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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.

In polygynandrous mating systems, in which females limit reproductive success, males can increase their success by investing in courtship. Earlier arrival at the spawning ground compared to when females arrive may increase their opportunities in competitive mating systems. In this study, we used passive telemetry to test whether a male minnow known as the asp, Leuciscus aspius, times its arrival at spawning grounds relative to the arrival of females. Males arrived in a model stream approximately five days earlier than females on average and left four to five days later than females over two years. Both sexes performed a daily migration between a staging ground (standing water, low energy costs) and the fluvial spawning ground (high energy costs). Fish abundance peaked twice a day, with a major peak at sunset and a minor peak at sunrise and with the evening peak abundance for males occurring 1 hour 40 minutes earlier than that of females. The number of females on the spawning ground never exceeded the number of males. While the degree of protandry is hypothesized to be influenced by the operational sex ratio (ranging from 0.5 to 1 in our study), our data did not support this theory.

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.

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.

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.

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

The distribution patterns of alpine bullhead Cottus poecilopus in three tributary streams of the Rožnovska Becva River (Danube basin) were studied with respect to temperature, oxygen concentration and saturation, shading, current, conductivity, total organic carbon (TOC), nitrates and phosphates, biochemical oxygen demand (BOD5 ), pH, redox potential, bottom grain structure, density of macroinvertebrates and the abundance of brown trout Salmo trutta. Sites with lower abundance per hectare of C. poecilopus differed significantly in dissolved oxygen saturation, density of macroinvertebrates during the autumn period (positive correlation with C. poecilopus) and in abundance per hectare of S. trutta (negative correlation). These results indicate that these factors significantly influence the distribution of this endangered species in the studied catchment and that stocking of S. trutta will impair its recovery.

Predator-prey relationships are often perceived simply as a situation in which a predator enhances its own fitness while reducing the fitness of its prey. However, this relationship may become reversed when the prey feeds on the juvenile predator stages. We investigated this phenomenon in a model asp (Leuciscus aspius; predator)-bleak (Alburnus alburnus; prey) relationship. The adhesive asp eggs are available for bleak predation after a spawning event for only tens of seconds before they adhere to the stones, where bleak do not forage. Gut content analysis demonstrated that eggs were utilized in high quantities, especially in the spawning peak of the asp reproductive season. Furthermore, using underwater video, we recorded the bleak feeding efficiency on naturally drifting asp eggs as the percentage of eggs eaten. Within the 40 cm egg trajectory captured by our cameras, total egg mortality was 21.2 ± 2.2% on average. The highest survival chances occurred among eggs drifting in aggregations, since the short drifting distance together with their aggregated distribution satiated bleak and part of the egg aggregation could attach to the spawning ground. This study emphasizes the potential efficiency of predator egg utilization by prey, which may have further consequences in predator-prey dynamics.

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.

We investigated the change in benthic fish communities in three artificial lakes of the Biesbosch area in the Netherlands between two time periods: before and after the invasion of round goby (Neogobius melanostomus). Native ruffe (Gymnocephalus cernua), the dominant species in benthic gillnet and littoral beach seining catches before the invasion, almost completely disappeared in all lakes only 2 years after the invasion. We found a significant increase in 0 + perch (Perca fluviatilis) and, in some lakes, pikeperch (Sander lucioperca) abundance in gillnet catches after invasion. In the post-invasion period, the 0 + fish community was dominated by perch, and the older fish community was dominated by round goby. The species richness of 0 + fish increased in the post-invasion period owing to the invasion of gobiids. However, it did not change for older fish between periods. Our results clearly show that, owing to a similar benthic lifestyle and high niche overlap, ruffe was the only species negatively influenced by the round goby invasion. The competitive superiority of round goby over ruffe is so strong that the once-dominant species of the overall benthic fish community collapsed after only a few years of coexistence.