Dissolved oxygen (DO) conditions within the hyporheic zone were investigated in a gravel stream (River Sieg) in North Rhine Westphalia, Germany, populated in the spawning season by recurring migratory fish species, like Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta trutta L.). The gravel bed was cleaned at three sites in an area of approximately 150 m 2 to a depth of 50 cm, reducing the quantity of grains <2 mm to below 0.2%. DO concentrations in cleaned and uncleaned sediments were monitored in situ at 10, 20 and 30 cm sediment depth from the end of November 2001 to the end of April 2002. DO showed only minor fluctuations at the cleaned sites and steadily decreased at all uncleaned sites over time. Fine sediment accumulation over 5 months in the cleaned sites was comparable to the proportion of material within the bed prior to the experimental cleaning and probably influenced the DO concentrations of the hyporheic water. Decreasing DO concentrations at all sites coincided with increasing water temperatures towards the end of the study period. The cleaning operation significantly improved the conditions of DO in the hyporheic zone of the three study sites.
Bream Abramis brama, white bream Blicca bjoerkna and roach Rutilus rutilus all undertook spawning migrations from the River Rhine into oxbows. Adult white bream and roach returned to the river after spawning. This indicates an annual change of habitat allowing the use of optimal conditions in oxbows for spawning and growth of juvenile fish, and the use of feeding grounds rich in benthos in the main river by the adults. In contrast, adult bream were still present in large numbers in the oxbows over the summer. Juvenile bream (0+ and 1+) migrated to the main river. Bream aged 2+ to 4+ were not found in the oxbows, whereas they occurred frequently in the river. Thus bream show ontogenetic changes in their use of the main river and oxbows. The young fish spend their long juvenile phase in the river after leaving the oxbows. Reproduction in oxbows is ensured by mature fish migrating in for a short period from the river as well as by other adults (d5+) that live permanently in the oxbows. The strong tendency of adult bream to develop permanent stocks in the oxbows gives this species a reproductive advantage, since many oxbows are accessible to Rhine fish only occasionally in the present hydrological situation. 1999 The Fisheries Society of the British Isles
During the last two decades an increasing number of sand and gravel-pits were established in the original flood plain of the River Rhine. Some of these are connected with the stream by a small canal for the transport of the spoil. Some of these dredged lakes have been studied during recent years at the ecological field station of the University of Köln, with regard to seasonal phytoplankton succession, productivity of copepods and cladocerans, occurrence of macrozoobenthos, and both spawning and recruitment of fish populations. These eutrophic lakes represent a valuable substitute biotope for lost lentic waters of the former flood plain. They offer via the canal connection an advantageous habitat to resident fish species of the Lower Rhine, (1) for reproduction and growth of the 0+-generation, and (2) for shelter during flood events. Recommendations for reclamation plans of such lakes are required.
The total fish abundances of two gravel-pit lakes connected with the River Rhine have been observed by a hydro-acoustic method (echo-sounder: Lowrance X16) for about one year. Diel (24-hour-period) as well as seasonal changes in fish distribution and aggregation behaviour have been established. Concerning the assessment of the fish stocks, these changes in fish behaviour have to be considered. Some clear fluctuations of fish numbers, which coincide with flood events of the river are interpreted as indications of fish migration from the river into the lentic water.
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