Abstract Shore-zone fish communities in the area of a nuclear power station at Millstone Point, Connecticut, on Long Island Sound, were studied from May 1969 through December 1975. Three areas affected by the thermal plume, and a control site, not affected by the plume, were sampled during February, May, July, September and December of each year from 1968 through 1972. In February 1973 an additional affected area and control site were added and sampled on the same schedule. In February 1974 the sampling schedule was changed to include samples in June, August and October. All samples were collected with a 30-ft seine of 1 4 -in mesh. Over 81 500 fish, representing at least 35 species, were collected. Species diversity, species richness, and species evenness indexes were calculated for each site and collecting period, as well as for each year. In addition, the degree of community similarity at each site from year to year was assessed. Although the most abundant species at each site was the Atlantic silver-side, Menidia menidia, the sites differed considerably from one another in their communities, depending on such factors as amount of exposure to wave action, vegetation, and substrate composition. In general, species diversity, richness, and evenness fluctuated at each site seasonally and from year to year such that no trends could be determined. There was a distinct shift in community similarity through 1974 and 1975 at most sites. Since no evidence could be found that operation of the power station was responsible for the changes, it was concluded that such fluctuations were natural, and would be observed in any ecosystem if it were studied for several consecutive years. It is suggested that the concept of community or ecosystem stability be re-examined and possibly redefined in light of these distinct fluctuations over 6 years.