Search

Freshwater protected areas are designated parts of the inland waters that restrict human activities. They were created as a mechanism to combat the decline of fauna and flora of the world. Some authors have questioned their actual effectiveness in terms of the purpose of protecting endangered fauna and flora. We conducted an experiment in Lipno reservoir in the Czech Republic to evaluate the impact of protection against angling pressure on the fish community. We selected data from two years of gill netting and analyzed the difference between areas of low anthropogenic impact (LAI) and those of high anthropogenic impact (HAI) in terms of abundance, biomass, standard length, and diversity indices. Three groups of fish were found to prefer protected areas with low anthropogenic pressure: 1. YOY (Young-of-the-year) perch (Perca fluviatilis), the dominant of the young-of-the-year fish community. 2. Pike (Esox lucius), wels catfish (Silurus glanis) and rudd (Scardinius erythrophthalmus), which were not found in HAI areas at all. 3. Larger individuals of pikeperch (Stizostedion lucioperca), which survived better in LAI areas. Some factors may affect LAI, such as illegal poaching or setting out food bait to attract the fish outside. Another factor that can be considered is the migration of fish, either to forage or to reproduce, since the LAI areas are open to the reservoir. The areas of LAI act as protective habitats for heavily exploited predatory fish species and increase fish diversity indexes. The example of the protected and low-impact areas of Lipno should be followed in other water bodies with high fishing pressure and anthropogenic impact.

Ligula intestinalis (Linnaeus, 1758) is a tapeworm parasite with a worldwide distribution that uses a wide variety of fish species as its second intermediate host. In the present study, we investigated the prevalence and population genetic structure of plerocercoids of L. intestinalis in five common cyprinoid species, roach Rutilus rutilus (Linnaeus), freshwater bream Abramis brama (Linnaeus), white bream Blicca bjoerkna (Linnaeus), bleak Alburnus alburnus (Linnaeus), and rudd Scardinius erythrophthalmus (Linnaeus), collected in six water bodies of the Czech Republic (Milada, Most, Medard, Jordán, Římov and Lipno). Of the six study sites, the highest frequency of parasitism was recorded in Lake Medard (15%). The overall prevalence rate among the species was as follows: roachrudd ≥ freshwater breambleakwhite bream. Two mitochondrial genes (cytb and COI) were used to compare the population genetic structure of parasite populations using selected samples from the five fish species. The results of the phylogenetic analysis indicated that all populations of L. intestinalis were placed in Clade A, previously identified as the most common in Europe. At a finer scale, haplotype network and PCoA analyses indicated the possible emergence of host specificity of several mtDNA haplotypes to the freshwater bream. Moreover, pairwise Fixation indices (F

Internal seiches are common in stratified lakes, with significant effects on stratification patterns, hydrodynamics and vertical nutrient transport. In particular, seiche can change the vertical distribution of the thermocline and the cold hypolimnetic and warm epilimnetic water masses by several meters on a timescale of a few hours. The results are rapid and strong changes in temperature profiles and oxygen availability that can have profound effects on vagrant and sessile organisms. Internal seiche dynamics could therefore affect fish communities directly through physiological stress and elevated mortality, and indirectly through prey distribution.The aim of this study was to analyze the effects of internal seiche dynamics on lacustrine fish behaviour, and to characterize fish reaction patterns, with the main focus on vertical movement of fish in the vicinity of a shifting thermocline, and avoidance of cold hypolimnetic water.The analysis was based on acoustic telemetry data from Lake Milada, a post-mining lake in the Czech Republic, with a total of 55 tracked individuals of four species: northern pike (Esox lucius), wels catfish (Silurus glanis), tench (Tinca tinca) and rudd (Scardinius erythropthalmus).The effects of seiche dynamics on the four species studied were weak but significant during the day, but only on rudd during the night. Upward seiche produced stronger reactions in fish than downward seiche, and the effects were manifested only during the strongest seiche events.Thermocline shifting during seiche events may induce a transient reduction in habitat for seiche-reacting species, thus potentially affecting predation and other inter- and intra-specific interactions, and probably affecting fish community dynamics.Graphical abstract

Sampling of benthic fish is complicated, especially in deep inland water bodies with a structured bottom. The catches were compared of rapidly spreading round goby (Neogobius melanostomus) using small fykes nets and benthic gillnets in three artificial lakes in The Netherlands over a two year period. Round gobies were captured at all depth layers in each sampled lake. Significantly larger individuals were captured in gillnets compared to fyke nets. Reference sampling in littoral areas captured a wide range in size of round gobies with beach seines. With fyke nets, the highest catches were usually achieved in the shallowest and deepest depth strata. Gillnets catch decreased at deeper layers. Both methods are passive sampling tools and did not provide the absolute catch per bottom area, however relative density estimates of round gobies at different depths or habitats are possible. Round gobies showed a significant size bias associated with capture method. Because it is important to understand the biology and ecology of invasive species like round goby, the combination of small fyke nets and gillnets appears to be a good solution to sample a variety of ranges in deep or/and structured benthic habitats.

