DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 247 



which have been sampled regularly since 1930, therefore refer mainly to 

 what may be called the adult phase of the population. These constitute the 

 longest series of quantitative data on the plaice population, and it is appro- 

 priate to see first whether there are features of the dynamics of the adult 

 population which could give a clue to the compensating mechanisms. 



The fact that the abundance of the adult plaice population has been 

 remarkably steady for most of the period since 1906 does not make it any 

 easier to detect the existence of possible compensating mechanisms. The 

 nearly complete cessation of fishing during each of the two wars, however, 

 caused marked, although temporary, increases in the density of the adult 

 population. These increases are not very noticeable in the landing statistics 

 shown in Figs, i and 2 because fishing itself did not recover its previous 

 intensity until a year or two after each war period, but they are clearly 

 shown from the data of catch per unit effort (Margetts & Holt, 1948). In fact, 

 when fishing restarted in the summer of 1945, the density of the adult plaice 

 population, in terms of weight of fish, was something like ten times the 

 average density during the 1930's, and about five or six times higher in 

 numbers. This increase in density was due purely to the absence during the 

 war period of a fishing mortality rate that had previously been in the region 

 of 50 per cent per year, but if density-compensating mechanisms are present 

 in the adult phase they should nevertheless have been detectable in such 

 circumstances; yet the secondary effects do not seem to have been very 

 marked. The growth of the older fish during the war period had indeed 

 been somewhat retarded; thus by 1946 the weight of plaice which had 

 reached maturity before the war was reduced by some 15-20 per cent 

 compared with fish of the same age under pre-war conditions. This is 

 evidence of some compensation of population biomass, but not necessarily 

 of any more far-reaching form of compensation. The growth of fish is 

 remarkably plastic and there are instances both from experimental studies 

 (e.g. Dawes, 1930, 193 1, for plaice) and in nature (e.g. Aim, 1946; Deelder, 

 1 951; for perch) in which much greater retardation than this did not have 

 any adverse effect on survival. Moreover, direct measurement of the natural 

 mortality rate of plaice over the whole war period was possible; certain 

 year-classes could be sampled both in 1939 before fishing stopped and again 

 in 1945 when fishing restarted (Beverton & Holt, I957)» and these gave 

 estimates of about 10-15 per cent per year. It is not possible to estabhsh 

 whether this figure is significantly higher than the natural mortality rate 

 under the conditions of reduced population density either before or after the 

 war, but even if it were so the effect would hardly be enough to produce 

 the degree of compensation observed. The retardation of growth may also 

 have influenced the efficiency of the reproductive processes in various ways, 



