New Zealand Journal of Marine and Freshwater Research abstracts

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

DAVID ROWE¹
MURRAY HICKS²
JODY RICHARDSON¹

¹National Institute for Water & Atmospheric
  Research Ltd
 P. O. Box 11 115
 Hamilton, New Zealand
 email: d.rowe@niwa.cri.nz
²National Institute for Water & Atmospheric
  Research Ltd
 P. O. Box 8602
 Christchurch, New Zealand

Abstract  Laboratory experiments demonstrated that migrant juvenile banded kokopu (Galaxias 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 adult abundance may decline in turbid rivers. To test this, we compared the abundance of diadromous native fish between turbid and clear rivers. The duration (% time) for which SS concentrations exceeded 120 mg litre^-1 (a critical 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 for over 20% of the time and clear rivers as those where SS concentrations exceeded 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 banded kokopu was reduced by 89.5% in turbid rivers and, although other diadromous fish species were also less common, banded kokopu was most affected. Densities of adult banded kokopu were also significantly lower in optimal stream habitats in three turbid compared with three matched clear rivers. We therefore 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.

Keywords  turbidity; suspended solids; native fish; diadromous fish; whitebait; estuary; river mouth; migration; recruitment

M99029
Received 24 May 1999; accepted 3 February 2000

PDF file of entire paper: medium quality (910K); (scanned from paper original: notes about this process)

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