Rheoreaction is the major behavioral response of fish in a stream. This inherited response consists in a tendency to swim against the current (Arnold, 1969; Pavlov, 1972Pavlov, , 1979. Movement is a component of nearly all behavioral responses of fish. The swimming speed is a common quantitative index of such reactions. At the same time, it permanently varies (pulsates) at each given point of water body. Such stream state is called turbulence. The nature of turbulence relies on formation and development of vortex systems in water flow. The indices of flow turbulence and its velocity are the main factors describing the hydrodynamic heterogeneity of fish environment.Previous studies of fish behavior were commonly concerned with just one aspect of this heterogeneityflow velocity. Even evaluations of fish swimming speed in a stream ignored turbulence. The first studies of the effect of turbulence on the functional indices of fish rheoreaction demonstrated that the threshold and critical flow rates decreased with turbulence (Pavlov et al ., 1982;Shtaf et al ., 1983).In this work, the mechanism of turbulence impact on fish locomotion indices was explored with the assumption that balance loss is one of factors decreasing fish swimming performance under conditions of high turbulence with hydrodynamic forces arising in a vortex flow. Clearly, these forces should have different impacts on fish with different body length. The impact of flow turbulence on the critical flow rate of perch ( Perca fluviatilis ) individuals with different body length was studied. MATERIALS AND METHODSExperiments were conducted in a special hydrodynamic trough (Fig. 1) in the Volga River. The working region was a passage 120 cm in length and 8 cm in width limited by a special mesh. A device controlling turbulent properties of the flow was placed in the upstream part. River water was pumped to the trough at a constant flow rate of 3.7 l/s. The flow rate in the trough depended on the depth, which was controlled by a flat paddle in the downstream part of the trough.The rate and turbulent properties of the flow were determined with a specially developed device including a velocity sensor and a signal converter/amplifier based on a single-chip processor . A microspinner with a fan diameter of 10 mm was used as the velocity sensor. This measuring complex allowed instant processing of data in the laboratory and field and controlling hydraulic conditions during experiments.The flow rate variations were evaluated as turbulence number and scale (Grinval'd and Nikora, 1988). Turbulence number K , which describes the changes in instantaneous speed in time, was determined from equation:where σ is standard pulsation or root-mean-square deviation of instantaneous speed from the time-average flow rate V c .A histogram of instantaneous speed frequency in a stream point within the averaging period (60 s) was used to illustrate the process of flow rate changes. The scale of turbulence ( L ), which describes the mean vortex size in a flow, was determined by one-dim...
This paper reviews results of experimental and field studies of downstream migration of young fish carried out in the Institute of Ecology and Evolution (Russian Academy of Sciences) as a part of the research programme on fish behaviour and ecology. Studies of fish migrations have been dominated by descriptions of migratory routes and proximate control of orientation. Behaviour and ecological factors influencing variability of downstream migrations of fish larvae and fry are still poorly understood. Life cycles of most of freshwater fishes can be roughly considered as a sequence of ''residential'' and ''migratory'' phases. Migrating, or just moving between microhabitats, fish are especially vulnerable to various threats including impacts of water abstraction systems. Abiotic (water flow velocity, rheogradients, turbulence, visual habitat heterogeneity) and biotic (foraging conditions, predators, competitors and parasites) factors can cause changes in fish spatial distribution as well as shift from residential to migratory behaviour. From the viewpoint of fish protection, these changes are important not only in the vicinity of water abstraction sites, but also in more distant parts of the aquatic system that should be taken into account in long-term assessments of impacts of abstraction on fish populations in regulated rivers. Besides macro-and mesoscale impacts that affect fish migrations and spatial distribution, micro-scale habitat heterogeneity (hydrodynamic and visual) are important as factors triggering and controlling various aspects of young fish behaviour. Interactions of water abstraction zones with fish populations have to be analysed at ecological (meso-scale spatial overlapping and longterm dynamics) and behavioural (effects of microscale structures and events) levels.
The complex influence of baro-, photo-and thermo-gradients on distribution and behaviour of young physostomous Leuciscus leuciscus L., leaciscus idus L. and physoclistous fish Perca fluviatilis L. was investigated. 40 different combinations consisting of 4 types of photogradients, 3 types of termo-gradients and 4 types of baro-gradients were tested. All considered factors influenced the distribution of physostomous and physoclistous fish with high degree of significance. Under a multi-factorial experiment fish behaviour and distribution are determined not only by a separately taken environmental factor but by the presence of other accompanying factors. Under different combinations of imposed factors, the reaction of fish to an individual factor and the character of the response (positive or negative) may change. Hydrostatic pressure is one of the strongest factors which influence fish distribution. The presence of its gradient may change the character of response of perch Perca fluviatilis to light (a changing of a sign of its photoreaction). Water temperature influences fish distribution more than illumination. However, the definite value of water temperature at which most fish prefer to stay may be changed depending on both the intensity of illumination and the value of hydrostatic pressure. 2000 The Fisheries Society of the British Isles
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