A novel sampling scheme, using a combination of electrofishing, visual exploration by scuba divers, two types of fyke nets and longlines, was tested in four reservoirs (including their inlets and outlets) to monitor a population of burbot Lota lota. This was supplemented by fry trawling and vertical hydro-acoustics, to detect L. lota larvae in two deep reservoirs that have had a long-term stocking programme. The majority of the L. lota detected were juveniles, captured by electrofishing in the littoral zones of the reservoirs and in running waters. Older individuals were rarely captured with longlines or fyke nets in deeper zones or structured habitats within the reservoirs. A combination of multiple sampling methods provided an assessment of the whole population. Population establishment could not be demonstrated as the age structure of the sampled fish corresponded with that of the stocked fish. Low post-stocking survival, migratory behaviour, interactions with other species and warmer water temperatures are considered the potential drivers for unsuccessful establishment of L. lota populations in these reservoirs.

In the Czech Republic, the Balkan spiny loach Sabanejewia balcanica was historically only known from the River Becva, a left tributary of the River Morava, itself a main tributary of the River Danube. Following its assumed extinction in the Czech Republic, a small population was found in a 1 km stretch of the River Vlara at the Slovak border, with individuals presumably having migrated from Slovakia. In 2016, we recorded six individuals in the downstream stretch of the River Jihlava. Based on present knowledge, this appears to be a population situated wholly outside of its previously known area of distribution, and the only population presently found in the 26000 km2 River Morava drainage basin. The distance of this population from the species' present distribution area and historic findings in the River Becva clearly indicate a much wider historical distribution of this species in the past.

Gas ebullition of river impoundments plays an increasingly significant role, particularly in transporting methane CH4 from their sediments to the atmosphere, and contributing to the global carbon budget and global warming. Quantifying stochastic and episodic nature of gas ebullition is complicated especially when conventionally conducted by using coverage-limited gas traps. Current knowledge of seasonality in a reservoir's gas ebullition is lacking in the literature. For this reason, advanced acoustic surveying was intensively applied to determine spatiotemporal distributions of gas ebullition in a European water-storage reservoir for two years. Additionally, the sampling was accompanied with gas collecting for analyzing gas composition. The gas released from the reservoir was primarily composed of CH4 (on average 52%, up to 94%). The longitudinal distribution of gas ebullition was mainly determined by a proximity to the river inflow as a source of organic matter. A magnitude of ebullitive fluxes within the reservoir varied up to 1,300 mL m−2 d−1 (30 mmol CH4 m−2 d−1). The most significant period of ebullition has turned out to be in fall, on average reaching a sevenfold ebullitive flux (70 mL m−2 d−1, 1.6 mmol CH4 m−2 d−1) higher than in the rest of the season. A substantial contribution to the fall peak was induced by an expansion of gas ebullition into greater depths, covering two thirds of the reservoir in late fall. The study demonstrates that the ebullitive fluxes of the temperate water storage reservoir were correlated to season, depth, and inflow proximity.

Monitoring of biota in heterogeneous ecosystems requires sampling in different habitats and across environmental gradients. The resulting multivariate community data are typically aggregated into one or several indicator values for the entire ecosystem, but the relationship between the robustness of such indicators and sampling effort, including the identification of minimum acceptable sampling designs, is not fully understood. We address this issue for multi-mesh gillnet sampling of freshwater fish communities in deep-valley reservoirs, using data from 29 detailed annual surveys in eight reservoirs in the Czech Republic that account for the inherent longitudinal and depth gradients and the qualitatively different benthic and pelagic habitats. We evaluate the performance of eight sampling scenarios, created by variously reducing the full dataset. To this end, we use 31 fish-based, community-, size- and species-level indicators calculated separately for benthic and pelagic habitats, and fit the relationships between the indicator values based on the reduced and full sampling design using Bayesian generalized linear models. The ability of reduced data to estimate the “true” indicator value across the entire dataset, expressed as the adjusted R 2 value of the best model for the given indicator, increased with sampling effort. However, the relationships differed between indicators: R 2 values were higher for abundance-based than for biomass-based indicators. We identified three suitable reduced sampling designs: (1) sampling the entire longitudinal profile in the epilimnion, yielding on average the highest R 2 values (0.97), (2) same as before but limited to one sampling layer closest to the surface (R 2 = 0.91), and (3) sampling all depth strata at the farthest points of the longitudinal gradient ( i.e. , dam and tributary, R 2 = 0.83). These results demonstrate that, in order to obtain robust estimates of fish community indicators, current gillnet sampling protocols can be optimized to reduce effort and minimize unwanted fish mortality.

