Fishy origins: how to protect īnanga spawning zones in 6 steps

Ever visited a stream and seen a school of īnanga? As our smallest and most abundant whitebait species, īnanga play an important role in the marine and freshwater food web as they migrate between salty and freshwater environments. Unfortunately, habitat loss means these little guys are struggling to keep their numbers up.

You might wonder, ‘how do you protect something that swims between rivers and the ocean?’ Part of the answer is to ensure īnanga have optimal spawning habitat. That's why we’ve developed a process to ensure these important fish have the best chance to reproduce and thrive.

Step 1: Find the saltwater wedge.

Īnanga will swim downstream to investigate ahead of spawning time for a suitable site. Usually, this is the saltwater wedge, the area where saltwater from the tides mixes with freshwater from the stream.

We use a salinity metre and depth probe to determine how salty the awa’s water is in parts per thousand, until we find the area where it transitions from salty to fully fresh. This is the top of the wedge, the favoured spot of īnanga preparing to spawn.

Step 2: Assess the habitat.

This stage is all about checking out the riparian area and assessing what the existing area provides for īnanga.

Some signs we look for include checking that the stream banks are not too steep (īnanga are poor climbers), and what the vegetation cover is composed of (īnanga need dense grass that will stay moist). Then, we give it a score on a scale of ‘healthy’ to ‘needs help’ and decide how to meet the needs of the stream.

Step 3: Create temporary habitat.

At key sites along the bank, we install hay bales using waratahs. Hay bales are effective as they are dense, creating shaded, moist areas that protect eggs from predators — a niche the rest of the habitat might not provide for yet.

Step 4: Look for eggs.

Next, our team search for īnanga eggs at known spawning sites. Here is where doing the saltwater wedge assessment beforehand becomes helpful, so we can mark out the area to search.

Eggs are found in areas where the tide rises about a metre to half a metre along the bank. Eggs are found deep amongst the roots of grass, where they stay moist as they await the next high tide.

Step 5: Control pests.

Rats and mice view īnanga eggs like we view M&Ms — one rat can wipe out an entire spawning site in a single night. Trapping begins between April-August for northern areas in New Zealand, and slightly earlier for more southern areas.

We space traps 20-25 metres apart, about 3.5 metres from the awa, wherever conditions make it possible.

Step 6: Restore the habitat.

Lastly, we work on long-term protection of īnanga, focusing restoration efforts on the area we surveyed right at the start: the top of the saltwater wedge.

We fence off the area for exclusion and plant it out with the ideal spawning habitat: native grass near the water’s edge (oioi, wiwi, raupō, and other sedges and rushes). Behind them come the larger plants to shelter and protect the stream: first, flaxes, tī kōuka/cabbage trees, and other shrubs. Then the larger trees (tītoki/New Zealand oak, māhoe/whiteywood, mānuka, kānuka), and furthest from the stream come the giants (tōtara, kauri, etc.).

With help from a range of partners, including the Ministry for the Environment via our Wai Connection project, we're putting these six essential steps into action all over Aotearoa New Zealand. Learn all about our smallest whitebait species and how you can protect them, too.