T. Jůza

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.

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.

Comparison of Gill Nets and Fixed-Frame Trawls for Sampling Threadfin Shad in Tropical Reservoirs

Abstract Threadfin shad Dorosoma petenense populations were sampled quarterly from 2010 to 2011 to determine appropriate sampling techniques for this species in tropical reservoirs of Puerto Rico. Offshore gill netting and night trawling were compared in terms of catch per unit of effort, size distribution, sampling precision, and bycatch. In total, 90 gill net–trawl pairs of catches were compared, which collected more than 80,000 threadfin shad. Gill-net and trawl catches were not correlated in either number or biomass. Coefficients of variation were greater in gill-net sampling (0.761 for numbers, 0.747 for biomass) than in trawl sampling (0.433 and 0.465, respectively) and were not dependent on reservoir, sampling season, reservoir section, or any combination of these factors for any gear. There was no correspondence in size distributions between gill-net and trawl catches. Gill nets collected threadfin shad in the range of 23–169xa0mm total length (TL) and displayed strong mesh size selectivity, causing...

Towards a better understanding of small scale distribution of littoral age-0 fish in a deep-valley reservoir: day or night surveys?

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

Occurrence of age-0 year dwarf pikeperch Sander lucioperca in late summer--an overlooked phenomenon in reservoirs.

Late summer sampling of pelagic age-0u2009year fish communities in five Czech reservoirs and one Dutch reservoir revealed extremely small age-0u2009year pikeperch Sander lucioperca (mean 24u2009mm standard length, LS , minimum 13u2009mm LS ) alongside more normal-sized S. lucioperca that are found at the end of the first growing season (mean 50u2009mm LS ), resulting in two clearly size-separated cohorts. Reference to such small age-0 year S. lucioperca in lakes or reservoirs at this time of year and in such large numbers are almost absent the scientific literature, and the presence of these small S. lucioperca is contradictory to the common understanding of the reproductive biology of this species. This overlooked phenomenon may have a major effect on the population dynamics of this valuable species because of size-dependent winter mortality.

Assessment of burbot Lota lota (L. 1758) population sustainability in central European reservoirs: LOTA LOTA POPULATIONS IN CZECH IMPOUNDMENTS

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