DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 243 



LONG-TERM TRENDS IN CERTAIN FISH POPULATIONS OF 

 THE NORTH SEA 



The physical nature of the marine environment and certain characteristics of 

 the biology and behaviour of fish often make it possible to identify a par- 

 ticular population as a self-perpetuating unit, to delimit its spatial distribution 

 and, given appropriate data, to obtain reliable comparative measures of its 

 size over long periods. Certain of the demersal fish species of the North Sea 

 exist as identifiable populations of this kind, whose members do not stray 

 to any extent beyond the confines of the North Sea and aggregate consistently 

 for spawning at a particular season and on characteristic spawning grounds. 



Early records of catches, when trawling began to spread into the North 

 Sea proper in the first half of the last century, show that the relative abundance 

 of the main species in the catch was broadly similar to that of the present day, 

 despite the major influence which fishing has undoubtedly had. By the 

 beginning of the present century the total amount of trawling in the North 

 Sea for the major species had increased to a level which was probably not 

 grealty different from that prevailing now, and which has not changed 

 much during the intervening period, except for the temporary cessation of 

 fishing during each of the two world wars. From 1906 onwards the Inter- 

 national Council for the Exploration of the Sea has pubhshed in its Bulletin 

 Statistique full records of the total landings of each species by all countries. 

 These constitute a unique set of data which it is possible to use, for certain 

 species, as a fairly reliable indicator of long-term trends in population size. 



Five species have been selected for the present analysis, namely plaice, cod 

 {Gadus callarias), haddock {Melanogrammus aeglefinus), sole (Solea vulgaris) and 

 turbot (^Rhombus maximus). Sole and plaice are typically more southern in 

 their distribution than cod, haddock and turbot, but there is considerable 

 overlap between the five species except between sole and haddock. Data of 

 landings per unit fishing effort would normally be preferable as indices of 

 population size, but these are not sufficiently rehable for all the five species 

 over the whole period to have much advantage over landings for the present 

 purpose. The relative market demand for these species has, however, 

 remained fairly steady and all of them are caught by similar gear. Moreover, 

 the transitory changes in their abundance resulting from the cessation of 

 fishing during each of the two wars were very pronounced in terms of 

 landings per unit effort but are not strictly relevant to the analysis of long- 

 term trends. For this particular purpose it was therefore decided to use 

 statistics of landings rather than of landings per unit effort, although the latter 

 are used for the more detailed interpretation of events in later years when it 

 is known that significant changes in fishing effort have occurred. 



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