Title:

Climbing Upstream: The Migration Struggle of Diadromous Freshwater Fish in the Hunua Ranges of New Zealand

Physical waterway barriers created by built infrastructure restrict freshwater fish from accessing upstream habitats, which has led to declines in freshwater fish biodiversity in New Zealand. The installation of fish passages can help to facilitate upstream migration if fish climbing ability is sufficient for use of the passage. The Hunua Ranges of New Zealand are influenced by physical waterway barriers, which impact native freshwater fish species including eels, bullies, and galaxiids. To determine whether fish passage culvert modifications in the Hunua Ranges have increased the success of fish passage upstream in the area over time, we used electrofishing to survey native freshwater fish species upstream and downstream of five culverts and at two sites above the Mangatāwhiri Reservoir. We found a greater abundance of species with weaker swimming abilities downstream of the culverts and a greater abundance of species with stronger swimming abilities upstream of the culverts. This indicates that the swimming strength of the species is a key factor in determining how effective the fish passages are for facilitating upstream migration, and the current fish passages installed in the Hunua Ranges may be ineffective at assisting the upstream migration of species with weaker climbing ability. Improvements in the effectiveness of fish passages on the culverts in the region could help increase certain key native freshwater fish populations.

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