Horizontally-aligned, fixed and mobile, transducers are routinely deployed at depths of 0.5–0.75 m to survey the surface layer of waterbodies for fish. However, simulations and measurements demonstrate that a smooth surface can cause serious errors in the target strength (TS) and split-beam angular position estimates. Errors in TS up to 10 dB and depth position up to 0.5 m were observed. Simulations suggested that multi-path signal propagation interfered with the direct path of that signal. Furthermore, when a standard target (calibration copper sphere) was moved away from the transducer at fixed depths, the estimated TS and depth of the target started to oscillate as a function of range. The amplitude increased with increasing range. The frequency decreased from the oscillation start until a certain range where the oscillation stopped. The region of oscillation depended on both the transducer and target depth. Horizontal observations of known fish echoes behaved similarly. Experiments in a lake showed that the influence from the surface disappeared when the surface became rippled due to wind.

Obtaining a representative sample is fundamental to the assessment of fish communities. In this study we used data from 99 different surveys of 29 artificial lakes in the Czech Republic to evaluate the precision of fish community indicator estimates based on reduced gillnet scenarios derived from triplet gillnets installed in each particular locality, depth strata and habitat. Forty-four indicators (based particularly on numbers and biomass per unit of effort – NPUE and BPUE, respectively) were compared between the full sampling design (three gillnets at every sampling location) and six reduced scenarios which were combinations removing one or two gillnets from the triplets. The whole community comparison found the accuracy (R2) between the full sampling design and reduced scenarios usually to be above 0.9. Strong accuracy was also found for species richness and mean fish weight. In individual species comparisons, accuracy above 0.75 was found for common species in a given habitat, whereas accuracy for less common species sometimes dropped below 0.5. Setting one gillnet in an appropriate depth strata and lake locality appears to be sufficient for unbiased overall NPUE, BPUE, and species composition estimates, and at the same time, reduces sampling effort and minimizes unwanted fish mortality. Capturing less common species in a given habitat is more or less accidental and additional sampling effort increased the probability of catching these species. On the other hand, methods targeting rare species of interest may be considered instead of increasing gillnet effort above the CEN standard.

Information about fish distribution and abundance in the upper part of the water column are often fundamental for both research and management. However, this information is extremely hard to obtain using conventional hydroacoustic methods. For this reason, the mobile hydroacoustic upward-looking system (38 kHz split-beam echosounder) in combination with a passive sampling method (gillnets) was tested to investigate the fish community (fish larger than 8 cm total length) in the upper 3 m of water column of the Řimov Reservoir (Czech Republic) during the growing season. We found most fish located in the depth layer closest to the surface down to 1 m – 50–78 % by acoustics (layer 0.3; 1 m) and 55-–1 m) and 55–71 % by gillnets. The size structure of both methods was generally similar, but the acoustic results contained a higher proportion of small fish (

We explored whether fin clips and scales can be used as potential non-lethal alternatives to muscle tissue for examining the isotopic composition of asp Leuciscus aspius, a locally threatened freshwater species. Dorsal fin clips, scales and muscle plugs were collected from two asp populations and subsequently analysed for nitrogen and carbon stable isotopes. Both fins and scales were consistently depleted in 15N and enriched in 13C relative to muscle. A linear regression found that the isotope values in asp fins and scales were significantly related to those in the muscle tissue. These results indicate that fins and scales have the potential to be a substitute for muscle in stable isotope studies of asp, thus providing a non-destructive sampling method for this species. Nevertheless, to determine reliable conversion factors between tissues, a subset of individuals covering a sufficiently wide range of body sizes may need to be sacrificed for any given population.

Ecological quality assessment of non-natural water bodies is, in contrast to natural systems, less developed and requires determining biological indicators that reliably reflect environmental conditions and anthropogenic pressures. This study was motivated to propose fish indicators appropriate for assessment of reservoir ecosystems in central Europe. We analysed changes in water quality, total biomass and the taxonomic, trophic and size composition of fish communities along the longitudinal axes of four elongated, deep-valley reservoirs. Due to high nutrient inputs from their catchments, the reservoirs exhibited pronounced within-system gradients in primary productivity and water transparency. Although fish communities were similar among the reservoirs and dominated by few native species, the community structure and biomass systematically changed along the longitudinal axes of the reservoirs. The biomass and proportion of planktivores/benthivores in the fish community were highest at eutrophic sites near the river inflow and declined substantially towards deep, more oligotrophic sites close to the dam. The biomass and proportion of piscivores significantly increased downstream within the reservoirs alongside improving water quality. At species level, perch Perca fluviatilis and bream Abramis brama responded most sensitively, although in opposite directions, to the longitudinal environmental gradient. The major longitudinal changes in fish community characteristics were found to be consistent between pelagic and benthic habitats. The results of this study suggest that fish communities are appropriate indicators of eutrophication and can be used for ecological quality assessment of non-natural lentic water bodies, such as reservoirs. Moreover, our results underline the necessity to consider within-system gradients in water quality and the fish community when planning sampling programmes for deep-valley reservoirs.

