Divergent Structure in Fish Taxocenes of North Temperate Streams

Abstract

The fish taxocenes of two typical river systems in Poland and Ontario differed in patterns of species richness, distribution, life history, and ecomorphology. In small streams there were more small nektonic and benthic fishes in Ontario. In Poland where large, nektonic cyprinids dominate, the fishes were larger, longer lived, later maturing, and absolutely and relatively more fecund and had longer reproductive spans, but egg diameters were similar. In Ontario, nester/guarders and brood hiders were common, whereas in Poland all but three species broadcast eggs. Small-stream fishes were small, early maturing, short lived, and sedentary and gave parental care to few, large eggs. Large-river fishes displayed opposite characteristics and often migrated to spawn in habitats suitable for young but not adults. The factor space of a principal components analysis of ecomorphological characteristics corresponded roughly to a longitudinal gradient in fluvial conditions. There was a significantly higher proportion of fishes from Poland in the downstream half of the space, and vice versa. The most prominent differences in taxocene structure generalize to west-central Europe and east-central North America. Evidently, several groups of North American fishes invaded and became adapted to small streams, facilitating speciation. European fishes retained the strategy of seasonal/ontogenetic longitudinal migration. Ancestors to the most diverse groups in North America inhabited both continents; consequently, long-term climatic patterns (glaciation) and/or the influence of lowland ichthyofauna are the most likely causes of the differences.