Implications of sediment for fish in the lower Wairoa River, Hawke’s Bay

The Wairoa River and its tributaries in Hawke’s Bay are vital for mahinga kai and hold high cultural value for the iwi and hapū of Te Rohe o Te Wairoa. Sediment entering the water poses a threat to fish and river health. This report synthesises scientific information about sediment risks in the Wairoa catchment. Investigations included sediment-related water quality, fish surveys, species sensitivity to sediment, erosion modeling, and the impacts of major storms such as Cyclone Gabrielle.

The report describes the impact of sediment, both suspended in the water column and deposited on the riverbed, on different fish species. Sensitive species like banded kōkopu, kōaro, and īnanga are particularly affected, with sediment disrupting their spawning habitats and food sources.

Meeting regulatory criteria for visual clarity requires a 40% to 76% reduction in suspended sediment. ‘Best-effort’ erosion control measures could reduce sediment loads by up to 60%, improving visual clarity and creating better conditions for sensitive fish species, especially for banded kōkopu. However, climate change is expected to increase sediment loads, potentially limiting the benefits of these measures.

Cyclone Gabrielle resulted in erosion of stream banks, removal of riparian vegetation, and stream-bed disturbance, causing habitat degradation and increased fine sediment deposition at some sites. While some fish species like smelt are expected to recover quickly, others, such as eels and īnanga, will recover more slowly. Management actions such as riparian planting could support habitat recovery and enhance the resilience of fish populations.