Temporal and spatial heterogeneity in the distribution of cladocerans in lakes could be caused by abiotic (wind, water currents) and biotic factors (reproduction, food resources, predation). Diel horizontal and vertical distribution of cladoceran assemblages was studied in two deep lakes (Milada and Most Lakes, Czech Republic) in early (June) and late (September) summer. The objective was to study diel vertical migration (DVM) and diel horizontal migration (DHM) of cladocerans under conditions of different macrophyte cover in littoral areas (rich in Milada Lake, poor in Most Lake) and fish assemblages (non-specialised planktivorous species in Milada Lake, and abundant planktivorous fish - maraena whitefish, Coregonus maraena - in open water habitats in Most Lake). Temporal variations in cladoceran assemblages were reported in both lakes in the two sampling periods. DVM was observed in the two lakes, performed by the most vulnerable species to fish predation (the larger Daphnia spp.), but with different patterns (direct and reverse) probably linked with the local fish community and other biotic and abiotic factors in each lake. Horizontal movements were only observed in Most Lake: D. longispina increase its abundance in open waters at night compared to the littoral points; while Ceriodaphnia spp. showed the inverse pattern. In both lakes, higher densities were often found at night in surface layers, producing a great "diurnal deficit": cladocerans remain undetected in some zones during the day (especially in the littoral areas) moving to surface layers at night.

Fish stocking is one of the most widespread and frequent management strategies in freshwater systems. However, the contribution of stocked fish to the population is seldom investigated, and hence the effectiveness of this strategy is virtually unknown for many populations. Understanding the contribution of stocked fish into the population is crucial because it allows the disentanglement of the confounding effects generated by allochthonous individuals into the estimation of survival and growth rates of the population. To discriminate between the allochthonous and autochthonous individuals in a population, the shape of sagittae otoliths from pikeperch (Sander lucioperca) where compared. Results indicated significant differences among stocked and non-stocked fish, with the former having smaller and wider sagittae otoliths than the latter. Our results suggest that this technique can be used to discriminate the natal origin of fish in a much faster and cheaper way than commonly used techniques.

Fish otoliths are conservative structures that are widely used on fishery science for multiple purposes. Despite its relevance in the research field, little is known about the ontogeny and inter-population effects on the otolith of freshwater fish. In this study we used otoliths from 1800 European perch (Perca fluviatilis) individuals from 9 different populations to analyze the ontogenetic and inter-population differences on the otolith shape using six morphometric indices. The relationship between fish and otolith length was fitted using three different regression models (linear, power and logistic) to identify the best allometric relationship. Our results show that there are strong ontogenetic and interpopulation differences in P. fluviatilis otolith shapes. We also show that the relationship between the fish and otolith length follows a logistic curve. The ontogenetic differences on otolith shape might be related to extrinsic factors (diet shift and intra and interspecific competition) in each ecosystem, given that the reservoirs are different and no clear pattern on the otolith shapes can be distinguished among populations. Our results imply that the available back-calculation models may not always provide accurate estimates of P. fluviatilis length and that a model that takes into account the real allometric relationship for the species can improve the fish length estimations of back-calculated fish lengths for the European perch.

Fish recruitment in riverine reservoirs is not fully understood because the long-term data series required for standard stock–recruitment models are often lacking. In this study, two unrelated piscivorous species with different ecologies, asp ( Leuciscus aspius ) and pikeperch ( Sander lucioperca ), were investigated over a 14-year period in a reservoir in the Czech Republic using a novel informative statistical approach based on dimension reduction methods. This method is useful for situations in which potential predictors are equal to, or exceed, the length of the time series. Recruitment of asp fry was affected by zooplankton abundance, predator density and temperature. Recruitment of pikeperch fry measured with seine and trawls was only affected by the number of predators, while recruitment of pikeperch fry estimated with gillnet data was also affected by temperature and water level fluctuation. Although gillnets are commonly used sampling method, it seems to be inappropriate for developing fry predicting model. This research also highlights the use of a novel approach to dimension reduction for analysis of factors affecting recruitment using shorter time series (in our case 13 years).

This study addresses fish behaviour at the mouth of a midwater trawl in two temperate reservoirs. Fish distribution and behaviour were monitored using a SIMRAD EK60 (38 kHz) split-beam echosounder with the transducer deployed at the water surface, attached to the surface trawl headrope. We were able to describe day and night patterns of vertical distribution of fish in the reservoirs, describe fish behaviour near the footrope (i.e. vertical swimming velocity, fish tortuosity, vertical direction of swimming), and find the proportion of fish performing avoidance reactions. Significant differences were found between day and night distributions and behaviours. In total, only 35 of the 1514 acoustically recorded fish performed apparent avoidance behaviour. Twenty-seven avoidance reactions were observed in the day (6.3-% of total fish) while eight were observed at night (0.7-% of total fish). Fish were more active in the daytime, while tending to stay still in the water column with no observable reactions at night. We found significant dependence between the size of fish directly avoiding the trawl and their swimming velocities. The greatest echogram slope values belonged to those fish with a strong avoidance reaction. When the footrope was perceived, the fish reacted from approximately 1 m vertical distances and escaped up to 5 m below the footrope (usually 1–2.5 m). Light, water transparency and temperature stratification were the dominant factors influencing vertical distribution and fish avoidance behaviour.

