Preliminary results of the salmonids monitoring in the river Rega (north-western Poland)

• ¹ West Pomeranian University of Technology, Poland
• ² Warsaw University of Life Sciences, Poland
• ³ Stanisław Sakowicz Inland Fisheries Institute, Poland

River Rega is 167.8 km long, of high average gradient (21.80 m3/s) which disembogues to the Baltic Sea. Rega, with well-oxygenated waters is extremely important ecological passageway for migrating for spawning salmonids (Salmo salar and Salmo trutta) as well as it is a perfect habitat for non-migratory salmonids species such a Salmo trutta fario or Thymallus thymallus. The main sites of salmon and sea trout spawning are below the hydropower Rejowice, below the Gryfice dam, in the region of Trzebiatów village, Brodziec and Lubieszowa streams. In 2018-2019 the stocktake of salmonids fish in the basin of Rega river was performed. All salmonids fish were measured (TL, total length) and weighed, the age was estimated by the scale reading; in addition the condition of fish was calculated (the Fulton and Clark coefficients) and the diet composition was examined. The electrofishing was performed in July 2018 in the rivers: Ukleja, Brzeźnicka Węgorza, Piaskowa, Sąpolna; in October 2018 in Rega, Mołstówka, and in January 2019 in Rega again. 25.86% of fish collected were salmonids (Salmo trutta, Salmo trutta fario, Thymallus thymallus). The sea trout contributed to 40.0% of the whole salmonid sample, the brown trout to 43.33% and European grayling to 16.67%. Salmo trutta were collected during the back migration after spawning in the river Rega. The range of total length was 51.0-61.0 cm (mean 55.8cm) and weight was ranged from 880.9 – 2169.9g (mean 1541.7g). The average value of the Fulton and Clark coefficient were 0.89±0.17 and 0.87±0.17, respectively. The stomachs and alimentary tracts were empty. The age ranged within 5-6 years, with average value of 5.25 (river age 2-3, sea age 2-3). Graylings were collected in July in the river Brzeźnicka Węgorza (tributary of Rega river) and in October 2018 in Rega. The range of total length was 20.5-31.1 cm (mean 25.24 cm) and weight was ranged from 74.6-295.6g (mean 166.04g). The average value of the Fulton and Clark coefficient were 0.94±0.10 and 0.83±0.10, respectively. In the alimentary tracts of T. thymallus the following diet components were found: representatives of Diptera (larvae of Chironomidae and Ephemeroptera, adults of Trichoptera, Ephemeroptera, Plecoptera, Coleoptera, Lepidoptera, and Crustacea (Copepoda Gammarus spp.). In addition, the presence of Annelida (Oligochaeta), Acari (Hydracarina), and Donata was recorded. The age ranged within 2-3 years (2 years old fish were immature). The brown trout S. trutta fario occupied 4 of 6 examined rivers. The range of total length was 12.2 – 23.0 cm (mean 17.77 cm) and weight was ranged from 17.2 -137.4g (mean 73.49g). The average value of the Fulton and Clark coefficient were 1.16±0.10 and 1.01±0.11, respectively. The high values of condition coefficients resulted from the pre-spawning period of sampling. In the alimentary tracts of S. trutta fario the following diet components were found: Trichoptera, Ephemeroptera, Diptera and Homoptera, as well as undetermined remains of Amphipoda and Molusca. The age ranged within 2-3 years (2 years old fish were immature and they were prevailing). Preliminary data obtained in this study show that river Rega is an important habitat both for migratory and non-migratory salmonids in the northern Poland. Besides the protection of these unique sites in this area, it is necessary to employ the active procedures of conservation, namely the more intensive stocking with the Salmo salar smolts as well as to arrange the natural spawning grounds to provide an opportunity to salmonids to spawn in the areas where the proper substrate is lacking.