ONTOGENIC CHANGES IN PREY SELECTION AND VISUAL-ACUITY OF THE YELLOW PERCH, PERCA-FLAVESCENS

Age-0 yellow perch, Perca flavescens, shift from pelagic to demersal w aters of Oneida Lake, New York, between late June to mid-July, when th ey reach standard lengths of 24-31 mm. The timing of this habitat shif t coincides with the size range over which yellow perch achieve a degr ee of visual resolution that nearly equals that of adult yellow perch, from 174 min of arc in newly hatched larvae to 9-12 min in adults. Th is visual improvement is reflected in the yellow perch's diet, which c onsists of an increasingly wider range of prey sizes and types. If twi n cones are counted as functionally separate photoreceptors, there is a significant improvement of the calculated visual acuity in larval fi sh with lenses <1 mm in diameter but not in older fish with larger len ses. During its rapid growth phase the most optimistic calculation of visual acuity in a young yellow perch is insufficient to explain the f eeding success necessary at this time. We suggest therefore that young yellow perch spend more time in search of prey than their adult count erparts.