Quantification of Chaoborus and small fish by mobile upward-looking echosounding

Chaoborus larvae inhabit frequently the water column of lakes, when they can be mistaken for small fish. Because larvae ascend up to the blind zone of downward-looking echo sounding at night, quantitative acoustic estimation of them is possible only with upward-looking approach. For this reason, the mobile hydroacoustic upward-looking system (120 and 38 kHz split-beam echosounder) in combination with a direct catch method (trawling) was tested to investigate the night community of invertebrates and juvenile fish in the surface layer of the Římov reservoir (Czech Republic). In the target strength range of invertebrates (smaller than -59 dB), the 38 kHz echosounder recorded only a small proportion of targets while the 120 kHz echosounder recorded distinct peaks corresponding to high densities of Chaoborus (target strength, TS range -70 to -60 dB, average TS -66 to -64 dB). At 120 kHz frequency, the TS distribution of smaller cohort of juvenile fish ( 1.5 ind.m-3.

A simple fish-based approach to assess the ecological quality of freshwater reservoirs in Central Europe

The assessment of ecological quality in freshwater ecosystems is a key issue in many countries, but conditions for the development of assessment methodologies are often country-specific. This study proposes a simple methodology for the assessment of the ecological potential of reservoirs based on fish communities using a dataset covering major environmental and pressure gradients in reservoirs in the Czech Republic. Fish data obtained by gillnet sampling were correlated with a proxy of eutrophication as a key indicator of anthropogenic pressure for selecting appropriate fish-based indicators, establishing scoring criteria and developing the index of ecological quality. Expert judgement was also used to select potential fish indicators. Nine indicators were selected for the final fish-based index, fulfilling the criteria required by the Water Framework Directive. Two steps were used to validate the fish-based index quantification of its inter annual stability and sensitivity analysis of individual indicators. Finally, the index was compared to a previously developed general index for Central and Western Europe. Our study demonstrates that a combination of expert judgement and strict validation methods can result in an informative assessment of the ecological conditions, which can help identify conservation and restoration priorities. © P. Blabolil

Seasonal habitat use of three predatory fishes in a freshwater ecosystem

To understand the spatiotemporal overlap in the habitat use of sympatric predators, we studied longitudinal activity and reservoir section and depth use of pike (Esox lucius), pikeperch (Sander lucioerca) and catfsh (Silurus glanis) in the Římov Reservoir, using an autonomous telemetry system for 11 months. We found signifcant diferences among these species in studied parameters that varied considerably over tracked period. Pike consistently used the same sections of the reservoir, while pikeperch and catfsh frequently visited a tributary during the warm season (late spring and early autumn), and moved closer to the dam during the cold season (late autumn to early spring). Pike longitudinal activity was highest in the cold season, pikeperch in the warm season, and catfsh activity peaked in both seasons. Overlap in the depth use among species was higher in the warm season, when all species used the upper layer of the water column, and lower in the cold season, when pikeperch and catfsh used deeper areas. These results demonstrated overlay and temporal variation of habitat use among these predators, as well as potential spatiotemporal space for their direct ecological interactions. Acoustic telemetry · Predators · Habitat use · Movement ecology · Winter ecologypublishedVersio

Long-term monitoring of fish in a freshwater reservoir: Different ways of weighting complex spatial samples

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. 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 (European Committee for Standardization) 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 (Czech Republic) 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 (L.). All approaches consistently showed an increasing trend in whole-reservoir fish abundance and a decreasing trend in biomass over the study period. 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

Fish detection with modern sonar systems

This dissertation thesis was focused on improving the methodology to detect fish with modern sonar systems in lakes and reservoirs. The first part of the thesis is aimed to the vertical beaming acoustics with a key focus on the acoustic dead zone and its practical solution. The second part deals with the fish orientation in reservoir?s open waters and its consequences in horizontal beaming acoustics. The last one dedicates to the DIDSON multi-beam sonar and its reliability in detection and sizing of fish

Hydroacoustic assessment of the fish stock in Lake Balaton in 2019.

The presented report summurizes results of hydroacoustic surveys in Lake Balaton, including both littoral and open-water regions in all three basins of the lake. The report is describing the spatial distribution and size-composition of the fish stock in Lake Balaton in autumn 2019. The results are presented in terms of fish density, biomass and fish size distribution for particular hydroacoustic transects and lake basins. In the report are summarized all available data from previous hydroacoustic surveys and the development of fish stock is discussed

Hydroacoustic assessment of the fish stock in Lake Balaton in 2014

The objective of this project was to survey fish assemblage in Lake Balaton using 120 kHz acoustics to estimate fish quantity in open water at night. In addition, the results from this work could further help answer what a proportion of Asian carps might occupy the waters of Lake Balaton based on the resultant size distribution of observed targets

Fish orientation along the longitudinal profile of the Rimov reservoir

The aim of this work was to verify the assumption of random fish orientation in the lacustrine zone of the ``canyon-shaped{\crqq} Rimov reservoir and to compare distributions of fish orientation in the lacustrine and tributary zone. The study confirmed that most fish were oriented randomly in the lacustrine zone of the reservoir, whereas in the tributary fish moved predominantly in parallel with the longitudinal axis of reservoir

Hydroacoustic assessment of the fish stock in Lake Balaton in 2017

This report describes a study of open-water fish assemblage in Lake Balaton, Hungary, in 2017. The lake was acoustically surveyed using two horizontally-oriented 120 kHz transducers. The observed fish stock was assessed on the basis of fish sizes, abundance, biomass, and spatial distribution in the lake

Orientace ryb v podélném profilu nádrže Římov během dne: Následky pro horizontální akustické průzkumy

The orientation of fish has important consequences for estimating their true size from horizontal acoustic records. The aim of this study was to verify the assumption of randomly orientated fish in the lacustrine zone of the canyon-shaped Římov reservoir during daytime and to compare distributions of fish orientation between the lacustrine and tributary (riverine) zones