The Seasonal Cycle in the Cornish Pilchard, Sardina Pilchardus Walbaum

Abstract

1. In the pilchard (Sardtha pilchardus Walbaum) of Cornwall there is a substantial seasonal variation in the mean weight of a fish of given length. The weight is highest in late autumn and early winter, and least at the end of winter and the early spring.2. These changes in weight are largely due to variations in the fat content of the fish, which are richest in fat at the beginning of the winter fast, and poorest at the end of it. But, especially among the larger fish, the 'residue', chiefly proteins, are drawn upon at the time of the exhaustion of the fat, and consequently at the time of minimum mean weight. The water and ash content show no seasonal variation, and therefore play no part in the observed seasonal variation in the mean weight of the entire fish.3. The gonads have their lowest mean weight in August, increase in weight slowly throughout the winter, and are at their maximum state of development from April to July. The spawning season, as deduced from these samples, is from March to September.4. The rate of feeding is greatest in the same months as the spawning months, namely, from April to July. From July to September the average weight of food per stomach decreases, probably not because the planktonic food is then less abundant, but because it is shared among a greater number offish. In October there is a rally in the rate of feeding, but from November to February or March is a period of fasting, when very little food is taken.5. The food of the pilchard is exclusively planktonic. In the spring, and again in the autumn, diatoms are abundant or even dominant in the food; in the summer, copepods and Crustacea are the chief items of food.