The efficiency of three fry trawls for sampling the freshwater pelagic fry community |
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| Institution: | 1. Biology Centre, Institute of Hydrobiology, Academy of Sciences of the Czech Republic, Na sádkách 7, 370 05 ?eské Budějovice, Czech Republic;2. Faculty of Biological Sciences, University of South Bohemia, Brani?ovská 31, 370 05 ?eské Budějovice, Czech Republic;1. Biology Centre of the Czech Academy of Sciences, v.v.i., Institute of Hydrobiology, Na Sádkách 7, 370 05 ?eské Budějovice, Czech Republic;2. University of South Bohemia, Faculty of Science, Brani?ovská 31, 370 05 ?eské Budějovice, Czech Republic;3. University of South Bohemia, Faculty of Economics, Studentská 13, 370 05 ?eské Budějovice, Czech Republic;1. Biology Centre of the Czech Academy of Sciences, Institute of Hydrobiology, Na Sádkách 7, 370 05, ?eské Budějovice, Czech Republic;2. University of South Bohemia, Faculty of Science, Brani?ovská 31, 370 05, ?eské Budějovice, Czech Republic;3. Fisheries and Oceans Canada, Gulf Fisheries Centre, 343 Avenue de l''Université, C.P. 5030, Moncton, NB, E1C 9B6, Canada;1. Biology Centre of the Academy of Sciences of the Czech Republic, v.v.i., Institute of Hydrobiology, Na Sádkách 7, 370 05 ?eské Budějovice, Czech Republic;2. Evides Water Company, PO Box 4472, 3006 AL Rotterdam, The Netherlands;3. AqWa, Oosthavendijk 43, 4475 AB Wilhelminadorp, The Netherlands;4. University of South Bohemia, Faculty of Economics, Studentská 13, 370 05 ?eské Budějovice, Czech Republic;5. University of South Bohemia, Faculty of Science, Department of Ecosystems Biology, Brani?ovská 31, 370 05 ?eské Budějovice, Czech Republic;6. Biology Centre of the Academy of Sciences of the Czech Republic, v.v.i., Institute of Entomology, Brani?ovská 31, 370 05 ?eské Budějovice, Czech Republic;7. Vltava River Authority, Denisovo náb?e?í 14, 304 20 Plzeň, Czech Republic |
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| Abstract: | Avoidance reactions of young-of-the-year fish assemblages were investigated in the offshore zone of three reservoirs in the Czech Republic during late summer. Pikeperch (Stizostedion lucioperca) strongly dominated in the Lipno Reservoir in 2003, bream (Abramis brama) and bleak (Alburnus alburnus) were the most abundant species in the Slapy Reservoir in 2004 and roach (Rutilus rutilus) and bleak dominated in the ?elivka Reservoir in 2004 and 2005. Densities of fry ranged between 1.09 ind./100 m3 in the Lipno Reservoir and 2.95 ind./100 m3 in the Slapy Reservoir. In each reservoir, three trawl nets of different widths (dimensions 1.5 m × 3 m, 3 m × 3 m, 6 m × 3 m) were used in the pelagic area at night. For each trawl the estimated fish density was determined as the catch per 100 m3 of water and was compared between all three trawls. The fish length–frequency distribution was also compared between all the trawls. Day catches were much lower compared with those taken at night. Adult fish showed very clear avoidance of fry trawls. At night, no important differences in estimated fry density, obtained by these trawls, were observed in any locality for any fish species, nor in the length distribution of important species between trawls. According to these results, the avoidance reactions of fry in the late summer (August, September), at night, are unimportant. At this time of year, nighttime pelagic trawling appears to be an effective method for quantitative fry sampling. We recommend the 3 m × 3 m trawl for quantitative night sampling of the fry community as a good compromise between assured efficiency and relatively easy handling. |
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