Constant magnetic field of low value slows down the embryonic development of trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss). Hatchlings were longer and heavier, and also more motile than those in the control experiment. Magnetic field induces a change in the circulation motion in embryos and larvae of pike (Esox lucius) and carp (Cyprinus carpio), and in the embryos of trout (5. tnitta). Exposure to magnetic field in embryos causes an increase in a heart rhythm, particularly during the early pulsation period of the forming heart. Magnetic fields cause a change in the gas exchange in embryos of a rainbow trout (O. mykiss). Intensity of breathing processes increase in a magnetic field, however, is dependent on stage of embryonic development, and it is especially manifested in the period of an advanced organogenesis. Embryos of rainbow trout (O. mykiss) and trout (S. tnttta) exhibit a sense of direction both in natural and artificially created magnetic field.

Most publications on the influence of environmental factors on macroinvertebrate communities focus on large rivers, whereas relatively few examine small watercourses in agricultural areas, which, due to their size and pressure from intensive agricultural production, are much more susceptible to the effects of unfavourable environmental conditions or anthropopressure. The aim of our study was to compare the biodiversity of macrozoobenthos assemblages and to determine the effects of physicochemical and hydrological conditions on their abundance and density in 10 small rivers in agricultural areas located in northwest (53°23′ N 15°14′ E) and central (52°11′ N 20°48′ E) Poland. In total, 105 taxa were recorded, with the majority being euryoecious. Among the assessed physicochemical parameters, oxygenation was found to affect the density and number of taxa; another important factor was the content of nitrate nitrogen. Sensitivity to changes in water temperature was observed in some macrozoobenthos taxa (especially Decapoda). Of the examined hydrological parameters, the greatest effects were exerted by speed, flow, and depth.