DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 253 



that the possibihty of compensation arising through an increase of spawning 

 area with population size can be rejected, at least for plaice. 



There are, however, other ways in which compensation may possibly 

 occur in connection with the spawning processes, although these cannot so 

 readily be tested. Direct interference in one or other stage of the spawning 

 act, if the adults become too crowded, can occur in fish such as salmon, 

 where a fairly elaborate behaviour pattern is involved and the extent of the 

 spawning beds may be very restricted. There is only one recorded observa- 

 tion of the spawning behaviour of plaice (Forster, 1953), but this does not 

 suggest that any very elaborate behaviour pattern is involved. There are also 

 instances in which overcrowding of eggs reduces the efficiency of hatching, 

 as has been shown, for example, by McKenzie (1947), for smelt; but even 

 the peak concentration of plaice eggs, which are dispersed throughout the 

 water column, seldom exceeds two per cubic metre, and this hardly seems 

 hkely to be a lethal concentration. A further possibility is that although the 

 growth of adult plaice has not been greatly influenced by large changes in 

 population density, the ability of the females to produce healthy and viable 

 eggs may have been much more affected. Certainly, the condition factor of 

 mature females, i.e. the ratio of weight to length, was some 10 per cent 

 lower immediately before the spawning seasons of 1946 and 1947, when the 

 density was high, than during pre-war years, but it is not known whether 

 such a change could have affected adversely the viabihty of eggs. It seems, a 

 priori, rather unlikely that this could have been solely responsible for the 

 graded degree of compensation which appears to exist over such a wide 

 range of population density, but the possibihty that it may be a contributory 

 factor at the highest densities cannot be ruled out. 



COMPENSATION DURING THE EARLY LIFE OF PLAICE 



Apart from these uncertainties concerning some of the events associated with 

 spawning, it seems therefore that the main compensation in plaice occurs at 

 some time during the first year of life. It has, of course, been suspected for a 

 long time that a very heavy mortahty occurs in many marme species during 

 the first few weeks after hatching (Hjort, 191 4), when the young larvae are 

 susceptible to relatively small changes in environmental conditions that 

 would be of little or no significance a few months later in hfe. In a few 

 instances the incidence of larval mortality has been followed quantitatively; 

 for example, by Sette (1943) for Atlantic mackerel {Scomber scombrus), and 

 by Ahlstrom (1954) for the Pacific sardine {Sarditwps caerulea), but there does 

 not yet seem to have been a study in which the contribution of the various 

 causes of death to the total mortality rate has been measured. Yet a distinction 



