Fish communities of the lower Fraser River (Canada) and a 21-year contrast

Fish communities were sampled at 14 sites along the lower 120 km of the Fra ser River, British Columbia by beach seine four times in 1972-1973 and thre e times in 1993-1994. Of the 37 species collected, peamouth chub, largescal e sucker, starry flounder, and northern squawfish were predominant in eithe r density or biomass. Densities and biomass of most species and all fish co mbined were higher in 1993-1994. Strong rank correlations of species abunda nce or biomass indicated that the overall fish community structure was very similar in both periods. At smaller spatial scales (reaches of 2-3 sites) and shorter time scales (by season), less than half the comparisons showed any significant correlation indicating changes in community composition. La rgescale sucker, one of the largest contributors to biomass in both periods , showed decreased abundance, i.e., lower density, lower biomass, and large differences in the size-frequency distribution, specifically very low repr esentation of small size classes. Other species, especially small-bodied fo rms, generally increased in numbers. Despite large changes in the lower Fra ser River ecosystem in the past 21 years, the overall fish community has sh own remarkably little change over that interval.