DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 249 



fact, these data fit closely to a hyperbola, which is what would be expected 

 if the data of Fig. 3 arc best represented by a horizontal line. Thus it seems 

 that the main compensating mechanisms in plaice are located somewhere 

 between the successive generations of adult fish. 



Apart from the lack of a relation between adult population size and subse- 

 quent recruitment, the degree of variation in the number of annual recruits 

 is itself not without significance. Thus, in plaice, over the series of twenty- 

 six year-classes whose abundance can be measured fairly rehably, the extreme 

 range of numbers has been about six-fold and the average variation only 

 about 36 per cent above and below the mean (see Fig. 3). The significance 

 of this degree of variation of recruit numbers can be judged from the fact 



75 100 125 



Index of adulL population weight 



Fig. 4. — Index of survival to recruitment in plaice plotted against biomass 

 of parent population. 



that because of the enormous fecundity of female plaice no more than roughly 

 ten fish can survive from every million eggs spawned, if the size of the adult 

 population is to remain steady. In other words, on average the progeny 

 from about 999,990 eggs out of every miUion die before reaching maturity; 

 but the observed variation of recruit numbers means that the extreme range 

 of deaths has been only from about 999,970 to 999,995, and the average 

 range only from about 999,987 to 999,993. This indicates a degree of 

 compensation which is remarkably close by any standards. In contrast, 

 recruitment to the haddock population over thirty year-classes has varied 

 over an extreme range of 500-fold (Parrish, 1956, and unpubhshed data); 

 this would seem to be consistent with the comparative lack of stability 



