Whitebait decline likely overstated
¯Inanga will remain viable for generations to come, write scientists David Schiel and Mike Hickford.
Our tiny whitebait are fighting a battle way above their weight class. They have huge cultural value to Ma¯ ori and Pa¯ keha¯ , and we like to eat them in their teeming millions. Fears about their decline, and wildly unfounded claims that they may go extinct, have turned them into the poster child for all that is besetting our freshwater environments.
While it is true there are many things affecting whitebait and their population dynamics, whether they are in serious decline and on a downward trajectory is highly debatable. We know this from many years of research on whitebait and their ecological requirements.
To understand the population dynamics of any species, you must consider their whole lifecycle. New Zealand whitebait are actually five species from the genus Galaxias, but about 90 per cent of the catch are ı¯nanga, G. maculatus. This is a lowland species that, unlike some of the others, is unable to climb over upstream rocks and boulders into higher catchments.
Adult ı¯nanga are generally one to two years old and grow to about 10cm long. During autumn they form loose spawning groups, but only in the salty brackish waters between a stream mouth and upstream freshwater. They spawn just after full and new moons when the tide is high, and lay tiny eggs at the bases of rushes and grasses on the banks of rivers and streams.
Each adult may spawn several times, but generally die shortly afterwards. Eggs develop for a lunar month if the temperature and humidity are optimal, which is why they are greatly affected by damaged vegetation along waterways.
They are inundated at the next full moon or new moon tide, promptly hatch into tiny larvae and are washed out to sea. There they grow for about six months in offshore ‘‘larval pools’’, feeding on plankton.
We know from our studies that they grow slower in the Bay of Plenty and Canterbury than on the West Coast of the South Island. When spring pulses of freshwater are pushed offshore from streams and rivers after heavy rains, the larvae sense the freshwater plume and follow the signal back to the coast.
As they arrive inshore, they quickly develop the ability to adjust their salt balance as they move from marine to freshwaters. It is these post-larval fish that swarm into rivers and form the whitebait catch – one of the few fisheries in the world that targets fish of this age.
Even if not caught by whitebaiters, the vast majority of whitebait die within several months of entering streams, as predation by birds and larger fish such as trout, and food limitation whittle down their numbers.
Most ı¯nanga die when they are between one and two years old, so to sustain our whitebait populations constant replenishment is required.
There is a general belief among scientists and the public that whitebait are not as plentiful as they once were.
The evidence for this, however, is piecemeal and entirely anecdotal. There are no records of whitebait catch, except for those of a few fishermen who keep diaries of their own catches.
Individual catches may be declining, but there are many more people fishing now than ever before. The fact that the great majority of whitebait will die soon after entering waterways makes it even harder to gauge what the effect of fishing might be – but it provides fertile ground for speculation.
We know, however, that many streams and rivers are homes for ‘‘sink’’ populations. Here the ı¯nanga lifecycle is often broken because riparian vegetation has been so altered in many places by urban development, barriers, stock grazing and other interventions that appropriate egglaying habitat is severely reduced or gone altogether.
As a result, even though adult populations may be robust, they cannot contribute to the larval pool and the next generation.
The feedbacks between egg laying and production of the next crop of whitebait are unknown because of the highly variable and complex environmental influences during their oceanic development.
Nevertheless, because the riparian zone of so many waterways is compromised, it is highly likely that this has negatively affected the whitebait run over the past several decades.
But there is much to be hopeful about. We are accumulating enough evidence to indicate that more area-specific regulations may assist this fishery, for example by taking account of the clear differences in size and age of Bay of Plenty and Westland whitebait.
As well, the push by Federated Farmers and the National Policy Statement for Freshwater Management to fence stock from waterways and plant riparian grasses is likely to produce positive benefits for whitebait production.
Cities such as Christchurch have stopped mowing the riparian margin along local rivers, which has resulted in significantly more egg laying by whitebait.
As with most environmental issues, increased knowledge and multiple avenues to solutions will probably work best.
In the case of whitebait, it is far from doom and gloom. These fish will remain a viable New Zealand icon for coming generations, and will also no doubt feature on the menus of discerning Kiwis for some time to come.