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We conducted a systematic evaluation of the correspondence in fish length data obtained from vertical hydroacoustics and gillnetting across seven European natural lakes differing in abiotic and biotic characteristics. Length data were analyzed as continuous size spectra characterized by their maximum-likelihood estimated exponents b. First, we examined the relationship between size spectra obtained from the two sampling methods. We then examined whether size spectra from the two methods were correlated with lake descriptors separately or in combination. The modeled relationship between the exponents b from the two methods showed that the exponent b from the hydroacoustics was, on average, the same as that from the gillnet sampling in the seven lakes. The exponents b from the hydroacoustics and gillnets, when averaged, were significantly correlated with lake depth, while their differences were significantly correlated with mean air temperature. To conclude, the overall good correspondence between the continuous size distributions obtained by both methods supports the application of vertical hydroacoustics in acquiring size structure of fish communities in lakes, but not in fully replacing the invasive gillnetting. Yet, some specific methodological details require further research.

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 ecology

Fish body orientation significantly influences the size obtained with hydroacoustic signals, and thus the estimate of fish size and biomass. For this reason, each characteristic of a target’s echo can be advantageous for developing algorithms to refine acoustic fish estimates. We measured pulse-compressed broadband acoustic signals from tethered fish (common bream Abramis brama) in different dorsal positions. Based on generalized additive mixed models (GAMM), we initially tested the influence of the fish dorsal aspect on the amplitude echo envelope and amplitude echo descriptors (amplitude maximum and amplitude echo length at seven different levels below the maximum) by altering the fish dorsal orientation. Our study confirmed that the dorsal aspect influenced the shapes of the amplitude echo envelopes in both fast- and slow-tapered pulses. Furthermore, we found that echo lengths approximately 15 dB below the amplitude maximum, especially for fast-tapered signals, could provide good characteristics of the echo-envelope shape for determining the fish dorsal aspect and facilitating thus the conversion between acoustic target strength and true fish length.

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.

To understand the conditions of coexistence in multiple-species predator community, we studied longitudinal and vertical movement of pike (Esox lucius), pikeperch (Sander lucioperca) and catfish (Silurus glanis) in the Římov Reservoir, using an autonomous telemetry system for 11 months. We found significant differences among these three species in movement and depth that varied considerably in time, with the greatest differences between warm (late spring and early autumn) and cold season (late autumn to early spring). Preference for different sections of the reservoir was stable for pike, while pikeperch and catfish frequently visited tributary during the warm season, and moved closer to the dam during the cold season. Pike longitudinal activity was highest in the cold season, pikeperch in the warm season, and catfish activity peaked in both the warm and cold seasons. Overlap in the depth used among species was higher in the warm season, when all species used the upper layer of the water column, and lower in cold season, when pikeperch and catfish used deeper areas. These results demonstrated ability of predators to actively inspect a large portion of the reservoir in both longitudinal and vertical dimensions, although differing in the timing of their habitat use and activity.

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

Fish stock monitoring is an important element for the sustainable management of inland water resources. A scarcity of data and the lack of systematic monitoring for Lake Trichonis precludes an up-to-date assessment. To assess the current status of pelagic fish stock, a hydroacousting survey was conducted for the first time in Lake Trichonis, Greece. In October 2019, the lake was acoustically surveyed with two, horizontally and vertically mounted, 120 kHz transducers during day and night. A decreasing gradient in pelagic fish density from the western to the eastern shores of the lake was observed. Fish density was significantly higher in the intermediate layers of the water column, in the eastern region, compared to the western region. The lake appears to host primarily communities of small-sized fish (TL: 0&ndash, 5 cm), whereas larger fish (TL: 5&ndash, 50 cm) are a small minority of the total fish stock. The overall average estimated fish length was approximately 2.4 cm. The adoption of routine inland fish stock monitoring through hydroacoustic methods could be a promising step in the effort to improve the understanding of unique inland water ecosystems with minimum impact on endemic species, as well as to mitigate human impact and achieve long-term sustainable management.

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.

Trawling juveniles is an effective method to estimate 0+ fish assemblages in inland bodies of water. Trawlers are usually moved between bodies of water by ground transport, which restricts the size, weight and type of the equipment able to be transported, and as such do not always come equipped with machinery for trawl operation. Having to lift the trawl onto the deck of the trawler after each tow also makes small-scale juvenile trawling time-consuming and personnel intensive and deep water trawling is especially hard work. This study introduces a new two-boat setup for continuous trawling that enables faster and easier operation of the trawl, even in deep water layers, and more accurate distance and depth measurements.

