An investigation of North Sea haddock (Melanogrammus aeglefinus) length-at-age data, with particular reference to the effects on growth of the outstanding year classes of 1962 and 1967

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Abstract
This paper investigates the growth of North Sea haddock with particular reference to the growth of the very strong year classes of 1962 and 1967. Lengths at age and growth rates were investigated with particular reference to these two strong year classes and to those that were produced immediately before and immediately after them. The results show that: 1. The very good year classes of 1962 and 1967 as well as those of 1963 and 1968 which immediately followed them exhibited smaller lengths at age than those of 1961 and 1966. This appearedto be primarily owing to a reduction in growth rate after the fish became demersal, and prior to the attainment of a length of about 28 cm. Prior to the attainment of about 28 cm in length the growth rates of the two strong year classes as well as those of the two year classes which immediately followed them, were as low as those observed during the late 1920s. Once these year classes had attained about 28 cm in length, however, growth rates were maintained at a level comparable to that determined for the 1950s, which was greater than that determined for the late 1920s. 2. In the long term, there has been an increase in mean lengths at age, and the lengths at age of the 1961 and 1966 year classes which immediately preceded the 1962 and 1967 year classes were amongst the highest recorded since sampling began in the 1920s. 3. During the late 1960s, due to the appearance of the 1962 and 1967 year classes, the biomass of the stock increased by about four to seven fold for nearly a decade. In spite of this, the growth rates for haddock greater than about 28 cm in length were not lower during the 1960s than they were during the 1950s, and they were higher than those during the 1920s. The results suggest that if the strong year classes caused intraspecific competition for food, the effect was felt primarily by individuals of the same age or younger. Older and larger individuals appear to have been scarcely affected, if at all. 4. Although larger haddock eat larger food particles, there was found to be a very considerable overlap of food particle size in the diets of fish of different sizes. Based on stomach content analysis, the opportunity for intraspecific competition between fish of different sizes would appear to be very high. The different responses, to changes in biomass, of fish of different sizes, are therefore presumably owing to intraspecific differences in the relative success at food gathering, rather than to differences in the food available for fish of different sizes.

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