It is whitebaiting season again. However, anytime is a good time to remember they are the juveniles of six species of fish. Five of these are migratory fish: inanga, banded kōkopu, giant kōkopu, kōaro and shortjaw kōkopu. (The sixth species is common smelt.) Transitioning between salt- and freshwater is essential for these galaxiid fish, and for shortfin and longfin eels. Culverts, weirs, and dams often delay or prevent fish migration. Over time, these man-made structures have resulted in a reduction in the distribution and abundance of some of our most iconic and valued freshwater species.
An action point in the Ōhiwa Harbour Strategy addresses this issue and aims to develop a greater understanding of indigenous fish species and restore connectivity in the Ōhiwa catchment. Most of the potential fish barriers (structures) in the catchment are culverts, which may have an outlet too high for fish to swim or climb (as in photo) or are angled too steeply for fish to be able to swim against the fast current. All these can be retrofitted to be more fish friendly. Of the 968 structures found since 2022, 295 of them were found to impact on the movement of fish. So far 88 of these have been retrofitted.
Further information has also been gathered to gain a better understanding of the impact of these barriers on the freshwater fish species in the Ōhiwa catchment. 22 sites in many streams around the catchment were sampled using the environmental DNA (eDNA) technique. Genetic material is constantly shed by organisms, including fish. By collecting water samples which will contain this discarded DNA and sequencing it, we can gain a picture of plants and animals living in and around the stream.
Some interesting findings: Crans bully were detected, this is believed to be the first record of this fish in the Ōhiwa catchment. The sampling also confirmed the continued presence of shortjaw kōkopu previously found in the upper Nukuhou stream. Shortjaw kōkopu are a threatened species. Perhaps best of all, native freshwater mussels were also detected for the first time.
All whitebait species are either declining or threatened. For their existence to continue, programmes such as this, seeking to restore fish passage are crucial. Equally important will be to stick to the whitebaiting rules so these tiny fish get a chance and do not all end up in a fritter. To halt the further decline of this precious species the whitebaiting season has been limited to 1 September to 30 October throughout mainland New Zealand.
The Ōhiwa Harbour Strategy partnership’s fish passage project continues with the retrofitting of the remaining structures that were identified as barriers.
