Jan Kubečka

Acoustic size vs. real size relationships for common species of riverine fish

Abstract Target strengths of brown trout, rainbow trout, roach, perch, dace and chub, crucian and common carp, bleak and bream were determined for two ultrasonic frequencies of 200 kHz and 420 kHz using dual-beam sonar directed horizontally at tethered individual stunned fish of known size whilst being rotated through 360° by means of a carousel structure. This provided a data base for each fish of real size in length or weight and target strength for side body aspect, head/tail aspect and mean all-aspect (the average acoustic size of an individual fish rotated through 360°). In all, 182 fish were insonified in this way with more than 800 sonar runs (=replicated rotations). In all individuals, the largest target strength was for side-aspect, the lowest for head/tail-aspect and the mean all-aspect was intermediate. Statistically significant linear regressions of target strength on log length or log weight ( Y = aX + c ) were fitted to the data for individual species with an adequate number of individuals of sufficiently wide size range. Regressions incorporating all three kinds of target strengths, three variants of length (standard, fork and total) and weight have been calculated for each of two frequencies. In addition to individual species regressions, significant pooled all-species regressions ( n =180) were obtained but these were less useful than were family regressions for salmonids ( n =41) and cyprinids plus perch ( n =114). The comparable family regressions differed significantly, either in slope or in elevation (where slopes were in parallel) and the salmonid regression lay lower than the cyprinid plus perch one. In some but not all species, comparable regressions at different frequencies were similar. This study demonstrates the importance of body aspect for describing relations between acoustic size and real size in freshwater fish species.

Long-term biomanipulation of Rimov Reservoir (Czech Republic)

The history of fish, zooplankton and phytoplankton in RimovReservoir during its 14 years existence is documented. Afterthe filling of the reservoir in autumn 1978, the biomass ofplanktivorous fish increased from near zero to 650 kg ha-1 bythe fourth year of the reservoir‘s existence, after which thefish stock continually decreased under various forms ofbiomanipulative pressure, down to a value of about 100 kg ha-1. Thisdevelopment of fish stock was accompanied by changes in zooplanktonsize structure, but a cascading effect down to reduction ofphytoplankton biomass was not pronounced. The changes ofzooplankton body size structure were: increasing or decreasingaverage size of Daphnia galeata (the main cladoceranherbivore species) and changes in relative proportion ofcladoceran species, but none of the main zooplankton species was replacedor disappeared. The variation in phytoplankton biomass wasexplained by variation in phosphorus concentration and not related tozooplankton structure. It was concluded that the criticalbiomass of planktivorous fish below which the effect of loweringphytoplankton biomass could be observed lies below 100 kg ofplanktivorous fish per hectare. An attempt is made to quantifythe relationship between fish stock density and the size structureof zooplankton potentially able to influence top-down effects onphytoplankton.

Fish migration between a temperate reservoir and its main tributary

Upstream- and downstream-directed migration between the Římov Reservoir and its only tributary, the Malše River (Czech Republic) was studied using two giant traps, sited at the mouth of the tributary, during the spring and summer periods, 2000–2002. The highest number of fish species was found in the tributary area (26 species and one hybrid). Over 10% of all fish biomass of the reservoir migrated through the tributary zone. The most important migration, in terms of fish numbers and intensity of movement, was the upstream spawning run. The fish species living in the reservoir can be divided into three groups: (1) the obligatory tributary-spawners (asp Aspius aspius(L.), bleak Alburnus alburnus (L.),chub Leuciscus cephalus(L.), and white bream Blicca bjoerkna(L.)); (2) generalists — fish spawning both in the tributary and the reservoir (bream Abramis brama(L.),roach Rutilus rutilus(L.), perch Perca fluviatilisL., pike Esox luciusL., and ruffe Gymnocephalus cernuus(L.)); and (3) fish spawning out of the tributary area (carp Cyprinus carpioL., pikeperch Stizostedion lucioperca(L.), catfish Silurus glanis L. and eel Anquilla anguilla(L.)). Tributary spawning is likely to support the strong roach population in the river upstream. Downstream migration from the river by salmonids appeared to be less important, in terms of relative numbers. Asp, roach and perch migrated as single spawners, bleak, bream and chub as multiple spawners. It was possible to define six periods of migration succession during spring and summer, which differed in species dominance, gonadal status and migration rates. The sequence starts with asp spawning and finishes after local movements of resident fish replace the spawning migrations. The most spectacular period during this sequence is the mass spawning of cyprinids when the roach, bleak, chub and bream migrate upstream and spawn nearly synchronically. During this period the daily migration rate exceeded 1000 individuals, which is over 100 times higher than during the feeding migration period.

