By Lyn Hamilton-Hunter
Despite the relatively urban environment at Snells Beach, there is an area where one of the whitebait adult species, inanga spawns amongst us. One of their main spawning periods is May, although there is now evidence that this species spawns most months of the year.
Inanga are one of the five whitebait species in New Zealand. The other four are giant kōkopu, banded kōkopu, shortjaw kōkopu and kōaro. All of these are either at risk/declining or at risk/vulnerable, or we are not worried but they are still in decline. Go figure!
So, inanga
These are the most widespread and prolific of the whitebait species around New Zealand. However, their numbers are declining, and much work has been done by Whitebait Connection to restore their spawning grounds in many streams around Rodney, including education work at Snells Beach.
The banks of the stream below Whisper Cove have lots of long grass hanging in the stream and this is ideal for the inanga to hide, spawn and leave their eggs to develop.
This makes it an important ecological site and protecting it vital to help sustain the small population of inanga at Snells Beach, not only for them to spawn but to have somewhere to return to.
Predation is an issue as inanga are affected by Snells Beach mammalian predators, in particular mice. But also rats, and hedgehogs. Snells Shoreline Conservation Community has a trapping programme around the stream and the shoreline, which recently has targeted mice. Humans impact too when we disturb or remove the vegetation.
Life cycle of New Zealand’s native freshwater fish – a brief explanation
Most of New Zealand’s freshwater fish have an amphidromous life cycle, that is they spend the first few months as larvae in saltwater and mature to adulthood in freshwater.
A little about the whitebait species is understood, and other species such as bullies, also have a similar life cycle.
Currently there is a reasonable amount of work being done in the Rodney region to understand and support the life cycle of the inanga. They tend to hang in the lower reaches of the freshwater streams and the saltwater wedge, where the fresh meets the salt at high tide and this is where, if habitat allows, they spawn. Less is known about the other adult whitebait species, banded, giant
and shortjaw kōkopu, and the kōaro spawn further upstream in the freshwater.
Further upstream, at the northern end of Snells Beach there are banded kōkopu in the freshwater streams at Brick Bay winery. There is reasonable cover for them up there and a few spots to spawn in.
For all species, the eggs are laid in the overhanging vegetation and after about four weeks they hatch and swim downstream to the salt water. They spend four to six months in the harbours or ocean feeding on zoo plankton and phytoplankton before returning to fresh water. The existence of a population in the stream will encourage the larval fish to head upstream, because it is thought that the pheromones guide them towards their own species. They are not necessarily heading back to their original birthplace, but from an oceanic pool.
On their return upstream they are still recognised as whitebait, clear larval fish, then as they move upstream, they metamorphose into a recognizable darkening fish and grow into adulthood, and so the cycle continues. Inanga have a shorter life span of about two years and will spawn once sometimes twice. The other species can be long lived and spawn every year once mature.
Niwa has a great poster Life cycles of Aotearoa-New Zealand's five galaxiid species.