The effects of disturbance and severe seasonal fluctuations in environmental conditions on north temperate shallow-water fish assemblages

Abstract

The shallow-water fish populations at one disturbed and one undisturbed site in the Oslofjord, Norway were sampled with a beach seine from September 1981 to September 1982. The inner site (inner fjord; Hvervenbukta) is in the most polluted part of the fjord and has a heavy recreational usage during the summer months, the outer site (Ellingstad) is in a least polluted part of the fjord and has only sporadic recreational usage. Physical conditions at the two sites differ primarily in seasonal changes of dissolved oxygen levels (Hvervenbukta 3·3–11·1 ml O 2 1 −1 and Ellingstad 5·3–7·1 ml O 2 1 −1). Thirty-three fish species were caught at Ellingstad compared with only 23 at Hvervenbukta, with 0·61 similarity in species lists. Both sites had approximately equal numbers of resident and non-resident species. The mean density of individuals was much greater at Hvervenbukta (0·36 m −2) than at Ellingstad (0·04 m −2). Both sites showed a high dominance factor with a few species comprising the majority of the abundance. Overall species diversity ( H′), evenness ( F′) and species richness ( D) were greater at Ellingstad, consistent with the reduced disturbance. The overall range in seasonal similarity was relatively high for both sites as a function of the severe seasonal cycle in temperature (ice cover to approximately 20 °C). These relatively high variations are to be expected for north temperate fish assemblages. The range in the seasonal similarity index was greater at Hvervenbukta reflecting increased disturbance. There were dissimilar patterns in the seasonal cycles of community parameters at the two sites. Changes in population structure were largest during the period of recruitment of new year classes.