Growth Studies on North Sea Herring: I. The second year's growth (I-group) of East Anglian Herring, 1939-63

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Abstract
The study of the major growth changes in North Sea herring which have taken place in the last fifteen years or so has until now been based mainly on mean length-for-age data.Such data have limitations, but despite the increased scope that back-calculation of annular rings on scales might be expected to allow, the use of methods incorporating data obtained from scales has been limited. This paper, the first of a series, describes a method which allows accurate and reliable comparisons of growth to be made for each year-class and each year of life, based entirely on back-calculated estimates of mean length-for-age. The method is applied to growth of I-group herring sampled in the East Anglian fishery, which is then compared over a period from 1939 to 1963. There were quite large changes in I-group growth in the 1940s, equally big changes in the 1950s, and in 1963 an increase of such a magnitude as to imply that a major change in growth conditions had occurred in that year.The precision of growth estimates derived from backreading can be determined because the reappearance of a year-class in the fishery each year allows successive, and to a large extent independent, estimates to be made of the same year's growth. In the data considered the precision reached is very satisfactory, allowing reasonably firm conclusions to be drawn for differences of 1 or 2% in growth between years. This is despite the occurrence of LEE'S phenomenon which, in fact, by revealing an age- (and length-) dependent bias in growth estimates, allows its effect to be estimated and removed, wholly or partly, when comparisons are made. The method does not rely on theoretical models of growth processes, for reasons which are discussed, but does allow the comparisons made to be defined in quasi-experimental terms.

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