Reduced abundance of banded kokopu (Galaxias fasciatus) and other native fish in turbid rivers of the North Island of New Zealand

Laboratory experiments demonstrated that migrant juvenile banded kokopu (Ga laxias fasciatus Gray) were more sensitive to suspended sediment (SS) than other native fish species. If juvenile migrants avoid waters made turbid by SS and their recruitment to adult habitats up stream is reduced, then adul t abundance may decline in turbid rivers. To test this, we compared the abu ndance of diadromous native fish between turbid and clear rivers. The durat ion (% time) for which SS concentrations exceeded 120 mg litre(-1) (a criti cal level from laboratory experiments) during the migration season (August- December) was estimated for over 150 New Zealand river sites. Turbid rivers were defined as those where SS concentrations exceeded 120 mg litre(-1) fo r over 20% of the time and clear rivers as those where SS concentrations ex ceeded 120 mg litre(-1) for less than 10% of the time. Eight turbid rivers and seven clear ones were identified where sufficient data on SS and native fish populations existed to permit a comparison. The mean occurrence of ba nded kokopu was reduced by 89.5% in turbid rivers and, although other diadr omous fish species were also less common, banded kokopu was most affected. Densities of adult banded kokopu were also significantly lower in optimal s tream habitats in three turbid compared with three matched clear rivers. We then:fore concluded that the abundance of adult banded kokopu was reduced in turbid rivers and propose that this is because of reduced recruitment of juveniles in turbid rivers.