Diel vertical migrations of bathypelagic perch fry

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Abstract
The behaviour of young‐of‐the‐year (YOY) perchPerca fluviatilisas a dominant species in the assemblage of fry in the pelagic of Slapy Reservoir (Czech Republic), was studied during late May and mid‐June 2002 using acoustic methods and complementary net catches. During the day, perch fry were present simultaneously in littoral, epipelagic and bathypelagic habitats. Bathypelagic perch fry, forming a scattering layer, migrated vertically each day between the epilimnion and hypolimnion, with an amplitude of 11·0 m in May and 12·5 m in June. At dusk, the migratory bathypelagic fry mixed in the epilimnion with non‐migrating epipelagic fry and spent the night close to the thermocline (abundance maximum at 3–4 m in May, 0–2 m in June). In June, shoaling behaviour by some of the bathypelagic perch fry was also observed: the shoaling fry remained higher in the water column than the non‐shoaling fry. Both depths of the scattering layer and the depths of the fry shoals were strongly controlled by the light intensity. The contribution of the bathypelagic part of the population to the total numbers of pelagic perch fry decreased from 28·1% in May to 4·7% in June, while the density of all pelagic perch fry increased (c. 96 000 individuals ha−1in May and 142 000 individuals ha−1in June). In May, the bathypelagic (average total length,LT, 11·9 mm) and epipelagic (averageLT14·6 mm) perch fry differed in size while, in June, the epipelagic fry were divided into two distinct size groups. The more abundant group, of small epipelagic perch fry (averageLT14·6 mm), was similar in size to the bathypelagic fry (averageLT14·6 mm) while the less abundant group, of larger epipelagic fry (averageLT34·4 mm), was similar in size to littoral perch fry (averageLT35·0 mm). The results suggest that in perch fry three different survival strategies with different risks can be used in the same locality, time and year.