Marie Prchalová

Acoustic study of fish and invertebrate behavior in a tropical reservoir

The fish and invertebrate behavior of the Ubol Ratana Reservoir, Thailand, were monitored using up- and downlooking split beam sonar located at a fixed location. In the same area and period, ichthyoplankton nets and multimesh gillnets were used. The bulk of targets, recorded by acoustics and direct capture, consisted both of fish 3‐4 cm long and insect larvae 0.2‐1 cm long. Diurnal patterns of behavior were very distinct: during the daytime, invertebrates were hidden in the bottom and most fish stayed in compact shoals. Time course of acoustic fish biomass and abundance was very variable due to shoaling. Only the largest fish were recorded as solitary targets. At night, the whole acoustic range was filled with targets and the time course of fish biomass (5‐15 kg ha ‐1 ) and abundance (20‐45 thousand individuals ha ‐1 ) were more constant. The biomass increased mostly at surface layers. Fish appeared in the evening in the water column 1 h earlier and stayed there in the morning 1 h longer than invertebrates. Dawn and dusk are good periods for studying fish before invertebrates outnumber them. Apart from fish, according to the target strength, swimming speed and depth distribution, at least four groups of water invertebrates were distinguished acoustically, some with extremely fast vertical movement (7‐9 cm s ‐1 vertical speed). Comparison of up- and downlooking observations gave comparable results in midwater layer outside the near-field of the transducer. The uplooking approach can be more suitable for night records; downlooking for the day.

The effect of hydropower on fish stocks: comparison between cascade and non-cascade reservoirs

Spatial distributions of fish were studied in two types of reservoirs by means of hydroacoustics during the summer. Different patterns of fish distribution were found in non-cascade (Římov, Želivka) and cascade (Kamýk, Slapy, Štěchovice, Vrané) reservoirs. Maximum biomass and density of fish were observed in the tributary area of the non-cascade reservoir. The biomass declined towards the dam area. Average weight of fish showed the opposite trend—maximum average weight was observed in the dam area and declined towards the tributary area. In the cascade reservoirs fish distribution was found to be more complicated but the maximum biomass and density of fish were observed in the dam areas, whereas the tributary areas were nearly fishless. Poor ecohydrological conditions in the tributaries of cascade reservoirs are likely to be responsible for the low abundance of fish in these areas. This was evident in the example of Vrané reservoir which has two major inflows. Low biomass and density of fish were observed in the cold and low oxygen saturated inflowing cascade Vltava River while higher biomass and density were observed in the warm and well oxygen saturated Sázava River inflow. Average weights of fish followed the same trend as in non-cascade reservoirs—the maximum was in the dam area and it declined towards the tributary area. The vertical distribution of fish is driven by the fully developed stratification of temperature and oxygen during the summer season in non-cascade reservoirs. A significantly lower biomass of fish was observed below the thermocline in vertical surveys in both non-cascade and cascade reservoirs. Summer stratification in cascade reservoirs is weakly developed or is not developed at all due to hypolimnetic releases of cold water and short retention times. High transparency suggests lower primary production in cascade reservoirs.

Fish habitat preferences in an artificial reservoir system

Spatial distribution o f fish in water bodies is not homogenous due to historical and physical constraints. In Europe, fish inhabiting reservoirs and lakes are of riverine origin, and they may choose habitats that are similar to those in rivers, such as tributary inlets and shallow littoral areas (FERNANDO & HoLciK 1991 ). In addition, most reservoirs are strictly thermally stratified, and fish prefer to inhabit epilimnion (VASEK et al. 2004). Experiments leading to understanding fish spatial distribution are difficult, so observations in simplified natural conditions rnay unravel important facts. The aim o f this study was to describe distributional pattems in three artificial reservoirs with no thermal stratification.