Clupeid populations of inshore waters of the west coast of Scotland

Abstract

Population dynamics and aspects of biology of the clupeid populations in the sea-lochs and the 'open' areas around Oban were studied from April 1970 to October 1972. The area investigated is an important nursery ground for young clupeids. The localised distribution of 0-group fish and the age structure of the sprat populations are mainly attributed to the biomass that can be supported by each locality. The composition of the commercial fishery was similar to that of the more 'open' areas. Growth rate of 0-group clupeids is not significantly different from locality to locality. A period of rapid growth in sprats after their first winter is evident. Growth curves using empirical data and back-calculated lengths were constructed. Autumn-spawned young herring probably originate from the Minch autumn spawning stock, and the spring-spawned ones from the Clyde stock. The sprat populations appear to be homogeneous in origin. Spawning in sprats lasts for a period of five to six months, starting in February-March. Minimum size of maturity is 88-90 mm in both sexes but males mature earlier in the season. Sprat shed their eggs in 7 to 10 batches and the significance of serial spawning is critically examined. The fecundity ranges from 8300 to 46600 in fish between 95 and 146 mm in length. It is significantly correlated to weight, length and age. 0-group clupeids tend to feed throughout the year, whereas nearly 40% of the older individuals do not feed during the winter months. The species composition of the diet in 0-group herring and sprats are identical. The diet is mainly crustacean, the main component being copepods. Qualitative and quantitative similarity of the diet and the overlap of the daily feeding periodicities in 0-group clupeids is indicative of potential competition for food between these two species, when and if the food supply is limiting. Two species of nematodes and one helminth were found to be parasitic in the body cavity and the gut respectively. In addition two species of the genus Lernaeenicus were parasitic on sprat.