Data from nine reservoirs in the Czech Republic were used to investigate density and size distribution patterns for dominant fish species of the juvenile pelagic community at night. Clear trends of increasing density along the longitudinal gradient for bream, roach, bleak, and pikeperch were observed in long (>5 km) reservoirs but were absent in short reservoirs. Increasing body size towards the tributary was observed for bream, perch, pikeperch, and ruffe. Cyprinids showed a clear affinity to the surface layer (0–3 m) of long reservoirs, while pikeperch and ruffe juveniles occurred homogeneously at depths down to six meters. Perch juveniles had the highest densities especially in 3–6 m water layer. Size did not change with sampling depth for any species in the reservoirs studied. Our data showed gentle spatial segregation of different juvenile fish species. Although the tributary area is usually the most productive, it is not able to support all juveniles leading to inter- and intra-specific competition and the relegation of juveniles of some species to less productive habitat. This trend was more apparent in long reservoirs with strong trophic gradients. The gradient of nutrients seems to be an important factor driving fish distribution along longitudinal profile of reservoirs.

Understanding spatial distribution of organisms in heterogeneous environment remains one of the chief issues in ecology. Spatial organization of freshwater fish was investigated predominantly on large-scale, neglecting important local conditions and ecological processes. However, small-scale processes are of an essential importance for individual habitat preferences and hence structuring trophic cascades and species coexistence. In this work, we analysed the real-time spatial distribution of pelagic freshwater fish in the Římov Reservoir (Czechia) observed by hydroacoustics in relation to important environmental predictors during 48 hours at 3-h interval. Effect of diurnal cycle was revealed of highest significance in all spatial models with inverse trends between fish distribution and predictors in day and night in general. Our findings highlighted daytime pelagic fish distribution as highly aggregated, with general fish preferences for central, deep and highly illuminated areas, whereas nighttime distribution was more disperse and fish preferred nearshore steep sloped areas with higher depth. This turnover suggests prominent movements of significant part of fish assemblage between pelagic and nearshore areas on a diel basis. In conclusion, hydroacoustics, GIS and spatial modelling proved as valuable tool for predicting local fish distribution and elucidate its drivers, which has far reaching implications for understanding freshwater ecosystem functioning.

Citation: Žák, J., Jůza, T., Blabolil, P., Baran, R., Bartoň, D., Draštík, V., Frouzová, J., Holubová, M., Ketelaars, H. A. M., Kočvara, L., Kubečka, J., Mrkvička, T., Muška, M., Říha, M., Sajdlová, Z., Šmejkal, M., Tušer, M., Vašek, M., Vejřík, L., Vejříková, I., Wagenvoort, A. J., J. 2018 Invasive round goby Neogobius melanostomus has sex dependent locomotor activity and is under-represented in catches from passive fishing gear compared with seine catches. J. Fish Biol. 93:147–152. DOI: 10.1111/jfb.13646 Invasive

We investigated diel habitat use of fish covering the littoral and pelagic zones of the Řimov Reservoir (Czech Republic) and analyzed the influence of predator presence and of shifting feeding habitats in all dominant species and age groups. Our sampling revealed distinctive diel changes of fish distribution in the reservoir, which were age- and species-dependent. The overall abundance of subadult fish in littoral habitats was significantly higher at night than during the daytime. Subadults were almost absent in pelagic habitat during the day and their presence increased during the night, although densities were smaller than in the littoral. Adults preferred the pelagic zone during the day and partly migrated to the littoral at night. Potential fish predators were most likely responsible for small fish avoidance of the littoral and pelagic zones during day. Higher availability of food in the littoral was the most important driver of the high occurrence of subadults at night. Day preference of pelagic zone by adults is most likely caused by higher profitability of this habitat in comparison with littoral. The reasons for night inshore migration of adults are not obvious, but the homogenization of their distribution or resting in the littoral could explain such behavior.

The sampling skills of three common European fish species (Barbus barbus, Chondrostoma nasus and Squalius cephalus) were tested to assess their potential as biomonitors of past changes in river water quality through the analysis of diatoms in fish guts. The study was performed on three rivers with different chemical and physical conditions. Comparison of similarity indices revealed low diatom assemblage resemblance between epilithic samples and samples collected from fish guts at the same locality. In contrast, a mixed-effect linear model identified significant differences between locations through comparison of diatom-based water quality indices. Among fish species, diatom indices calculated from gut samples of B. barbus and C. nasus were in most cases not significantly different from those from epilithic samples while those from S. cephalus were often significantly different. The results of the study demonstrate that diatom analysis of fish guts provides a clear distinction between eutrophic and hypereutrophic rivers. Finer nutrient variations within hypereutrophic conditions were also found to be significant for some diatom indices. In this context, choice of appropriate diatom water quality index proved to be crucial in maximizing the sensitivity of the method.

