Diel Fluctuations of a Shallow Water Fish Community in the Inner Oslofjord, Norway

Abstract. A shallow water fish community in the inner Oslofjord, Norway, was sampled with a beach seine at 4h intervals over two 36 h periods (25-26th May and 28-29th September, 1982). Night length varied from 1 h in May to 11 h in September. Differences in the community composition were observed between May and September. The diversity of the fish community changed between the two months although the interpretation of the change was dependent on the method used to analyse the community. The three dominant species (Gasterosteus aculeatus, Sprattus sprattus, and Ammodytes tobianus) showed diel variations in catch rate which were out of phase with each other. The percentage overlap in abundance of the dominant species was low. The summation of the individual periodicities resulted in aperiodicity at the community level. There were fluctuations, over a diel cycle, in all the community parameters (number of individuals, number of species, species diversity [H'J, species richness [D] and evenness [J']) which affect measures of resource partitioning in fish communities and which should therefore be considered in ecological studies. Diel fluctuations in abundance of some individual shallow water species varied between seasons causing problems in population estimates.