By Bill Morris
On a glorious hot North Otago day over the weekend I dropped a cicada pattern in front of a cruising brown trout – instantly the fish turned and went for my fly. In haste I struck, but too early. The cicada yanked out of the predator’s jaws and I was left empty-handed and cursing loudly, adrenalin and disappointment mingling in that moment. The spooked trout charged off into the depths of the stream, pushing an ominous bow-wave ahead of it. As the ripples of the small drama settled, the water returned to the calm of the morning heat, its glassy surface broken only by the occasional swirl of another rising trout.
I grew up along these waterways and love the thrill of chasing trout here. The river is an ancient stream whose bends and channels have nurtured fish since the uplift of the mountains – native galaxiids, bullies and mud-fat eels, many of which made their way yearly up this river from the sea to breed.
Brown and rainbow trout were introduced to the river sometime in the late 1800’s and established themselves with gusto – today this is a world-renowned fishing stream and anglers come yearly from the United States, Europe and elsewhere to stalk its clear cold waters.
The construction of hydro-electric dams downstream from the 1930’s onward halted the migration of native fish species here. Eels are now rarely seen and those that are spotted are enormous. It is likely these monsters have lived here since the first dam went up, which would make them something like 80 years old. (for a powerful examination of the life of an eel in our rivers, have a look at Longfin, an award-winning Centre for Science Communication film from 2006.)
Fishing is so entrenched in our national culture that it’s sometimes easy to forget that trout are an introduced (and therefore invasive) species that has done enormous damage to our freshwater eco-systems. To the small native fish that once thrived in these streams, their introduction was the equivalent of releasing leopards into a field of lambs. While most people are well aware of the impact of introduced predators like possums and stoats in the mountains and forests of New Zealand, relatively few are as attuned to the threats facing our unique and endangered native fish species. While rare native birds are given world-class and expensive protection status, native fish such as eels and whitebait (which are the juvenile form of some of our galaxiid species) are harvested in large quantities and sold as a delicacy both domestically and internationally.
Our native fish contain within their bodies the dramatic story of our land, a story that encompasses the separation of continents, the thrusting up of enormous mountain ranges and rivers that turned tail and ran. Caught amongst this geological mayhem were the tiny galaxiids, of which New Zealand has 25 species. 20% of New Zealand’s galaxiid species are diadromous, which means they spend at least part of their lives at sea. Larvae hatched in rivers and streams are swept out to sea where they spend their first six months. These larvae have been found up to 700 miles offshore. As juvenile “whitebait,” they congregate around river mouths before making their way upstream. It is here that they are harvested by whitebaiters who can sell their catch for around NZ$70 a kilo.
At various stages, most galaxiid species in New Zealand apparently abandoned the difficult marine phase of their development and began breeding solely in the streams, rivers and lakes they called home. Isolated from one another, they evolved into the separate species we find living here today.
New Zealand, however, is no geological millpond. The clashing of continental plates threw up the Southern Alps in the last 30 million years, a process that is still happening today. As the landscape buckled, tore and subsequently fell apart, water, along with the galaxiids it contained, was moved and pushed across the land in different ways according to the new topographies. In several places in the South Island, whole rivers changed direction as uplift at one end sent the water flowing back the other way.
This reversal of one of these rivers, the Nevis in Central Otago, is reflected in the genetics of its fish. Molecular data has shown that the Smeagol galaxiid (an isolated and distinct form of the Gollum galaxiid, found only in the Nevis River) is more closely related to fish in the southern rivers the Nevis used to feed than to those in the Kawarau, into which it now drains. This is evidence this fish has evolved in isolation ever since the Nevis river changed direction hundreds of thousands of years ago. (In 2010 the presence of this little fish was instrumental in preventing the planned damming of the Nevis River for hydro-electric production.)
In other places, rivers were “captured” by neighbouring catchments as erosion broke down the ranges that divided them, or as the huge lakes of ice that joined their headwaters melted at the end of the last Ice Age. This allowed genetically separate species of galaxiid to inhabit the same waterways. Where these dramatic upheavals caused major environmental changes, such as the damming of a river to form a lake, or the reverse, other galaxiid populations would have rapidly gone extinct.
And so, like drops of mercury on a trampoline, populations of galaxiids diverged, evolved, re-connected and disappeared all across the landscape. Today these little fish can be found all over the country. In almost every branch of every river system they are genetically different. In some cases populations evolved to fit different environmental niches in the same waterway – ‘flathead’ galaxiids well adapted to riffles and rapids have been recorded living just a few metres away from ‘roundhead’ gollum galaxiids, which are better suited to the slower-moving water immediately downstream or upstream.
By studying the mitochondrial DNA of genetically different galaxiids, biologists, working in tandem with geologists, can construct an accurate picture of how New Zealand’s landscapes have changed over millions of years, and how the fish have evolved in reaction to these changes. Where geologists are in disagreement over the date of a particular event, evidence from the genetic material of galaxiids can sometimes provide the answer. The story of our country’s birth is recorded in their genes.
The arrival of trout and salmon, combined with habitat destruction and pollution of our streams and rivers, poses a major threat to our native fish. Our sports fish are here to stay and their presence in our rivers is safeguarded by legislation drafted at a time before native fish like galaxiids were regarded as having any inherent value. As I discussed in my last post, we often encounter a dilemma in New Zealand in which the value (commercial or otherwise) of an introduced species can eclipse the conservation imperatives of a native one. Here is another example. I will always enjoy trout fishing, but I’d love to think that one day there would be places that our native fish species could thrive unmolested by these fierce predators.
As University of Otago freshwater biologist Jon Waters (whose work forms much of the basis for this article) told me, if trout weren’t already here, we would probably still introduce them today;
“We just wouldn’t put them everywhere.”