AVOIDANCE OF SUSPENDED SEDIMENT BY THE JUVENILE MIGRATORY STAGE OF 6 NEW-ZEALAND NATIVE FISH SPECIES

The avoidance response of the migratory juvenile stage of six species of diadromous fish to different levels of suspended solid (SS) was det ermined in the laboratory using a two-choice avoidance tank. Trials we re mostly conducted with resuspended sediment from the outlet of lake Waahi, New Zealand, but a series of tests was also conducted with kaol in for comparison. Banded kokopu (Galaxias fasciatus) was the most sen sitive species, displaying a 50% avoidance response to kaolin and Waah i silt at 17 and 25 NTU, respectively. Koaro (Galaxias brevipinnis) an d inanga (Galaxias maculatus) were found to be less sensitive than ban ded kokopu, with a 50% avoidance response to Waahi silt at 70 and 420 NTU, respectively. Shortfinned and longfinned elvers (Anguilla austral is and A. dieffenbachii), and redfinned bullies (Gobiomorphus huttoni) showed no avoidance behaviour, even at the highest turbidities (1100 NTU) tested. Based on these laboratory results, and until field testin g is undertaken, a limit of about 15 NTU in otherwise clear waterways should ensure that the upstream migration of some of the most common N ew Zealand native freshwater species will not be affected.