PENCZAK, Tadeusz; GOMES, Luiz Carlos. Impact of engineering on fish diversity and community structure in the Gwda River basin, north Poland. Polskie Archiwum Hydrobiologii, Lomianki, v.47, n.1, p.131-141, 2000.Many mechanisms can determine the structure and functioning of fish assemblages. These were investigated for the Gwda River and its tributaries in Poland from samples collected during 1983-85 and 1995-97. The same 57 sites were sampled in both periods, using the same equipment (electric fishing) and team. Of the site characteristics examined only river engineering (canalization) explained the qualitative and quantitative changes in fish assemblages. In the two periods species richness was greater in the natural than in the canalized sites. Diversity was greater in natural sites in both periods but evenness was greater only in the second period. The position of dominants remained the same in both periods but numbers of rare species declined in the last period. A greater abundance of fish was recorded in the first period in natural sites. There was some evidence that canalization had a deleterious effect on obligatory riverine species.