To address the lack of information on spatial distribution of different fish larvae species, the effect of weirs of the river Overijsselse Vecht on migratory fish and the habitat structure favored by different fish larvae, the MSc thesis research was executed under a framework of a Ph-D research project "the effect of fish passes on the Ide, Leuciscus idus population and other Rheophilic fish species in the Overijsselse Vecht. The river known as a Bream zone was blocked by 6 weirs, and migratory fish were able to migrate upstream only when weirs were put down during the period of high discharge. Since 1994 fishways have been constructed along each weir to increase the migration possibilities. So, it is expected that migratory species would increase in number and in population size if barriers of the weir no longer exist. A sampling work over the 60 km of the river was conducted during 2 periods (period I: 8-17 May 1996, and period II: 28 May to 2 June 1996). The river is divided in 7 compartments (stretches) by the weirs present. In total, 143 sampling points were randomly selected over the compartments. Weirs, tributary, canal, and oxbowe were included in the sampling points. Effects of period and compartment or stretch were tested with a student-t test (a = 0.05). A total of 40,905 fish larvae (representing 9 species) were caught, of which 36,816 were Eurytopic species, 4,088 were Rheophilic species and 1 was Limnophilic species. Species of which less then 20 in total (Smelt = 15, Pike = 2, Bleak = 1 and Rudd = 1) were excluded from further analysis. The percid species were found more abundant in the first sampling period, while cyprinids were found in higher numbers during the second period. Only for Pikeperch an effect of compartment was significant, showing higher numbers of larvae down stream. The species Perch, Roach and Ide did not show a dispersion over the river (no significant of stretch was shown). During the second sampling period, Bream was more abundant in some stretches then others. No significant effect of water type (mainstream and backwater), submerged vegetation and bottom type could be shown. The population of the Rheophilic, Ide larvae was larger when compared with Bream and Roach. The larvae were equally dispersed over the stretches showing no effect of the weirs by good functioning of the fishways. Some other species of fish larvae were not caught due to different spawning times and limitation of gear effectiveness. Therefore, analysis on them were not available. |