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.

In this study, a remote sensing-based method of mapping and predicting fish spatial distribution in inland waters is developed. A combination of Earth Observation data, in-situ measurements, and hydroacoustics is used to relate fish biomass distribution and water-quality parameters along the longitudinal transect of the Ř&iacute, mov Reservoir (Czech Republic) using statistical and machine learning techniques. Parameter variations and biomass distribution are estimated and validated, and apparent trends are explored and discussed, together with potential limitations and weaknesses. Water-quality parameters exhibit longitudinal gradients along the reservoir, while calculations reveal a distinct fish assemblage pattern observed as a patchy overall biomass distribution. Although the proposed methodology has a great potential for sustainable water management, careful planning is needed to ensure the simultaneous acquisition of remote sensing and in-situ data to maximize calibration accuracy.

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 (

DIDSON acoustic cameras provide a way to collect temporally dense, high-resolution imaging data, similar to videos. Detection of fish targets on those videos takes place in a manual or semi-automated manner, typically assisted by specialised software. Exploiting the visual nature of the recordings, tools and techniques from the field of computer vision can be applied in order to facilitate the relatively involved workflows. Furthermore, machine learning techniques can be used to minimise user intervention and optimise for specific detection and tracking scenarios. This study explored the feasibility of combining optical flow with a genetic algorithm, with the aim of automating motion detection and optimising target-to-background segmentation (masking) under custom criteria, expressed in terms of the result. A 1000-frame video sequence sample with sparse, smoothly moving targets, reconstructed from a 125 s DIDSON recording, was analysed under two distinct scenarios, and an elementary detection method was used to assess and compare the resulting foreground (target) masks. The results indicate a high sensitivity to motion, as well as to the visual characteristics of targets, with the resulting foreground masks generally capturing fish targets on the majority of frames, potentially with small gaps of undetected targets, lasting for no more than a few frames. Despite the high computational overhead, implementation refinements could increase computational feasibility, while an extension of the algorithms, in order to include the steps of target detection and tracking, could further improve automation and potentially provide an efficient tool for the automated preliminary assessment of voluminous DIDSON data recordings.

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.

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.

The effect of target strength oscillations, generated by surface or bottom-induced sound multi-pathing, can cause serious errors in estimates of fish biomass in horizontal acoustic surveys of extremely shallow inland waters (depth 1.7–2 m). In this study, the oscillations were shown to occur in both windy and calm weather conditions, and depended on the target’s depth and range from a transducer. In calm weather the oscillations were more pronounced and targets that were located close to the water surface and further from the transducer were most affected. Errors in target strength reached up to an 8 dB difference from the theoretical value. Fish abundance and size structure estimates are likely to be affected even more than biomass estimates. To avoid boundary-induced errors in target strength, simulations of fish biomass surveys demonstrated that the data-processing range should be limited to 10 m from the transducer for surveys conducted in extremely shallow waters.

We conducted multiple small (2011–2012) and one large sampling campaign (2013) at selected profiles along the Elbe River. With the data we were able to outline spatial and temporal variability of methane concentration, oxidation and emissions in one of the major rivers of Central Europe. The highest methane concentrations were found in human-altered riverine habitats, i.e., in a harbor (1,888 nmol L−1), in a lock and weirs (1409 ± 1545 nmol L−1), and in general in the whole “impounded” river segment (383 ± 215 nmol L−1). On the other hand, the lowest methane concentrations were found in the “lowland” river segment (86 ± 56 nmol L−1). The methane oxidation rate was more efficient in the “natural” segment (71 ± 113 nmol L−1day−1, which means a turnover time of 49 ± 83 day−1) than in the “lowland” segment (4 ± 3 nmol L−1day−1, which means a turnover time of 39 ± 45 day−1). Methane emissions from the surface water into the atmosphere ranged from 0.4 to 11.9 mg m−2 day−1 (mean 2.1 ± 0.6 mg m−2 day−1) with the highest CH4 emissions at the Meissen harbor (94 kg CH4 year−1). Such human-altered riverine habitats (i.e., harbors and similar) have not been taken into consideration in the CH4 budget before, despite them being part of the river ecosystems, they may be significant CH4 hot-spots. The total CH4 diffusive flux from the whole Elbe was estimated to be approximately 97 t CH4 year−1.

