Introduced trout are steadily killing off native fish.
Angus mcintosh of Canterbury University roamed South Island high-country streams, and has confirmed that brown and rainbow trout (first introduced in the late 1860s) have decimated many non-migratory populations of native galaxiids. Trout compete for the same food in the same habitat and even eat the galaxiids themselves—a small trout can consume up to 135 newly hatched fry in 24 hours. Trout larger than 150 millimetres prey on all sizes of non-migratory galaxiids, says McIntosh, and there’s no hope for peaceful co-existence.
Introduced species, land-use intensification, forest clearance and wetland drainage now threaten 75 per cent of galaxiid species in New Zealand.
“In the tributaries of the upper Waimakariri River in Canterbury, we know that greater than 95 per cent of the fish-occupied river length contains trout. Given the effect of trout on non-migratory galaxiids, that equates to a 95 per cent decline in habitat where galaxiids can successfully reproduce,” said McIntosh.
His study suggested placing physical barriers to trout downstream to protect galaxiids. McIntosh says that rock chutes (less vertical than a waterfall) are ideal—galaxiids can climb by crawling up the edges of the flow with their fins, whereas trout require a deep pool from which to leap past the barrier.