EFFECTS OF TURBIDITY ON THE FEEDING ABILITY OF THE JUVENILE MIGRANT STAGE OF 6 NEW-ZEALAND FRESH-WATER FISH SPECIES

Laboratory tank experiments were used to determine the effect of turbi dity levels ranging from 0-640 Nephelometric Turbidity Units (NTU) on mean feeding rates for each species. Mean feeding rates for banded kok opu (Galaxias fasciatus), smelt (Retropinna retropinna), inanga (G. ma culatus), and common bullies (Gobiomorphus cotidianus) were all reduce d by increased turbidity, but significantly so only for banded kokopu and inanga. Banded kokopu was the most sensitive species as feeding ra tes at 20 NTU were significantly lower than in clear water, compared w ith levels over 160 NTU for the other species. Feeding rates for redfi nned bullies (Gobiomorphus huttoni) increased with turbidity up to 40 NTU then decreased. Reductions in feeding rate at elevated turbidity l evels were the result of a reduced inability to feed, rather than to s tress, as exposure to the highest turbidity level did not reduce feedi ng motivation or appetite. Feeding tests in the dark indicated that, w hereas vision was important for feeding by all species, banded kokopu and common bullies had a comparatively well-developed ability to feed using non-visual senses. Non-visual feeding may have reduced the impac t of high turbidity levels on feeding for these two species, and could confound attempts to determine the effects of turbidity on visual fee ding on live prey.