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.

Round goby is generally accepted to be a typical benthic dwelling fish species both in its native and invasive ranges. Nevertheless, mid-water night trawling in the Biesbosch reservoirs (The Netherlands) identified pelagic occurrence of invasive round goby juveniles, which is the first observation of pelagic utilization in Europe. Subsequent diet analysis revealed the presence of benthic and also pelagic zooplanktonic prey in the digestive tracts of investigated fish, which further confirmed active feeding in the pelagial. Round goby juveniles almost completely avoided the surface water layer and their maximum density always occurred below 6 m in all reservoirs. The observed size spectrum (8–58 mm standard length) included the largest pelagic round goby to ever be mentioned in the scientific literature. Our data also showed that round goby prevailed over other species soon after its invasion into the reservoirs, with native species disappearing from the fry community. This study documents the first evidence of the pelagic occurrence and feeding of round goby juveniles, and confirms the importance of pelagic habitats for early stages of this invasive species in European lentic systems.

Methane represents an important greenhouse gas, and its ebullition is a significant way of releasing gas from bottom sediments of shallow fresh waters to the atmosphere. Estimation of ebullition is complicated because of high spatiotemporal variability; however, a hydroacoustical survey represents an effective method for quantifying it. Commonly used vertical beaming in deep waters can be quite limited in very shallow waters. This study was thus aimed to investigate the possibility of using a horizontally oriented sonar beam for gas bubble quantification. Artificially prepared methane bubbles of various sizes, ranging from 2.5 to 905 × 10−3 mL (1.7–12 mm of equivalent spherical diameter), were released from a depth of 6 m in a freshwater reservoir. The acoustic target strength (TS) of these bubbles was observed using both the vertical and horizontal beams of a 120 kHz frequency split-beam sonar. TS obtained in both the vertical and horizontal modes increase with growing bubble size. However, for identical bubble size, the vertical observation gives stronger TS than the horizontal one. Further, TS distribution around mean value is wider with an increase in bubble size, and this distribution is greater in case of the horizontal mode of observation than vertical. It was observed that during bubble rise TS changes for both the vertical and horizontal mode of observation lie within the range of standard deviation of TS measurement; hence, depth is not relevant for TS regression models used in depth up to 6 m.

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 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.

Methane (CH4) is an important atmospheric trace gas mostly released from wet anoxic soils and sediments. While many studies have focused on relatively homogenous environments like rice fields and lake sediments, the changing contribution of heterogeneous sediments e.g. along the longitudinal profile of a rivers has not been covered very frequently. Here we investigated sediment samples from 11 locations of the Elbe River. Sediments were incubated to measure methanogenic/methanotrophic potentials and contribution of individual methanogenic pathways using isotope analysis of δ13C. Additionally, we determined the diversity of the methanogenic communities (analysis of T-RFLP targeting the mcr-A gene in the sediment samples), while abundances of archaea, methanogens and methanotrophs were determined by qPCR. The CH4 production was detected in six samples (out of 11 examined) and ranged from 0.12 to 644.72 nmol gDW−1 d−1. Methanotrophy was found in all examined sediment samples and ranged from 654 to 10,875 nmol gDW−1 d−1. Abundance of methanogens and methanotrophs (Mcr-A and pmo-A gene copy numbers) was not significantly different and quite stable around 106 to 107 copies gDW−1. The group specific qPCR showed high fluctuations, while the highest counts were reported for Methanomicrobiales and Methanosarcinales (105 to 108 copies per gram dry sediment), followed by Methanobacteriales (103 to 105 copies per gram dry sediment). A significant proportion of unidentified methanogens was found in almost every locality. Isotope analysis of δ13C showed that (CH4) is produced mainly by hydrogenotrophic methanogens. We see no trend in the studied parameters along the Elbe River. The molecular data showed no spatial characteristics, while we found hotspots of the measured CH4 processes (CH4 production and oxidation) due to other local driving factors (e.g. carbon content). Thus, out results indicate that the observed variability of the CH4 production and oxidation rates is only indirectly linked to the presence or quantities of different microbial guilds.

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).

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.

