294 E- D- L^ CREN 



recruitment. This has been done for the data presented above and is shown 



in Table IV. From the two lower lines of this Table it will be seen that about 



Table IV. Estimates of production, biomass and efficiency 



80 per cent of the whole production is involved in reproduction and recruit 

 rearing, and that only a small part of this production (13-42 per cent) may 

 actually survive to the recruitment stage. Additional data for the proportion 

 of pre-recruit production that survives are given by the computations for 

 sockeye salmon and brown trout fry where roughly comparable percentages 

 are 28 per cent and 33 per cent respectively. 



There are several practical implications of these fmdings. Firstly, it is clear 

 that the early part of the Hfe offish deserves much more study; some of the 

 data presented in this paper are Uttle more than informed guesses. Secondly, 

 the rearing of fish to the recruit stage (or to the migratory stage in the case 

 of anadromous salmonids) is obviously an inefficient process that absorbs 

 much of the capacity of the water to produce fish flesh. This inefficiency is 

 largely due to the mortahty suffered by the young, and any management 

 practices that tended to reduce this mortaUty should be worth while. Some- 

 times the young fish may act as a necessary link in the food chain ('forage 

 fish') for older and larger individuals of the same population, but this will 

 not normally be so. Thirdly, it is clear that in normal fish populations there 

 is an unnecessarily large expenditure of 'effort' on gonad products and the 

 whole reproductive and rearing process. The data from sockeye salmon and 

 trout show that the 'efficiency' of reproduction and rearing falls with an 

 increase in the density of eggs and young fish, and above a certain level 

 there may be no appreciable increase in the biomass of 'recruits' produced. It 

 is true that for migratory species it may be desirable to produce the maximum 

 numher of migrants (smolts) rather than the maximum biomass, but there is 

 some evidence which suggests that survival for salmon in the marine phase 



