Martin Čech

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

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

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

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

The effect of hydropower on fish stocks: comparison between cascade and non-cascade reservoirs

Spatial distributions of fish were studied in two types of reservoirs by means of hydroacoustics during the summer. Different patterns of fish distribution were found in non-cascade (Římov, Želivka) and cascade (Kamýk, Slapy, Štěchovice, Vrané) reservoirs. Maximum biomass and density of fish were observed in the tributary area of the non-cascade reservoir. The biomass declined towards the dam area. Average weight of fish showed the opposite trend—maximum average weight was observed in the dam area and declined towards the tributary area. In the cascade reservoirs fish distribution was found to be more complicated but the maximum biomass and density of fish were observed in the dam areas, whereas the tributary areas were nearly fishless. Poor ecohydrological conditions in the tributaries of cascade reservoirs are likely to be responsible for the low abundance of fish in these areas. This was evident in the example of Vrané reservoir which has two major inflows. Low biomass and density of fish were observed in the cold and low oxygen saturated inflowing cascade Vltava River while higher biomass and density were observed in the warm and well oxygen saturated Sázava River inflow. Average weights of fish followed the same trend as in non-cascade reservoirs—the maximum was in the dam area and it declined towards the tributary area. The vertical distribution of fish is driven by the fully developed stratification of temperature and oxygen during the summer season in non-cascade reservoirs. A significantly lower biomass of fish was observed below the thermocline in vertical surveys in both non-cascade and cascade reservoirs. Summer stratification in cascade reservoirs is weakly developed or is not developed at all due to hypolimnetic releases of cold water and short retention times. High transparency suggests lower primary production in cascade reservoirs.

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).

Littoral age 0+ fish distribution in relation to multi-scale spatial heterogeneity of a deep-valley reservoir

Littoral age 0+ fish were studied with respect to spatio-temporal heterogeneity in the deep-valley Římov 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.

Biomass and Abundance Biases in European Standard Gillnet Sampling

The European Standard EN 14757 recommends gillnet mesh sizes that range from 5 to 55mm (knot-to-knot) for the standard monitoring of fish assemblages and suggests adding gillnets with larger mesh sizes if necessary. Our research showed that the recommended range of mesh sizes did not provide a representative picture of fish sizes for larger species that commonly occur in continental Europe. We developed a novel, large mesh gillnet which consists of mesh sizes 70, 90, 110 and 135mm (knot to knot, 10m panels) and assessed its added value for monitoring purposes. From selectivity curves obtained by sampling with single mesh size gillnets (11 mesh sizes 6 – 55mm) and large mesh gillnets, we identified the threshold length of bream (Abramis brama) above which this widespread large species was underestimated by European standard gillnet catches. We tested the European Standard gillnet by comparing its size composition with that obtained during concurrent pelagic trawling and purse seining in a cyprinid-dominated reservoir and found that the European Standard underestimated fish larger than 292mm by 26 times. The inclusion of large mesh gillnets in the sampling design removed this underestimation. We analysed the length-age relationship of bream in the Římov Reservoir, and concluded that catches of bream larger than 292mm and older than five years were seriously underrepresented in European Standard gillnet catches. The Římov Reservoir is a typical cyprinid-dominated water body where the biomass of bream > 292mm formed 70% of the pelagic trawl and purse seine catch. The species-specific relationships between the large mesh gillnet catch and European Standard catch suggested that the presence of carp (Cyprinus carpio), European catfish (Silurus glanis), tench (Tinca tinca) or bream warrants the use of both gillnet types. We suggest extending the gillnet series in the European Standard to avoid misinterpretation of fish community biomass estimates.

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.

The occurrence of non-native tubenose goby Proterorhinus semilunaris in the pelagic 0+ year fish assemblage of a central European reservoir

In July 2008, early juvenile tubenose goby Proterorhinus semilunaris were found in nocturnal pelagic waters of the Vranov Reservoir, Czech Republic. Presence of benthic-living prey in the guts of these fish suggested migration between benthic and pelagic habitats.

Distribution of Herbivorous Fish Is Frozen by Low Temperature

The number of herbivores in populations of ectothermic vertebrates decreases with increasing latitude. At higher latitudes, fish consuming plant matter are exclusively omnivorous. We assess whether omnivorous fish readily shift to herbivory or whether animal prey is typically preferred. We address temperature as the key factor causing their absence at higher latitudes and discuss the potential poleward dispersion caused by climate changes. A controlled experiment illustrates that rudd (Scardinius erythrophthalmus) readily utilize plant matter at water temperatures above 20 °C and avoid its consumption below 20 °C. Field data support these results, showing that plant matter dominates rudd diets during the summer and is absent during the spring. Utilizing cellulose requires the enzyme cellulase, which is produced by microorganisms growing at temperatures of 15–42 °C. Water temperatures at higher latitudes do not reach 15 °C year-round; at our latitude of 50°N~150 days/year. Hence, the species richness of omnivorous fish decreases dramatically above 55° latitude. Our results provide support for the hypothesis that strict herbivorous specialists have developed only in the tropics. Temperatures below 15 °C, even for a short time period, inactivate cellulase and cause diet limitations for omnivorous fish. However, we may expect increases in herbivory at higher latitudes caused by climate change.

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.