246 R. J. H. BEVERTON 



become comparable in size to those of the three major species shown in 

 Fig. I. 



In contrast, the trend in haddock shown at the bottom of Fig. 2 is clearly- 

 downward, especially when it is remembered that the removal of fishing 

 pressure during the two wars allowed the population to recover to several 

 times its previous abundance on each occasion. Here again there is little 

 doubt that the impression gained from the trends in landings represents a 

 real change in population size, as during the period there has been a progres- 

 sive contraction of the main haddock fishing area and it is now rare to find 

 haddock in any quantity south of the Dogger Bank. 



The trends in landings of the three remaining species are less marked, but 

 there are signs of an upward one in turbot. It is known that the fishing 

 pressure on both cod and plaice has been rather less since the last war than 

 previously, and that this reduced fishing mortality has allowed the population 

 of adult fish of these two species to increase by something of the order of 

 50 per cent compared with earlier years. If allowance for this is made in the 

 data of Fig. 2 since 1950, the overall trend in cod becomes a downward one, 

 while the graph for plaice becomes virtually horizontal throughout the 

 period. 



Thus, of the five species considered, the plaice shows the greatest degree 

 of stability and the sole the most marked trend. It can also be seen from 

 Fig. 2 that the sole data show the greatest degree of fluctuation about the 

 overall trend and those of plaice the least, with the other three species 

 intermediate. Thus there appears to be an association between the degree of 

 long-term trend and the degree of short-term fluctuation about that trend, a 

 point which is referred to again later. It would have been instructive at this 

 juncture to have attempted a comparative study of the population ecology 

 and dynamics of the five species, but this is not possible in the present state 

 of knowledge. The information on plaice is the most comprehensive (see 

 Wimpenny, 1953, for a general account) ; on the assumption that the evidence 

 presented in Figs, i and 2 indicates considerable stability in the plaice popula- 

 tion, an attempt is made below to locate the phase of the life-history during 

 which compensation occurs and to identify the mechanisms which could be 

 responsible for it. 



COMPENSATION IN THE ADULT PHASE OF THE NORTH 

 SEA PLAICE POPULATION 



Plaice do not appear in the commercial catches until they are about three to 

 four years of age, on the average, and the majority attain maturity within a 

 further year. The age and length composition of the landings of plaice, 