The DIDSON multi-beam sonar has become a popular tool for monitoring fish. The aim of this study was to determine how well the DIDSON sonar can detect and measure fish during stationary applications in lakes. Cyprinid fish of known sizes (10–60 cm long) and with horizontal body orientations were deployed in known positions at two ranges (6.3 and 9.5 m) within a DIDSON high-frequency array of beams. A new method for estimating fish length from multi-beam sonars was thus developed and applied. At both ranges, all the deployed fish were invariably detected when they were aligned with their sides perpendicular to the beam. Increasing fish body angle, however, reduced the ability of the system to detect fish. Only the largest fish were detectable when the fish were parallel to the beam (head or tail aspect). The estimated fish length agreed well with the actual length when the fish were positioned perpendicular to the center of the multi-beam array. The lengths were underestimated for fish that were at the edges of the array, further away from the transducer or that had an increased body aspect, especially small-sized fish. Additionally, we observed that the wide girth of large fish can shadow the rest of the body, resulting in a decreased estimated length. We showed that determining the actual length of fish is challenging and is not a trivial task, and we raise the question of where to define fish length along the echo intensity of fish. We conclude that including the error functions for length estimates allows the DIDSON to obtain more reliable and accurate biological information.

The effect of diel period and littoral habitats on the distribution of age-0 fish was tested in a deep-valley reservoir using boat-modified point abundance sampling by electrofishing (PASE). Day and night samplings unveiled differences in abundance of age-0 fish while recognizing most of the commonly present age-0 fish species in the littoral zone. Night survey provided better information about the abundance of age-0 fish since most species appeared in higher numbers at night. Alternatively, night sampling underestimated bleak (Alburnus alburnus) and gudgeon (Gobio gobio), which were predominantly found in the littoral zone in the daytime. The structure of the age-0 fish assemblage was determined primarily by the characteristics of the littoral habitats, i.e. slope steepness and structure, and three different patterns of habitat use were detected among the fish species. To obtain the most comprehensive assessment of a littoral age-0 fish assemblage and to register all species-specific patterns of habitat use, it is necessary that all littoral habitats in the reservoir are sampled during both day and night periods. The boat-modified PASE could be also used as a standard monitoring tool for routine age-0 fish sampling under difficult conditions of steep-sloped shorelines in large inland waterbodies.

Large year-to-year variability in different fish species recruitment has been confirmed by previous studies while diurnal patterns of occupation in two basic reservoir habitats (pelagic and littoral) by different age-0 fish species in late summer are still unclear. Data collected over an 11-year period regarding late-summer age-0 fish assemblages in pelagic and littoral habitats of a reservoir were used to test the recruitment instability and to investigate diurnal habitat use. Trawling was conducted in the pelagic habitat at night while beach seining was conducted in the littoral habitat during day and night. Fluctuations in age-0 fish abundance and species composition were observed with both sampling methods; however, the following spatio-temporal patterns were relatively stable in most investigated years: (1) pelagic species (pikeperch; Sander lucioperca, small perch; Perca fluviatilis, bream; Abramis brama at night), (2) littoral species (large perch, asp; Leuciscus aspius, dace; Leuciscus leuciscus), (3) migratory species likely performing diel horizontal migrations (bleak; Alburnus alburnus), (4) species abundant in the littoral habitat both during day and night and also in pelagic habitat at night (roach; Rutilus rutilus) and (5) species detected in both habitats exclusively at night (ruffe; Gymnocephalus cernuus).

Fish avoidance behaviour in the trawl mouth at night was investigated in the extremely shallow and turbid Lake Neusiedl in Austria. To evaluate fish reactions, a fixed frame benthic trawl with three electricity modes (without electricity, with continuous electricity and with interrupted electricity) was used and the captured fish abundances, biomasses and size structures were compared between modes. Simultaneously, the dual-frequency identification sonar (DIDSON) monitored fish abundance and size in front of and beside the trawl mouth during all tows. White bream ( Blicca bjoerkna ), bleak ( Alburnus alburnus ) and razor fish ( Pelecus cultratus ) were dominant fish species in the trawl catches. We did not find differences in fish abundance or biomass when the tows with different electricity modes were compared. The length frequency distribution of fish was not significantly different between modes, but trawls with the two electrified modes contained a higher proportion of fish larger than 130 mm than trawls with the non-electrified mode. Additionally, the DIDSON recordings did not display any significant differences in abundance, length frequency distribution and from length–weight relationships calculated biomass between the electrical modes. Not even the two avoidance behaviour categories used as indicators of trawling error displayed significant differences between avoidance and biomass. Our results indicate that based on trawling abundance and biomass comparisons, and supported by observations by DIDSON, all electrical modes were similarly effective for all size groups of fish. The study found minimal avoidance reactions by the dominant fish species in the trawl mouth opening when a relatively small fixed frame trawl was used to sample fish in a shallow and turbid lake at night.

