Fig. 3. Height of contact with the divider wall for each group. The centre of the aperture (arrow) was maintained 25.5 cm from the aquarium floor. Data represent the mean heights ± S.E.M. per trial-day. At some points, the error bars are smaller than the symbols. Trial-days 1-10: water depth was held constant at 35 cm. H₂O up: water level increased by 10 cm, height of contact was recorded for the first 3 min; medium landmark size was maintained for large and small fish. The controls increased the height at which they contacted the divider wall, while the fish with a landmark did not.