Summary In lentic freshwater systems, vertical acoustics may underestimate fish abundance in the acoustic dead zone where fish detection capability is limited. To estimate this bias, the height of fish above the lead-line of a benthic multi-mesh gillnet (1.5 m high) was used to quantify both the vertical distribution of fish near the bottom and the proportion residing within the acoustic dead zone. The study was carried out at the percid-dominated Biesbosch Reservoirs in the Netherlands. Acoustic dead zones were estimated at 7 cm above flat bottoms, and 12–34 cm above 8° sloped bottoms at depths of 5–27 m, respectively. Depending on the habitat, 36 to 75% of the gillnet catch by number was present in the acoustic dead zone, representing 5–51% of the biomass. Near-bottom depths were highly preferred by ruffe Gymnocephalus cernua, often used by perch Perca fluviatilis and pikeperch Sander lucioperca, plus seemingly devoid of smelt Osmerus eperlanus. The total amount of fish hidden in the acoustic dead zone was estimated to be 13–39% of the whole water column. The proportion of biomass obscured in the dead zone was lower (1–12%). The conclusion is that undetected fish in the acoustic dead zone can seriously bias density assessment, which can be corrected by concurrent sampling with benthic gillnets.

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.

Observing and quantifying fish behaviour towards active sampling gears like trawls is challenging. A high-frequency imaging sonar (DIDSON) was applied to directly observe and record fish avoidance behaviour in front of the mouth of an active pelagic surface trawl in the meso- to eutrophic Czech reservoir Želivka in August 2009. In this multi-species environment the fish displayed a variety of behavioural patterns. We classified 11 different categories of avoidance ( n = 1051) based on fish reaction and swimming behaviour towards the active trawl using Sonar5-Pro software. Classification of these categories was based on eight continuous variables: (1) total fish length, (2) distance to the trawl, (3) tortuosity, (4) mean speed of total track, (5) mean speed of first avoidance track, (6) mean speed of total avoidance track, (7) angle between initial and first avoidance track and (8) angle between initial and total avoidance track. The acoustic analysis showed that 76% of the fish in trawling area was captured and 24% swam out of it. A higher proportion of the fish showed an avoidance reaction during the day (44%) than during the night (6%), indicating that ambient light conditions trigger this behaviour during the day. A two-step cluster analysis of the tracks revealed three distinct clusters that corresponded to the most abundant fish species in the reservoir: ‘bleak-cluster’, ‘bream-cluster’ and ‘silver carp-cluster’.

Diel changes in littoral fish assemblage were studied in four reservoirs in the Czech Republic (CentralEurope). The sampling was performed by beach seining in an unstructured littoral zone. Perch, Perca fluviatilis L.,roach, Rutilus rutilus (L.), bream, Abramis brama (L.), carp, Cyprinus carpio L., ruffe, Gymnocephalus cernua (L.),pikeperch, Sander lucioperca (L.), eel, Anguilla anguilla (L.) and A. brama · R. rutilus hybrids exhibited higherdensities at night. Only bleak, Alburnus alburnus (L.) exhibited higher densities during the day. The number ofspecies was higher in night hauls, and mean body size of roach and bleak was also higher at night. Diel changes infish densities resulted in the change in species composition between day and night. The study has implication forthe sampling design to assess littoral fish assemblages using beach seine netting and recommends night samplingfor a representative assessment or sampling during both diel periods for a robust assessment. KEYWORDS

– Spatial distribution of the fish community in the deep canyon-shaped Řimov Reservoir, Czech Republic, was studied using overnight gillnetting fishing in 1999–2007. Effects of depth, distance from the dam to the tributary and habitat type (benthic and pelagic) on fish community structure, catch per unit of effort (CPUE), biomass per unit of effort (BPUE) and average weight were tested. Fish were recorded in all sampled depths and parts of the reservoir. Redundancy analysis revealed that effects of three environmental variables were significant and most variability was explained by depth, then by distance from the dam and habitat type. CPUE and BPUE of all species decreased with depth and responses of individual species to depth were similar for juvenile and adult fish. Number of species, CPUE and BPUE of all species except perch increased heading towards the tributary and peaked close to or at the tributary part of the reservoir. Responses of juvenile fish to distance from the dam differed from that of adult fish. Structure of fish community differed in benthic and pelagic habitats with species preferably occupying epipelagic (bleak, asp, rudd and juvenile bleak, roach and bream) or littoral waters (perch, pikeperch, ruffe, roach, bream and juvenile percids). This study showed that fish distribution in the reservoir followed distinct patterns, which were probably shaped by a combination of physiological constraints plus trade-off between food resources and competition.

