usually claimed to be 

 known and has a maximum at some intermediate 

 level of spawning stock. The catches are adjusted 

 to obtain some desired level of escapement. This 

 might be the level giving — under average con- 

 ditions — the maximum recruitment, though a 

 more nearly optimum strategy would be to take 

 a rather larger catch, such that the immediate 

 gain in increased catch would be more than the 

 expected loss (with or without any discounting). 

 In such a situation it is possible to define, 

 purely on biological grounds, a catch for maxi- 

 mum recruitment; and also, using both bio- 

 logical and economic considerations, a catch 

 for optimum spawning (or optimum escape- 

 ment) .2 



The nature of the Pacific salmon fisheries — 

 the movement of the prespawning fish through 

 the fishing area during a short season and the 

 spawning and subsequent death of all those not 

 caught — makes relatively easy the visualization 

 of the catches as defined here, since they are 

 equal to the number offish arriving at the fishing 



2 This outline of the scientific rationale of salmon 

 management ignores the great practical difficulties of 

 management of any specific salmon fishery — the mixture 

 at sea of fish from several different stocks, the difficulty of 

 obtaining accurate measures of the incoming run before 

 the start of the very short season, etc. These are outside 

 the scope of the present note. 



grounds, less the escapement required for 

 maximum recruitment, or the optimum escape- 

 ment respectively. However, the same definition 

 could apply to any fishery, though in practice it 

 can be applied only in those few fisheries for 

 which the stock/recruit relationship is well 

 known. It may be noted that these definitions 

 lead to very great fluctuations in the catch that 

 should be taken. Very large catches may be 

 taken from good runs, but the allowable catch 

 would fall to a very low level, or even zero, 

 when the run is poor, and little, if at all, above 

 the required escapement. The proportion of the 

 stock (run) that is harvested decreases rapidly 

 with decreasing run and falls to zero as soon 

 as the size of the run drops to or below the target 

 escapement. 



Unfortunately, for most fisheries there is little 

 firm information on the relation between adult 

 stock and average recruitment. An obvious 

 example of the resultant difficulties in defining 

 objectively any specific catch quota is occurring 

 for both the herring and haddock on Georges 

 Bank (subarea 5 of ICNAF). For the haddock 

 there has been an unprecedented mn of poor 

 year classes (those of 1964 to 1970 inclusive) 

 which, combined with exceptionally heavy fishing 

 in 1966 and 1967, has reduced the stock to a 

 very low level. Though the more recent of the 

 weak year classes (since about 1968) were 

 associated with a low parent abundance, the 

 earlier ones came from moderate to large stocks 

 — in fact stocks of about the same abundance as 

 those giving the very large year classes of 1962 

 and 1963. Thus although it is highly probable 

 that the decline in adult stock is among the 

 causes of the run of poor year classes, it is 

 certainly not the only cause. 



The ICNAF scientists have, therefore, pointed 

 out that the sensible policy is to take action to 



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