Barriers to native fish migration

From November 2018 to February 2019, four Project Parore catchments were surveyed by BOP Regional Council for fish passage barriers as part of our “Hills to Ocean” project. Stream structures and crossings such as fords, culverts and bridges from Woodland Road north of Katikati to Lund Road south of Katikati were identified using aerial maps on ArcGIS, Geo View and Google Earth.

Following that study, a contractor carried out remediation work on 43 impediments to fish passage with almost immediate results.

Lawrie Donald commented “In one case, mussel ropes were installed at a ford which appeared to be a complete barrier to migratory fish. The work was finished by 3pm. After dark at about 8pm we inspected the ropes to find fish already climbing up and heading upstream. It was great, an instantaneous success.”

Often called “fish ladders”, the systems installed to enable fish to navigate man-made barriers are designed around the particular obstacle and may include ropes from mussel farms, ramps or baffles. The idea is to enable the tiny fish to “climb” or swim around a barrier and then rest in quiet water before making the next dash up stream.

Whitebait are the juveniles of six species of fish. Five of these are migratory galaxiids: inanga, banded kōkopu, giant kōkopu, kōaro and shortjaw kōkopu. The sixth species is common smelt.  It is hard to get reliable information on the size of ‘the catch’ from season to season because it is unregulated but one long-term local fisher has given up, saying it is hardly worth the effort. He remembers catching all he wanted in the 70s and 80s and the gradual decline since then has turned into a slump. Now whitebait are classed as ‘threatened’.

The bulk of the whitebait catch is comprised of īnanga which breed only once in their short lives, laying their eggs in vegetation beside estuaries and river mouths in late summer and autumn. When the eggs hatch, they are carried downstream as larvae and spend the next six months at sea. In the spring they migrate upstream as whitebait and grow into adult fish. Kōaro and three species of kōkopu migrate from the estuary into upper catchment sites where they can live for 10 years.

That’s why, says Lawrie, removing barriers to their upstream migration, and ensuring there is habitat for them when they get there, is vital to restoring viable population numbers. “Helping more white bait migrate up stream also helps another stream inhabitant – the fresh water mussel or kakahi, which uses fish to transport larvae upstream.”

“There is a need to educate landowners and the community about what a barrier to native fish looks like. Training will also be made available to contractors who may be installing stream crossings for subdivision purposes. Advice and demonstrations to this group will hopefully avoid future barriers being constructed within streams in the local area.”

The BOPRC report can be found here.

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