For ethical reasons and animal welfare, it is becoming increasingly more important to carry out ecological surveys with a non-invasive approach. Information about fish distribution and abundance in the upper water column is often fundamental. However, this information is extremely hard to obtain using classical hydroacoustic methods. We developed a rigid frame system for pushing upward looking transducers of the scientific echo sounder (38 and 120 kHz) in front of the research vessel. The efficiency of the new approach for monitoring juvenile fish at night was investigated by comparing the results with a quantitative fry trawl in the Římov Reservoir in the Czech Republic. The experimental setup enabled comparisons for the 0–3 m and 3–6 m depth layers, which are utilized by almost all juvenile fish in summer. No statistically significant differences in the estimated abundance of juveniles were found between the two sampling methods. The comparison of abundance estimates gathered by the two frequencies were also not significantly different. The predicted mean lengths from acoustic sampling and the trawl catches differed by less than 10 mm in all comparisons. Results suggest that mobile hydroacoustic upward-looking systems can fill the methodological gap in non-invasive surveying of surface fishes.

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.

This paper is concerned with parameter estimation for the Neyman-Scott point process with inhomogeneous cluster centers. Inhomogeneity depends on spatial covariates. The regression parameters are estimated at the first step using a Poisson likelihood score function. Three estimation procedures (minimum contrast method based on a modified K function, composite likelihood and Bayesian methods) are introduced for estimation of clustering parameters at the second step. The performance of the estimation methods are studied and compared via a simulation study. This work has been motivated and illustrated by ecological studies of fish spatial distribution in an inland reservoir.

The aim of this study was to test the selectivity of a trawl main body, especially for small fish, during night sampling. The tested trawl had a mesh size of 80/40/20 mm in the main body, a mouth opening of approximately 100 m2, and a length of 48 m and was originally designed for sampling vendace, Coregonus albula L. Trawl selectivity was determined by comparing the catch using this gear with that from a purse seine net (mesh size 6–10 mm, length 120 m, height 12 m) at the same sites and time. The size distributions of the total catch and densities of three dominant species (common bream, Abramis brama (L.), roach, Rutilus rutilus (L.) and bleak, Alburnus alburnus (L.)) were compared between the two types of gear. Densities of these species were divided into size groups before comparison: (i) small fish (60–120 mm, standard length SL); (ii) middle-sized fish (125–180 mm SL); and (iii) large fish (>180 mm SL), which included only roach and bream. Significant differences in the fish density between the trawl and purse seine nets were found. Generally, the densities in all size categories of bleak and small roach were underestimated in trawl catches, whereas the densities of middle- and large-sized bream were underestimated in the purse seine catch. We conclude that both types of gear are size selective – small fish escaped through the mesh of the trawl main body, whereas larger bream were able to escape from the purse seine before closing. The trawl with the given mesh sizes is recommended for quantitative sampling only of fish larger than 180 mm (SL) under low light-intensity conditions.

Littoral age 0+ fish were studied with respect to spatio-temporal heterogeneity in the deep-valley Řimov Reservoir (Czech Republic) from June to October 2007 using point abundance sampling by electrofishing. The abundance and diversity of age 0+ fish in different types of littoral habitats were examined along the longitudinal gradient of the reservoir during daytime. The impact of some physical attributes of the studied littoral habitats, e.g. slope steepness and substrate size, along with the season was the main factors affecting the distribution of age 0+ fish. The level of structural complexity was not a strong determinant, because the overall diversity and structural complexity of the available littoral habitats were relatively too low to have greater impact on the age 0+ fish distribution. The physical factors markedly influenced the spatial segregation between the two most important taxa in the reservoir—percids and cyprinids. Perch was the only representative of age 0+ percids in the littoral zone, which occupied steep-sloped habitats early in the season. In contrast, gently sloped habitats were mainly occupied by cyprinids later in the season. Species diversity was reflected in the occurrence of age 0+ cyprinids, achieving a maximum in gently sloped habitats in October. The effect of the longitudinal gradients on age 0+ fish distribution and diversity was generally far less significant, but was rather decisive during the period of a pronounced longitudinal trophic gradient during the summer season.