Brown trout populations of three Scottish lochs estimated by horizontal sonar and multimesh gill nets

Abstract Brown trout (Salmo trutta) populations of the three Orkney freshwater lochs—Boardhouse, Harray and Swannay—were studied in late September 1991 using horizontalll oriented sonar. Dual-beam echo counting during mobile surveys gave the following densities of fish per 100 m3: 2.94 in Boardhouse, 1.94 in Harray and 1.08 in Swannay. Multiple fish targets and reflections from submerged macrophytes were filtered out. The ranking of the estimated densities corresponds with the anglers yield which was the highest in Boardhouse and the lowest in Swannay. The acoustic signal size distribution obtained by mobile survey was interpreted by a deconvolution procedure analogous to analysis of target strengths by single-beam sonar. This revealed different fish length frequency distributions between the lochs. The sonar technique was sampling the same length range of fish as the multimesh gill nets earlier in the year but it estimated a higher proportion of the smallest fish. Results from the application of the deconvolution procedure to mobile survey data were compared with measurement of the target strengths of side-aspect tracked fish from fixed location data.

Diurnal changes of fish behaviour in a lowland river monitored by a dual-beam echosounder

The behaviour of fish was monitored in one of the lower reaches of the River Thames (England), using a BioSonics 102 dual-beam echosounder (420 kHz) with two transducers. One transducer was located in littoral (L) depths of the river whilst the other was sited in 3-m depth of water in mid-river and placed on a especially designed lifting device which allowed different 1-m depth strata to be sampled: surface (S), middle (M) and deep (D). These depth strata were sampled hourly over 24 h. The highest fish densities and biomass were recorded in the S and L strata during the night and early morning hours from 2230 to 0700 when the fish density in the M and D strata was much lower. During the night hours, the largest fish tended to move to the surface (S) and towards the littoral (L), resulting in a marked increase in fish biomass. Most of these fish seemed to move to the deeper strata during the daytime so that many of them became undetectable by horizontal sonar as they were too close to the bottom. Swimming upstream and downstream was more marked in the open water (S,M and D strata) than in the littoral (L), where movements were more random.

Trophic structure of nine Czech reservoirs regularly stocked with piscivorous fish

For nearly 20 years, most Czech reservoirs supplying drinking water have been under statutory protection which permitted reservoir managers to manipulate fish stocks in order to maintain a sustainable water quality. The most common biomanipulative measure adopted was stocking with piscivorous fish (mostly 5 cm fry) using an annual stocking level of approx. 25 000 fish per reservoir. Nine reservoirs were studied for signs of top-down food web effects, as predicted by the trophic cascade hypothesis based on levels of total phosphorus (TP), chlorophyll a (Chl a), zooplankton biomass (ZB) and zooplankton community structure. In all nine reservoirs, only small Daphnia species were recorded, such as D. galeata and D. cucullata. The proportion of large-bodied daphnids retained on a 0.71 mm sieve was less than 10% of the total crustacean biomass in all reservoirs. The relationship of Chl a level – TP, and of ZB – Chl a, was positive under enhanced piscivory and did not differ statistically from the relationships in other reservoirs with natural fish stocks. This implies that bottom-up forces remained stronger than top-down ones in the studied reservoirs, despite the stocking of piscivorous fish. The failure of this attempt at biomanipulation may be due to an insufficient stocking rate of predatory fish and/or inadequate data on the resident planktivorous fish levels.

Size Selectivity in Summer and Winter Diets of Great Cormorant (Phalacrocorax carbo ): Does it Reflect Season-Dependent Difference in Foraging Efficiency?

Abstract Diet of the Great Cormorant (Phalacrocorax carbo) was studied using regurgitated pellets, individual fish bones and fish remains collected from below roosting trees at Želivka and Slapy Reservoirs, Czech Republic, during summer, a warm winter and a cold winter. Both reservoirs have the same trophic status and similar fish assemblages. Using diagnostic bones (os pharyngeum, dentale, praeoperculare) and our own linear regression equations relating dimensions of the diagnostic bone and fish total length (LT), a total of 2,055 fish of 18 species and four families were identified in the diet of Great Cormorants and their size was reconstructed. Both fish total length and weight differed significantly between seasons being, on average, 12.0 cm and 30 g during summer, 18.3 cm and 109 g during a warm winter and 22.8 cm and 157 g during the cold winter. The average weight of fish taken by Great Cormorants significantly increased with decreasing air and water temperature. The contribution of the dominant “large growing”, torpedo-shaped fish species in the diet of Great Cormorants dramatically increased from summer to the cold winter. In contrast, the contribution of dominant “small growing”, torpedo-shaped species, or humped body-shaped species, showed completely the opposite tendency. Great Cormorants seem to consume all fish of appropriate size that they are able to catch in summer and select for larger fish in winter. Thus, the winter elevation of foraging efficiency described for Great Cormorants in the literature is due to capturing larger fish not to capturing more fish.

