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Wet and dry calorific values (energy densities) and total energy content of lesser sandeel Ammodytes marinus, sprat Sprattus sprattus, Atlantic herring Clupea harengus, whiting Merlangius merlangus, saithe Pollachius virens and cod Gadus morhua, were measured. Calorific values varied both within and between species. Larger fish tended to have higher calorific values than small ones, particularly when considered in terms of wet weight, but there was considerable temporal variation. The calorific values and body weights of sandeels larger than 10 cm showed marked seasonal trends and in consequence the total energy content of a sandeel of given length in summer is approximately double the spring value. The calorific values of herring and sprat also varied from month to month but seasonal cycles were less obvious. Whiting varied least. Both calorific values and total energy content of individual sprat were very variable in summer (the spawning season of this species). Because there is so much intraspecific variation, care must be taken when assessing the relative merits (in terms of energy) of different species as prey. The practical difficulties of obtaining reliable data on wet calorific values are discussed and the use of dry calorific values and dry weight/length relationships is recommended.

Individual haddock, Melanogrammus aeglefinus (L.) were maintained at different feeding levels in an aquarium from November until the completion of spawning. The mean duration of spawning was 33.2 days (range 19-59) during which an average of 16.6 (range 10-25) batches of eggs were produced. The size and dry weight of the eggs declined during the spawning period. Egg production and feeding level were correlated positively. There was some suggestion that when female haddock received low rations (< 5 kcal day-') a lower proportion spawned, and the dry weights of the eggs were lower compared with females on high rations (> 13 kcal day-'). The relation between daily growth in wet weight, g, and daily surplus energy intake, kcal, was: G=(0.295E)-0.328. When food energy is restricted, haddock appear to achieve a balance between somatic growth and reproduction. I. INTRODUCTIONIt is often found that the fecundities of individual female fish of the same size and age, drawn from the same population, differ considerably. The cause of this variability is not yet understood but it seems that fecundity might be influenced by factors such as the availability of space or food, and may be part of a density dependent population control mechanism.There are few published results of experiments to determine if there is a relationship between food intake and fecundity. Scott (1962), showed that when groups of rainbow trout, Salmo gairdneri Richardson, were kept on different ration levels, shortage of food reduced both the percentage of females spawning, and their fecundity. Scott did not detect significant differences between the wet weights of the intra-ovarian eggs of fish on different rations. Bagenal(1969), working with batches of brown trout, Salmo frutta L. also found that a reduction in diet lowered the percentage of spawning females and their fecundity. However, Bagenal found the dry weights of the intraovarian eggs of the low ration fish to be significantly higher than those of the high ration fish. Wootten (1973) kept pairs of female three-spined sticklebacks, Gasterosfeus aculeatus L. at three different ration levels. He found the percentage of females spawning, and the number of eggs produced, increased with ration size but he did not detect a relationship between food intake and the dry weight of spawned eggs. The experiments described in this paper differ from those referred to in that data were obtained on the influence of feeding level on individual females, rather than on groups of fish.

The diameter and dry weight of the eggs of haddock, Melunogrummus auglefinus, are positively correlated with fish length. The correlations are largely due to the fact that many of the smaller fish that were sampled were 2 years old, and the eggs of these young haddock, which can bc regarded as precocious spawners, are significantly smaller and lighter than those of older fish. The relative fecundity of 2-year-old haddock (274 eggs g-') is also lower than that of the other age classes (493 eggs g-'); this has important implications for the estimation of egg production from female spawning stock biomass. It is pointed out that in some years precocious spawners represent a large component of the North Sea haddock spawning stock. When the annual egg production of this stock is calculated. it may be appropriate to apply a weighting factor to the number of eggs contributed by the 2-year-olds, on the assumption that the small eggs of these fish produce larvae that are less viable than those of older haddock.