250 R. J. H. BEVERTON 



shown by this population (Fig. 2). Although no long series of data are 

 available for sole, it is known that recruitment to this population also can 

 fluctuate widely, and this population is the one which shows the most 

 definite evidence of a long-term trend. Thus the association noted earlier 

 from Fig. 2 between the degree of long-term trend and the degree of short- 

 term fluctuation about that trend, appears to be a function of the degree of 

 compensation that occurs, in effect, between successive adult generations. 

 Representative age-composition data cannot be extended to plaice younger 

 than three years of age from market samples ; but Dutch fishery scientists 

 have carried out quantitative research vessel sampling of young plaice on 

 the nursery grounds off the Dutch coast for some years. Their results have 

 shown that the same low natural mortality rate mentioned previously for 

 adult fish, of about 10 per cent per year, is found from the end of the first 

 year of life onwards. The sampling did not cover fish younger than this, but 

 it seems reasonable to conclude from this evidence that if compensation does 

 occur among juvenile plaice it must be restricted to the first year of life, or 

 else that it must be concerned in some way with the reproductive processes. 



COMPENSATION IN THE REPRODUCTIVE PROCESSES 



OF PLAICE 



In many fish, including plaice, fecundity is closely proportional to body 

 weight (Simpson, 195 1), so that if the sex ratio remains constant the total 

 weight of the adult population can be taken as an index of the total number 

 of eggs deposited during each spawning season. In fact, the increases in adult 

 populations of plaice resulting from the cessation of fishing during the war 

 were made up of a relatively greater increase in males than females, because 

 the former are more susceptible to fishing and so responded more when 

 fishing stopped (Wimpenny, 1953). Nevertheless, the data of total adult 

 population weight shown in Figs. 3 and 4 can be taken as roughly equivalent 

 to the total egg production. When compared with this range of egg produc- 

 tion, the effect of the retardation of growth on fecundity noted previously, 

 which probably reduced the fecundity at age by about 15-20 per cent, 

 seems of little practical significance as a compensating mechanism. 



It is possible, however, that compensating mechanisms may occur in the 

 spawning processes, quite apart from the total number of eggs produced; one 

 of these is that the extent of the spawning area may itself change with the 

 size of the mature population and give rise to compensation. North Sea 

 plaice spawn in two main areas, in the Heligoland Bight and in the Southern 

 North Sea. The distribution of eggs in the latter has been mapped by Hensen 

 net sampling in a number of years, and it has been estabhshed that the eggs 



