LONG-TERM DYNAMICS OF CERTAIN NORTH SEA FISH 



POPULATIONS 



R. J. H. Beverton 



Fisheries Laboratory, Lowestoft 



INTRODUCTION 



Despite recent advances in knowledge of the dynamics of marine fish 

 populations, the fundamental questions of whether the populations are in a 

 state of long-term balance and, if they are, of identifying the mechanisms 

 responsible for that balance, remain largely unanswered. There are several 

 reasons for this. One is that the abundance of successive year-classes often 

 fluctuates widely, due primarily to the influence of year-to-year variation 

 in environmental conditions, which makes it difficult to establish the 

 existence of true long-term trends except from a very long series of data. 

 Another is that the natural life-span of many of the species which exist as 

 large population units supporting major commercial fisheries (and therefore 

 for which the records and information are most extensive), extends for at 

 least ten years and in some cases up to thirty years (Brown, 1957; Beverton 

 & Holt, 1959). Consequently, even if many years of reliable data on the size 

 of the population are available, they may cover a span of only a very few 

 generations. A further difficulty is that although commercial fishing records 

 can often provide valuable comparative indices of population abundance, 

 they refer only to sizes of fish which can be retained by the gear and are of 

 commercial importance. In many instances, fish do not reach such a size until 

 they are approaching or have attained sexual maturity; yet the indications 

 are that if compensatory factors are operative their influence may well be 

 confined to the juvenile or even larval phase of the life-history, about which 

 relatively little quantitative information exists. 



It is the purpose of this paper to examine the evidence for long-term 

 stability, or lack of it, in certain fish populations of the North Sea for which 

 particularly good records are available, and to attempt to delimit the possible 

 compensating mechanisms in one of these populations about which most is 

 known of the various phases of the life-history, namely the plaice [Pleuronectes 

 platessa L.). 



