Tomáš Jůza

Chaos and stability of age-0 fish assemblages in a temperate deep reservoir: unpredictable success and stable habitat use

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).

Patterns in diel habitat use of fish covering the littoral and pelagic zones in a reservoir

We investigated diel habitat use of fish covering the littoral and pelagic zones of the Římov 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.

Sampling of deep benthic perch fry: insight into the diel vertical migrations

At depths smaller than those of the daytime bathypelagic layers, the vertically migrating perch Perca fluviatilis fry form a benthic community. Modified Breder traps were used for sampling these deep benthic perch fry during their daylight stay in the benthic habitat of Římov Reservoir, Czech Republic, in spring 2007 and 2009. The size and condition of perch caught in the traps (the catch was removed in late afternoon) was compared to that of fish caught by trawl in the bathypelagic habitat at midday and by electrofishing in the littoral habitat at midday. Perch from the traps were in significantly worse condition compared to those from the trawl and electrofishing. All fish left in the traps overnight were dead. It is likely that the evening migration to the epilimnion is essential for perch fry performing diel vertical migrations in stratified canyon-shaped reservoirs. The sampling efficiency of traps is strongly affected by the abundance of targeted fish and their swimming activity in a cold and dark benthic habitat.

Who Is Who: An Anomalous Predator-Prey Role Exchange between Cyprinids and Perch

Piscivory in cyprinids (Cyprinidae) is extremely rare. Specifically, common bream (Abramis brama) and common carp (Cyprinus carpio) are zooplanktivorous fish in deep lentic waters. Nevertheless, we observed predation by these two cyprinids under natural conditions in the Vír Reservoir, Czech Republic. We conducted diet analysis for cyprinids caught by trawling and gillnets and the large amount of young-of-the-year (YOY) perch (Perca fluviatilis), with sizes of 37–52 mm standard length, were found in their digestive tracts. In 2010, a large amount of YOY perch caused a significant decrease in Daphnia spp. size and abundance in the reservoir. Hence, a food deficit was induced for the cyprinids, apparent also from the poor nutritional condition of common bream which was much worse than the condition of those in similar reservoirs. Common carp and common bream shifted to forced piscivory, and they utilized the YOY perch as an alternative food source. In contrast, smaller species, such as roach (Rutilus rutilus) and bleak (Alburnus alburnus), widely utilized planktonic cyanobacteria. In the following year, YOY perch occurred in significantly lower numbers and conversely, Daphnia spp. size and abundance were significantly higher. The forced piscivory was not observed. Our results indicate a switch to forced piscivory by cyprinids, which was caused by a shortage of their natural food source. Moreover, this phenomenon presents an effective mechanism for reduction in the numbers of YOY perch, ensuring the stability of the ecosystem.

Invasive round goby Neogobius melanostomus has sex-dependent locomotor activity and is under-represented in catches from passive fishing gear compared with seine catches

The higher proportion of males of the invasive round goby Neogobius melanostomus in samples from two activity selective passive fishing gears compared with one activity non-selective fishing gear in three Dutch lakes is related to higher male locomotory activity and is a sex-dependent trait. This difference in activity reflects the different ecology of male and female N. melanostomus.

Seasonal and Spatial Dynamics of Gas Ebullition in a Temperate Water‐Storage Reservoir

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 reservoirs 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.

Condition and feeding behaviour of subadult burbot (Lota lota) in riverine and lacustrine environments

The condition and feeding behaviour of burbot, a widespread potamodromous species in riverine and lacustrine environments, were compared in order to evaluate the importance of both in three artificial systems. Subadult burbot were sampled in three temperate reservoirs in spring, and one of them also in summer and autumn. Standardised abundance and sizes of burbot were comparable between the reservoirs, but the conditions were significantly different. The Clark’s condition coefficient and index of fullness were independent of burbot size and individuals. Feeding behaviour in terms of abundance and composition of consumed food was environment-dependent. The most common prey category was aquatic insect larvae, dominating in the riverine environment. Permanent water invertebrates, microcrustaceans, beetles and crayfish were less common food and eaten more often in the lacustrine environment. Prey-fish were usually the most common species of suitable size. The food analyses demonstrated opportunistic feeding behaviour with selection of prey associated with benthic habitat and suitable size. Burbot is therefore flexible not only in environments utilisation, but also feeding strategy.

A novel upward-looking hydroacoustic method for improving pelagic fish surveys

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.

Correction: Biomass and abundance biases in European standard gillnet sampling.

The following information is missing from the Funding section: This study received institutional support from the Czech Academy of Sciences (RVO:60077344, http://www.avcr.cz/) and the CEKOPOT project (CZ.1.07/2.3.00/20.0204), co-financed by the European Social Fund and the state budget of the Czech Republic. Additionally, this study was supported by the Grant Agency of the Czech Republic (Project GPP505/12/P647). Daniel Ricard was supported by project CZ.1.07/2.3.00/30.0032 (Promotion of postdoctoral positions in the Biology Centre of the Czech Academy of Sciences), co-financed by the European Social Fund and the state budget of the Czech Republic. The complete, correct funding statement should read: This study received institutional support from the Czech Academy of Sciences (RVO:60077344, http://www.avcr.cz/) and the CEKOPOT project (CZ.1.07/2.3.00/20.0204), co-financed by the European Social Fund and the state budget of the Czech Republic. Additionally, this study was supported by the Grant Agency of the Czech Republic (Project GPP505/12/P647). Daniel Ricard was supported by project CZ.1.07/2.3.00/30.0032 (Promotion of postdoctoral positions in the Biology Centre of the Czech Academy of Sciences), co-financed by the European Social Fund and the state budget of the Czech Republic. Ecohydros S.L. provided support in the form of salaries for authors [AMH], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section.