FOOD AND FEEDING-BEHAVIOR OF ROACH (RUTILUS-RUTILUS, LINNE 1758) JUVENILES IN RELATION TO MORPHOLOGICAL CHANGE

To study trophic relationships between O+ roach (Rutilus rutilus) and available food in Lake Aydat morphological changes and feeling behaviour were studied in roach fry. Five ontogenic stages were distinguished according to their morphological development. During two fishing expeditions (June and August 1988) we collected O+ roach with body lengths between 11 mm (when they begin active feeding) and 35.9 mm (at metamorphosis). The contents of a total of 172 fry intestines were analyzed. Between 11 and 14.9 mm, fins are only slightly developed, consequently limiting roach fry mobility; feeding behaviour may be passive. Food is essentially algae (diatoms) and some zooplankton. At 18 mm, a greater protrusibility of the mouth allows ingestion of larger prey. At 23 nun, a morphological "break" occurs, which could be attributed to an intermediate phase before the juvenile stage. Up to 30 mm, intestine content analysis revealed a diet based on diatoms. At this stage, changes in food habits could be linked to development of well distinguished fins, allowing fry displacement and thus the possible search for food. From 30 mm onwards, when morphological development is finished, the fish seem to become omnivorous opportunists.