Long term development of fish community of Římov Reservoir
Large lakes and reservoirs represent complicated three-dimensional systems difficult to assess. Fish sampling is one of the most challenging as different species differ in their spatial preferences and behavior. In the same time, anthropogenic activities continue to pose the greatest challenges to freshwater ecosystems. Therefore, long-term monitoring is essential for the management and conservation of these resources. Monitoring programs for freshwater bodies often use a range of indicators, including biological elements such as fish. Římov Reservoir, Czechia (210 hectares, max depth 43 m), serves as reference waterbody for long term ecological studies and for testing different approaches of sampling freshwater life. In this study we made maximum use of gillnetting according to extended European standard CEN (2015). Water quality – sampling of fish with multi-mesh gillnets. European standard. European Committee for Standardization. Ref. No. EN 14757. The main goal was to assess the most realistically the development of fish community of the reservoir as a whole.
Existing European standard provides a depth-stratified gillnet sampling approach mainly in benthic habitats and at the deepest part of lakes to account for the uneven distribution of fish. However, the commonly used CEN protocol does not weight sufficiently habitat volumes and underrepresent pelagic habitats to calculate whole-lake catch and biomass per unit effort (CPUE and BPUE, respectively). Extended European standard gillnet (4 larger mesh-sizes added in the geometric series) catch data collected over 18 years (2004 - 2021) in Římov Reservoir were used for a method comparison on indices for relative abundance and biomass of fish: CEN protocol without volume-weighting and two volume-weighted approaches. We also evaluated changes in species composition and trends in these fish population over time. Results indicated interannual changes in species composition, relative abundance, and biomass of fish community. The CEN protocol tended to put greater emphasis on benthic habitats which generally have larger CPUE and BPUE. Consequently, the two volume-weighting approaches produced lower estimates of the two parameters, with the exception of the most dominant pelagic bleak (Alburnus alburnus). All approaches consistently showed an increasing trend in whole-reservoir fish abundance and a decreasing trend in biomass over the study period. This change is likely to consist with climate change. Following our assessment, we put forward the volume-weighting approach that considers the Volume of the depth Stratum (VOST) for weighting as the most realistic approximation of fish populations and therefore recommend its use.
Fig. 1: Setting of gillnets at Římov Reservoir (A), schematic representation of habitat definitions on the reservoir transversal cross-section with depth strata for middle and dam localities as an example (B), and schematic representation of habitats on the reservoir cross-section with depth strata and gillnet settings of the three methods (CEN, VOST and VOCOM) (C). The number of depth strata decreases from the dam (the deepest point) to the tributary. CEN refers to the European Committee for Standardization protocol, VOST is a method that weights catch rates using the volume of the depth stratum relative to the volume of the reservoir, while VOCOM is a method that weights catch rates using the volume of the depth compartment in each locality and stratum relative to the volume of the reservoir.
Fig. 2: Abundance (left) and biomass (right) composition of fish community of Římov Reservoir during 2004-2021.
Tesfaye, G.C., Souza, A.T.S., Bartoň, D., Blabolil, P., Čech, M., Draštík, V., Frouzová, J., Holubová, M., Kočvara, L., Kolařík, T., Martinez Von Dossow, C., Moraes, K., Muška, M., Prchalová, M., Říha, M., Sajdlová, Z., Soukalová, K., Šmejkal, M., Tušer, M., Vašek, M., Vejřík, L., Vejříková, I., Peterka, J., Jůza, T., Kubečka, J. (2022). Long-term monitoring of fish in a freshwater reservoir: different ways of weighting complex spatial sampling. Frontiers in Environmental Science 10:1000087.