ONTOGENIC CHANGES IN BEHAVIORAL AND HISTOLOGICAL MEASURES OF VISUAL-ACUITY IN 3 SPECIES OF FISH

Vision plays an important role in the early life history of fishes. We investigated the ontogenetic changes in visual acuity of early life h istory stages of alewife, Alosa pseudoharengus, yellow perch, Perca fl avescens and bloater, Coregonus hoyi, across a range of sizes. Acuitie s were determined through histological examination of the retinae of l arvae. Reactive distances of larvae to prey were estimated through vid eophotography of their response to prey and were then converted to mea surements of visual angle. Both measures of visual ability improved wi th size (age) for all species. When behavioural and anatomical measure s of ability were compared as a function of size, the data indicate th at fish are anatomically more capable of seeing objects than the behav ioural response suggests. In two of the three species, the relationshi p between histological acuity and visual angle was not constant. These results may indicate that while vision may limit initial rates of enc ounter and feeding, increases in visual acuity mean that in older stag es limitations on encounter and feeding are more likely to be behaviou ral. Furthermore, these results indicate that encounter rates based up on histological estimates of visual acuity will be greater than compar able estimates based upon reactive distances. We recommend calculation of encounter rates based upon reactive distances.