FEEDING OF JUVENILE TWAITE SHAD (ALOSA FALLAX LACÉPÈDE, 1803) IN THE ELBE ESTUARY.

Seasonal, diurnal and size-dependent trends of food composition, prey selection and daily ration of juvenile twaite shad were investigated in the Elbe estuary. Juvenile twaite shad were caught for diet analysis with framed gape stow nets in the mesohaline region of the Elbe estuary in April, July and August 1993. Copepods, mysids and fish were identified as main food items of juvenile twaite shad. In August, the importance of mysids and fish as main prey organisms increased with increasing total length of twaite shad, whereas the proportion of copepods decreased. During night fishes were consumed to a lesser extent than during day. Insects were mainly eaten during night. In the early morning copepods were main food items. Juvenile twaite shad preferred the mysidacean Mesopodopsis slabberi and the sprat Sprattus sprattus. Cladocerans were ingested corresponding to their availability, whereas all other food items were avoided. Daily ration according to WINBERG (1956) showed peak values in spring and summer and decreased during the following season. The maximum daily ration of about 45 % of body wet weight was calculated in May. Daily ration decreased with increasing age of juvenile twaite shad

The foraging ecology of larval and juvenile fishes

Knowledge of the foraging ecology of fishes is fundamental both to understanding the processes that function at the individual, population and community levels, and for the management and conservation of their populations and habitats. Furthermore, the factors that influence the acquisition and assimilation of food can have significant consequences for the condition, growth, survival and recruitment of fishes. The majority of marine and freshwater fish species are planktivorous at the onset of exogenous nutrition and have a limited ability to detect, capture, ingest and digest prey. Improvements in vision, development of fins and associated improvements in swimming performance, increases in gape size and development of the alimentary tract during ontogeny often lead to shifts in diet composition. Prey size, morphology, behaviour and abundance can all influence the prey selection of larval and juvenile fishes. Differences in feeding behaviour between fish species, individuals or during ontogeny can also be important, as can inter- and intraspecific interactions (competition, predation risk). Temporal (diel, seasonal, annual) and spatial (microhabitat, mesohabitat, macrohabitat, regional) variations in prey availability can have important implications for the prey selection, diet composition, growth, survival, condition and, ultimately, recruitment success of fishes. For fish populations to persist, habitat must be available in sufficient quality and quantity for the range of activities undertaken during all periods of development. Habitats that enhance the diversity, size ranges and abundance of zooplankton should ensure that sufficient food resources are available to larval and juvenile fishes