The orientation of fish in a horizontal plane has important consequences for estimating their true size from horizontal acoustic records. The aim of this work was to verify the assumption that during the daytime fish are randomly orientated in the lacustrine zone of the canyon-shaped Řimov reservoir and to compare distributions of fish orientation between the lacustrine and tributary (riverine) zones. Fish orientation was acoustically surveyed at fixed locations using the SIMRAD EK 60 split-beam echo sounder (elliptical beam, 120 kHz) with a horizontally aligned transducer. The horizontal aspect (angle between the fish body and the transducer axis) was used to describe their orientation. The conventional single-echo detector (SED) and the cross-filter detector (CFD) were applied. No trend was found along the reservoir when comparing records from four sites processed with the conventional SED. At all sites, most fish appeared to move predominantly in directions perpendicular to the central axis of the acoustic beam, i.e. the side-aspects (90°) of fish prevailed over other aspects. The CFD registered tracked fish several times more often than the SED. In the lacustrine zone the frequency distribution of fish aspect appears very similar when recorded by sonar beams oriented parallel to and across the longitudinal axis of the reservoir (criss-cross-beaming experiment). In the tributary zone, beaming perpendicular to the longitudinal axis of the reservoir revealed a significantly higher proportion of fish moving along the longitudinal axis. Therefore, the assumption of random fish orientation is not applicable in the tributary zones of such reservoirs.

Des campagnes acoustiques nocturnes et durant le jour ont ete effectuees pour comparer les estimations de stock de poissons de 8 lacs artificiels et un puits de mine inondee d'Europe centrale. Durant l'ete, de jour comme de nuit, des biomasses plus elevees de poissons ont ete observees dans la partie haute (jusqu'a 4 m de profondeur) de la colonne d'eau, par sondage acoustique horizontal que dans la partie plus profonde par sondage vertical. Des comparaisons du coefficient de retro-(reverberation) en fonction du volume d'eau (s v ), de la biomasse de poissons (kg ha -1 ), de leur abondance (ind. ha -1 ) et du poids moyen des poissons (g) ne montrent pas de schemas communs entre ces reservoirs mais varient significativement en fonction de la periode du jour ou de la nuit. Pour la plupart des reservoirs, les campagnes nocturnes presentent des estimations plus importantes que celles effectuees de jour. Cependant, le contraire est aussi observe pour certains reservoirs. Ces differences ne peuvent s'expliquer par la composition en especes des stocks de poissons. La repartition en tailles des poissons est differente entre la nuit et le jour. Une plus grande proportion de plus gros poissons (1+) est trouvee durant la nuit pour deux des trois lacs reservoirs et du lac minier. En general, il n'est pas possible de decider si des campagnes de jour ou de nuit fournissent des estimations plus fiables des stocks de poissons. Nous conseillons d'effectuer des transects de nuit et de jour avant de decider de la periode d'evaluation. Pour une estimation complete, il est souhaitable de combiner des campagnes acoustiques mobiles avec un echantillonnage au moyen d'un engin de peche.

The aim of this study was to quantify the efficiency of different lengths of beach seine nets for each diel period in freshwater reservoirs. Only fish older than young-of-the-year were considered. Nets of 10, 20 and 50 m length ("in-nets") were tested in an enclosed area framed by a 200 m long net (block net). The net efficiency estimate was calculated as the ratio of fish catches with the in-net and block net divided by the ratio of their respective areas. The net efficiency estimate was significantly different between day and night catches. At night, the efficiency estimate of nets depended on the size of the fish. The efficiency estimate of a 10 m long net decreased significantly in reverse correlation with fish size. A similar trend was found when using a 20 m long net. The efficiency estimate of a 50 m long net was independent of fish size. The variance in efficiency estimate between samples with a given net length was high, but decreased with longer nets. Of five species tested, only the efficiency estimate for catching bream (Abramis brama) increased significantly with the length of net. The biomass and abundance of larger fish was generally higher at night, although especially short nets exhibited a spuriously high efficiency estimate during the day, probably due to the concentrating (chasing) effect of the hauling ropes. We therefore recommend the use of a 50 m long net, since its nighttime efficiency estimate was about 0.9 in terms of both sampling abundance and biomass. A model relating the efficiency estimate and net length was developed with the data acquired.