256 R. J. H. BEVERTON 



be reached at the present time. The weight of available evidence points to 

 the pelagic larval phase as the critical one, with the direct or indirect conse- 

 quences of competition for food among the larvae as the basic mechanism; 

 yet it has to be admitted that most of the evidence brought forward in 

 support of this is inferential. It has not yet been demonstrated conclusively, 

 for instance, that the consumption of Oikopleura by plaice larvae is the main 

 factor responsible for reducing the abundance of Oikopleura to the point at 

 which the survival of the larvae is adversely affected, although this is perhaps 

 the essential test. Again, there is little quantitative information about the 

 early demersal life of plaice from the time they metamorphose and terminate 

 their pelagic phase until they first appear in samples taken close inshore some 

 months later. The reason for this is primarily the practical problem of 

 sampling quantitatively for juvenile plaice of only a few miUimetres in 

 length while they are still dispersed in relatively deep water, and such 

 relevant information as exists on this phase of the hfe-history comes, in fact, 

 from experimental studies on the rearing of plaice (Shelbourne, personal 

 communication). These show very clearly, nevertheless, that the mortality 

 rate of plaice larvae is much reduced after metamorphosis; they become 

 progressively more resistant to adverse environmental conditions of all kinds 

 which would have been lethal before metamorphosis, and in particular are 

 much less susceptible to variations in the kind and amount of their food 

 supply. Of course, this does not prove that compensation does not occur 

 during this early demersal phase in the sea, and it may be that the concentra- 

 tion of the juvenile plaice in coastal waters that occurs when they are a few 

 months old gives rise to compensation, despite their greater resistance to 

 adverse conditions. Nevertheless, the evidence is perhaps sufficient to justify 

 exploring more thoroughly in the first instance the dynamics of the pelagic 

 phase which, difficult though it is to attack, is more accessible to quantitative 

 study than is the early demersal phase. 



Some of the many questions which need answering in the search for 

 compensating mechanisms in the pelagic larval phase have been touched on 

 previously, but there is one particular feature of the relationship between 

 adult population size and number of surviving recruits in plaice which may 

 be of special significance. It is that over the wide range of adult population 

 size that has occurred, there is no clear evidence of what may be called 'over- 

 compensation', that is, of the higher adult densities resulting in a reduced 

 number of progeny. The same is true for most other marine fish populations 

 so far as can be judged from existing data. There is probably some tendency 

 towards over-compensation in certain salmon populations, where the 

 spawning grounds and environment for the young are much more restricted 

 than in marine populations; but the only striking evidence of this 



