The effect of littoral complexity on the diel distribution of early juvenile fish communities in temperate freshwater reservoirs

The diel distribution of early juvenile fish communities from two temperate freshwater reservoirs that differed in cover of submerged aquatic vegetation was investigated. In the daytime, about 99.9% of the juvenile fish community from the plant-rich Žlutice Reservoir was found in the nearshore belt of submerged aquatic vegetation reaching an average abundance of ~7982 inds. 1000 m⁻³. In contrast, in the pelagic habitat, few fish were found at day. In the plant-poor Římov Reservoir, 98.9% of juveniles occupied the pelagic habitat during daytime and reached an average abundance of ~333 inds. 1000 m⁻³. Vertical distribution of fish in the pelagic habitat in both water bodies was affected by steep thermal and oxygen stratification and no individuals occurred below 6 m depth. At night, 88.8% of the juvenile fish community from the plant-rich Žlutice Reservoir was in the pelagic habitat with an average abundance of ~1423 inds. 1000 m⁻³. Their diel horizontal shifts were much more pronounced compared to the community from the Římov Reservoir that reached an average nighttime pelagic abundance of ~523 inds. 1000 m⁻³. Only 1.1% of the juvenile fish community in the Římov Reservoir was found in the littoral during the daytime and 0.7% at night reaching an average daytime abundance of ~1688 inds. 1000 m⁻³ and average nighttime time abundance of ~1664 inds. 1000 m⁻³. This study indicates that if a well-developed littoral zone with abundant aquatic vegetation is present, early juvenile fish will perform diel horizontal movement in temperate stratified reservoirs.