Īnanga (whitebait) spawning in the Motueka catchment

Written by MCCadmin

28/11/2024

Monique Patterson, who works for Tasman Bay Guardians (TBG) has been involved in a project looking into īnanga spawning sites along the top of the South Island. I went and talked to her about this important TBG project.

What are īnanga?

Īnanga are a native fish. They’re the most common and smallest of the whitebait species. When caught for whitebait they’re in their juvenile form, but they can grow up to 11 cm in length. They’re one of a number of migratory galaxiids living in our rivers and are classified as “At risk – declining” by the Department of Conservation.

Where are their spawning sites?

Īnanga spawning sites on the Motueka River

Īnanga spawning sites (where eggs are laid) are always at the bottom of river catchments where the salt water from the sea meets the freshwater from the river. The approximate location is found at what is called the saltwater wedge. In the Motueka catchment this is generally found no further inland from the coastal margin than 1km, in both the smaller tributaries and larger rivers. The map shows the spawning area on the Motueka River. It is important to note just how small the spawning zone actually is within a catchment and if the habitat is unavailable or unsuitable the survival rate will keep diminishing.

What sort of habitat do īnanga need to spawn?

As I said, īnanga spawn at the bottom of catchments where there aren’t any rapids or hard to swim up areas.

The eggs are laid at the base of densely rooted grasses, sedges and rushes and sometimes even into the roots at the time of a very high tide. The eggs need to be kept damp and moist for a month until the next high tide comes and washes them out to sea.

What affects spawning:

There are a number of things that people do that can affect the quality of the spawning site, and therefore how many eggs are able to be laid and survive through to the next high tide cycle:

Disturbed spawning sites on the Motueka River

Sediment affects spawning. This can be a big issue. If eggs get covered in sediment they die, or if the grass where they spawn has high sediment loading, the spawning habitat becomes degraded and less suitable for laying eggs.

Disturbing the area around a spawning site. The photos below show a disturbed spawning site in the Motueka catchment.

Contaminants from the rest of the catchment, or from localised discharges resulting in poor water quality.

Overshading. If trees are planted on river banks at spawning sites, this can shade grasses resulting in poor growth and lack of densely rooted areas for eggs to be laid.

Temperature of the water, which can be affected by climate change or rivers not having sufficient shading.

What does the Tasman Bay Guardians project involve and how has it been going?

Hay bales used to create spawning habitat

TBG has been running the Whitebait Connection programme with schools in Tasman for about 8 years. This involves educating school children about īnanga, why they’re special, and how they live. TBG through the recent Wai Connection funding have also had an opportunity to learn more about our local īnanga spawning sites. TBG have done a sweep of all known and some not so known spawning sites along the top of the south.

We identified some priority sites within the region; these sites were visited monthly around the spawning season to see what’s been going on there. We are hoping to add to the local knowledge by sharing what we have learnt both with the community and with our local Council and Department of Conservation. There are two īnanga habitat restoration projects on the go in the Motueka catchment and a few more in the pipeline. Below is an example of where we’ve created artificial habitat using hay bales, which has been really successful.

Interesting facts about īnanga and other fish in our rivers

There are five different galaxiids in the whitebait catch. Īnanga make up about 90% of catch. There is also giant kokopu, banded and short jaw kōkopu, and kōaro. All have threatened status apart from banded. Short jaw kōkopu is most threatened.

Spawning time is generally in April and May. However at some of the spawning sites, we’ve noticed a longer spawning season – more like February to early September. We’re looking into why this is including measuring the temperature of the water and observing what grasses are there.

Adults lay eggs in the grasses above the usual high tide mark. At the next large high tide (full moon or new moon) the eggs get washed out to sea as larvae. They become part of the food web for fish etc. When juvenile they then swim back to the river, and that’s what whitebaiters catch.

Īnanga that make it past the whitebaiters go upstream. They feed on macroinvertebrates and grow into adult īnanga.

What can Motueka catchment residents and visitors do to protect īnanga and other spawning fish:

Do:

Find out if there are any spawning sites near where you live
Erect signage around spawning sites so people know they’re there
Tell others about the importance of whitebait/īnanga and how we can look after them
If you’re a teacher or a school student, sign up to the Whitebait Connection programme through Tasman Bay Guardians.

Don’t :

drive in the area
remove the grasses/sedges in the spawning area
plant species that create too much shade near spawning areas as this means the grasses are more sparse
mow around spawning sites during the spawning season
let animals near the river where the spawning sites are

Where could I go to find out more information?

Here are links to some great pages and resources about īnanga and īnanga spawning:

Īnanga (whitebait) spawning in the Motueka catchment

Soggy second Old Man's Beard working bee

Looking back over 2024 for MCC