Cyprinid predation on zooplankton along the longitudinal profile of a canyon-shaped reservoir

In the deep, elongated řimov Reservoir (South Bohemia, Czech Republic), the diet composition of the three most abundant cyprinid species, as well as the fish and zooplankton distribution. were investigated at three sites (dam, middle, tributary) located along the longitudinal transect of the reservoir, and during two periods of the year. The studied fish were roach (Rutilus rutilus) and bream (Abramis brama) of two length classes (10-18 cm SL and >18 cm SL), and bleak (Alburnus alburnus) of one length class (10-18 cm SL). The diets of all cyprinids along the entire longitudinal transect constituted almost exclusively crustacean zooplankton. In May, Daphnia galeata was a highly preferred prey item of all fish, except for those of the 10-18 cm length class in the tributary station, which consumed mainly bosminids. In August, bleak and roach of both length classes preyed on D. galeata and Leptodora kindtii. Larger bream (> 18 cm SL) consumed these prey items as well and, moreover, fed significantly on Diaphanosoma brachyurum. Proportions of L. kindtii in the cyprinid diets were particularly conspicuous in the dam and middle sites In the tributary site, cyclopoid copepods were an important prey item in the diets of both length classes of bream, in contrast to their infrequency in the diets of roach and bleak. Densities of epilimnetic zooplankton declined gradually downstream in the reservoir, they were about two-fold higher in the tributary site compared to the dam site in both months, respectively. A relatively higher total fish catch per unit effort was found in the tributary site in comparison to the dam or middle site, and a decreasing mean size of D. galeata in zooplankton was observed from the dam to the tributary site in both months. Such results indicated stronger fish predation pressure in the upstream part of the reservoir during the two periods of the year. Moreover, lower diet overlaps within the cyprinid assemblage of the tributary site than within the cyprinid assemblages of the dam and middle sites suggested intense competition for food at the upstream end of the reservoir.

Ontogenetic changes in the bathypelagic distribution of European perch fry Perca fluviatilis monitored by hydroacoustic methods

The behaviour of bathypelagic perch fry (BPF) was monitored during 21 May, 24 June, 8 August and 5 September 1996 along the longitudinal profile of Římov reservoir (Czech Republic), using acoustic methods (SIMRAD EY 500 echosounder, elliptical split-beam transducer, Sonar5 postprocessing software). In May, the BPF layer comprised a mass of solitary perch larvae at an abundance of over 31,000 ind. ha−1. In contrast, in June, the BPF layer consisted almost exclusively of shoaling individuals. The abundance of BPF increased up to 166,000 ind. ha−1. Similary, in August, fish in shoals prevailed in the BPF layer but the abundance dropped to less than 3,000 ind. ha−1. In September, the BPF layer consisted of both individuals and small groups of fish. At this time the abundance of BPF slightly increased to almost 5,000 ind. ha−1. Individual parameters describing the BPF layer, depth of the main layer, abundance of BPF in the main layer and thickness of the whole layer, differed significantly among months. The negligible abundance of potential predators below the thermocline, both in June and in August (exclusively adult perch), suggests that the BPF shoals were more likely to be created as a result of a social need rather than as a defense against predators. During similar surveys carried out prior to (25 April, 5 May, 7 May, 12 May, 15 May) and after this period (3 October, 7 November, 2 December), the BPF layer was not recorded.

Adverse ecological effects of small hydropower stations in the Czech Republic .1. Bypass plants

Adverse effects of 23 bypass- or mill stream-type small hydropower stations (SHPSs) have been studied by traditional benthological and ichthyological techniques. Four locations, if available, were distinguished for each SHPS: the natural flow sections (NFS) above and below the SHPS area, the mill stream or diversion channel bringing water to the turbine, and the reduced discharge section (RDS) of the former stream course. Most SHPSs took a high percentage of the stream discharge (turbine intake capacity was usually bigger than average yearly discharge of the stream at a given point). Despite this, the decrease in the stream-wetted area was relatively low (usually 0–30%). The changes of benthic communities did not show any distinct trend explicable by the SHPS operation. Mechanical damage of fish by turbines was found rather rarely with SHPSs. The diverting weir (which usually distributes the discharge between the RDS and the mill stream), in combination with the abstraction of water for SHPSs, was an important migration barrier for resident fish in 30% of SHPSs. Water abstraction causes succession from large-bodied fish species (adult brown trout, chub, dace, grayling, etc.) towards small-bodied fish (trout fry, minnow, bullhead, stone loach, gudgeon). Average individual weight and fish biomass decreased four times in RDSs. Fish biomass losses exhibit significant negative relationships to the ratio of turbine intake capacity and stream discharge. Several mitigation measures are suggested.