A hydroacoustic survey and supplementary gillnet investigation were carried out in the open water of the central part of Lake Turkana in September 2009. Overall acoustic fish density and biomass were assessed as 1381 ind./ha and 30 kg/ha, respectively. The fish density estimate was lower than the results from two previous investigations in the 1970s and 1980s (long-term average 3739 ind./ha), but the biomass remained relatively unchanged (long-term average 25.4 kg/ha). A decreasing gradient in pelagic fish density from the western to eastern shore of the lake was observed. Fish were distributed unevenly within the water column. During the day, a majority of fish aggregated in the mid-water layers (10–12 m below the water surface), creating on echograms the so-called Midwater Scattering Layer. This feature dissipated completely during dusk and the majority of fish occurred in the surface layers at night. These diel vertical fish migrations influenced day and night hydroacoustic estimates of the total fish abundance. Synodontis spp., Lates spp. and Schilbe uranoscopus dominated the catch of both mid-water and bottom gillnets installed in open water areas. Hydrocynus forskalii and Brycinus spp. contributed significantly to the catch of mid-water gillnets while Bagrus bayad and the endemic Haplochromis macconneli occurred only in the catch of the bottom gillnets.

Inland water bodies contain extremely steep-sloped bottoms compared with those typically occurring during marine vertical surveys. These steep bottom slopes can cause high acoustic dead zones, biasing our estimates of living organisms. The studies so far have used the assumption that the first contact between the acoustic wavefront and bottom will be at the point where the radial ray from the transducer is normal to the bottom (90°), which we refer to as the normal ray assumption (NRA). Nevertheless, as acoustic energy dramatically decreases laterally due to the beam's pattern, this assumption may not be fulfilled further from the acoustic axis. It is reasonable to believe that the methods assuming the NRA can fail at quite steep slopes. We installed a calibration benthic gillnet of known height at sites with different bottom slopes, ranging from 12° to 50°. The gillnet's float-line served as a good visible marker above the monitored lake bottom and was successfully used for measuring the acoustic dead-zone height empirically. By comparing the observed and modeled dead zones based on the NRA, we can show that the current methods for their estimation are invalid at quite sloping angles. We conclude that the current dead-zone estimation methods are not always applicable for surveying inland water bodies with extremely steep bottom slopes. Installing a simple calibration net as an off-bottom marker can provide help for in-situ dead-zone measurements.

Fish activity was monitored analyzing the catches of gillnets exposed for short time intervals at night. Experiments were done in two Central European reservoirs with different turbidity and fish density. Cyprinids dominated in both reservoirs. The aims of the study were to describe and model a pattern of fish activity in general and on species and age group levels, and in turbid and clear water. Fish activity showed two distinct peaks around sunset and sunrise and was low during night. This pattern was the same in clear and turbid reservoirs, as well as for all species and age groups tested. This study established some justifications and standards for gillnet sampling: (i) it is possible to compare overnight gillnet catches between different lowland European species and waterbodies, as the pattern of fish activity is general; (ii) gillnets should be set 2–3 h before sunset and lifted 2–3 h after sunrise to cover activity peaks, thus getting reliable and comparable results; (iii) depicting gillnet catches per average hour of exposition produces biased and incomparable results.

The reproduction biology of perch Perca fluviatilis was studied in relation to lake hydrology, spawning substrate and female size using SCUBA divers during late April and mid-May 2009 in Chabařovice Lake, Czech Republic. An extreme displacement of water mass, induced by a long-lasting strong wind, caused the abundance of egg strands to differ significantly between individual parts of the lake. On average, 91% of perch spawning activity occurred at depths greater than 3 m. The mean concentration of dissolved organic carbon (DOC) indicated that the lake belongs to the category where DOC influencing the penetration of ultraviolet radiation, is not responsible for the deep deposition of egg strands by perch. Most probably, the avoidance of shallow depths (

The roles of wind protected bays, presence of littoral vegetation and light attenuation in the water column on spawning site selection and depth of egg strands deposition by perch Perca fluviatilis was studied in Římov Reservoir, Czech Republic, in the years 2007 and 2011 using boat observation and SCUBA divers. The data were compared with results from Chabařovice Lake, Czech Republic, where similar monitoring took place in 2007–2010 and 2012. In shallow water of Římov Reservoir, the density of egg strands was significantly higher in grass bays compared to both rocky bays and the main reservoir body. Most egg strands were deposited in water less than 0.5 m deep on reed canarygrass Phalaris arudinacea. In year when the littoral vegetation was absent perch were forced to spawn significantly deeper on various types of woody structures. In Římov Reservoir, which is less vulnerable to wind, 91.1% of egg strands were spawned in water ≤3 m deep. In contrast, in the wind exposed Chabařovice Lake, even in the presence of littoral vegetation, 90.5% of egg strands were found at depths greater than 3 m. In Chabařovice Lake, the light penetrated to three times greater depth compared to Římov Reservoir and, similarly, the depth limit to which 95% of egg strands were spawned was three times greater in this lake compared to Římov Reservoir. This study is the first contribution showing the role of water transparency in controlling the depth distribution of perch egg strands in lakes and reservoirs.