Summary:

In this work analysis and assessment of physico-chemical status and fish structure of the Mała Wełna River for a period of 2000-2009 are presented. Based on the values of oxygen indexes (dissolved oxygen, BOD5 and COD) and biogenic indexes (ammonia nitrogen, nitrate), noted during research period, waters of the Mała Wełna River was classified as moderate quality. Results of the fish structure study carried out on 7 reaches situated along course of the river showed that both abundance and biomass varied among particular measurements points (Shannon's index amount 0.9). Fish population was represented by 18 species, among them in 2008 pike constituted 47% of the biomass but the highest abundance showed sticklebed (32%) Among 18 species occurring in the waters of the studied river
15 of them were classified to a category of lower risk of endanger and 3 as en-danger species. Similar to water quality also ecological status of the river was evaluated as moderate.