286 E. D. LE CREN 



protein and fat gonad development requirements will be relatively larger 

 than simple gonad weight will indicate (Swift, 1955). For the Pacific salmon 

 {Oncorhynchus spp.), where the adults die immediately after spawning, the 

 'production cost' of spawning should also include the weight of the spawners* 

 bodies. The total biomass of the spawning escapement is lost to the stock of 

 salmon; in most other species there is little or no mortality associated 

 especially with spawning. 



PRODUCTION IN THE LARVAL AND JUVENILE PHASES 



Estimates of production, and indeed any understanding of population 

 dynamics, require measures of the basic parameters; mortality and growth. 

 These have been estimated with reasonable degrees of accuracy for the adult 

 or the exploited phases of a number of fish populations, but data for larval, 

 juvenile or pre-recruit phases are much scarcer. This is partly because young 

 fish have not been studied so much as older fish and partly because the 

 necessary data are much more difficult to obtain. In spite of this, however, 

 the intrinsic interest and importance of the young stages make it worth 

 while attempting some rough estimates of their population parameters, 

 including the magnitude of production in the early phases. 



Even though data on population numbers and survival in young fish may 

 not be available, it is frequently possible to estimate the number of eggs 

 from information on the abundance and fecundity of adult females. It is 

 thus possible to pin-point for each brood or year-class its initial abundance 

 as well as the number and size of the survivors when they are recruited into 

 the adult or fished stock. It only remains to fill in the changes in mortality 

 and growth in the intervening period; this may be difficult but a reasonable 

 range of probabihties can often be suggested. 



Frost & Smyly (1952) have pubhshed data on the brown trout {Salmo 

 trutta) in a small lake, that can provide an example of this method of 

 estimating production. The lake was netted to give twelve successive annual 

 samples, each of which averaged roughly half the population, and the fish 

 caught were analyzed for age. The catches of one- and two-year-old fish 

 were subject to net selection and are therefore not representative, but the 

 seine effectively caught fish that were three years old or older and so the 

 age data give a good estimate of the average mortality after the third year. 

 In the fourth year the instantaneous mortality rate, Z, is 0-63. An estimate 

 (based on published and unpubhshed data) of egg deposition by the number 

 of mature females in the average total population comes to 3,670. If a graph 

 is drawn of survival (Fig. i) it is clear that survival from the egg to three 

 years old must be less than subsequently. Three possible estimates of the 



