Effects of turbidity on the feeding ability of the juvenile migrant stage of six New Zealand freshwater fish species

Laboratory tank experiments were used to determine the effect of turbidity levels ranging from 0-640 Nephelometric Turbidity Units (NTU) on mean feeding rates for each species. Mean feeding rates for banded kokopu (Galaxias fasciatus), smelt (Retropinna retropinna), inanga (G. maculatus), and common bullies (Gobiomorphus cotidianus) were all reduced by increased turbidity, but significantly so only for banded kokopu and inanga. Banded kokopu was the most sensitive species as feeding rates at 20 NTU were significantly lower than in clear water, compared with levels over 160 NTU for the other species. Feeding rates for redfinned bullies (Gobiomorphus huttoni) increased with turbidity up to 40 NTU then decreased. Reductions in feeding rate at elevated turbidity levels were the result of a reduced inability to feed, rather than to stress, as exposure to the highest turbidity level did not reduce feeding motivation or appetite. Feeding tests in the dark indicated that, whereas 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 impact of high turbidity levels on feeding for these two species, and could confound attempts to determine the effects of turbidity on visual feeding on live prey.