By Guest Work 13/02/2017

New Zealand’s trout and salmon species are here to stay and every year they are evolving to become more a part of the unique fabric of this country, both culturally and genetically. In this two part series Dr Jamie Steer says it’s time to recognise this and celebrate our wildlife – both new and old. 

In 1895, an article entitled ‘New Zealand Trout – the Development of a New Species’ appeared in the Bay of Plenty Times. It reported that “considerable interest has been taken at Home lately regarding the trout which are in New Zealand waters”, noting that it was “now a vexed question among scientific piscatorialists what the nature is of the fish that are in our waters”.

It was reported that introduced trout in New Zealand had “acquired a new character and new habits, and should be designated by the scientific term of ‘salmo Australis [sic]”.

A new species of trout?

New Zealand trout?
New Zealand trout? Author provided.

A few years later, this discussion was furthered in an article in the Transactions of the New Zealand Institute by the chairman of the Wellington Acclimatisation Society, A.J. Rutherfurd. He wrote that: “All these fish are gradually accommodating themselves to their new environment, and becoming very like the varieties found in corresponding northern latitudes… Even within the limits of a single species (so-called) no two are found to be exactly similar, but there is a tendency to diverge from the original type in such direction as to preserve and increase useful varieties – a law of variability by adaptation, which is destined to modify every organism so as to fit it for new conditions of existence… my theory is that, whatever variety we liberate of the ordinary species of trout, it will develop into a Salmo novae-zealandiae, suited to the water in which it is liberated…”

It sounded like a reasonable enough proposition and certainly one that was in line with the evolutionary theory of Charles Darwin. However, it was largely ignored. In 1922, scientist George Thomson dismissed Rutherfurd’s argument in his renowned book on The Naturalisation of Plants and Animals in New Zealand. Like most of his contemporaries, Thomson felt that evolution was a slow process that occurred over extremely long timeframes – certainly not human ones. Furthermore, the notion that humans could have a hand in the evolution of new species was seen as impossible. Humans could select for new domestic varieties like dog or cattle breeds, but this was categorically artificial selection and not the natural selection that characterised evolution. Accepting the familiar biblical narrative, scientists mostly believed that humans could interact with the natural world but they were never part of it.

‘100 years of adaptation’

It was not until around 70 years later that the thesis of evolutionary adaptation in introduced salmonids in New Zealand was seriously entertained. In 1991, popular hunting and fishing author Tony Orman wrote in his book, Fishing the Wild Places of New Zealand, that trout in this country were “the result of 100 years of adaptation”. He recounted a discussion with fishing guide Jack McKenzie: “[He], along with New Zealand’s top fisheries biologists, considers the wilderness rivers are a resource that has evolved over the century since trout were liberated. ‘What is unique,’ explained Jack, ‘is that the headwaters of these rivers often hold a population of rainbows, and to a lesser extent browns, of incredible size, that are the result of 100 years of natural selection’.”

Nevertheless, other than a 1971 study on life history differences between Chinook salmon in their native range relative to those introduced to New Zealand, there had actually been little research into the matter. New Zealand’s top fisheries biologist R.M. McDowall raised the question again in a 1991 article in the New Zealand Journal of Marine and Freshwater Research but provided no support either way. As freshwater scientist Martin Unwin reflected in a 1999 article in Fish & Game New Zealand, “Unfortunately, while this question had been asked often enough by New Zealand fisheries biologists, most of us tended to assume the answer was an automatic ‘no’. The theory of evolution is based on the premise that species gradually adapt in response to their local environment, where the time-scale implied by ‘gradually’ is usually thought of in geologic terms.”

By those standards, the 90-plus years since most salmonid introductions seemed far too short a period for any significant change to evolve. Unwin noted, however, that a 1992 visit by American fisheries scientist Tom Quinn began to question that idea.

South Island salmon

Chinook salmon
Chinook salmon (Oncorhynchus tshawytscha). Wikimedia / Zureks.

Based at the University of Washington, Quinn conducted research on variation in Chinook salmon which had been introduced to the South Island. In a resulting paper, together with Unwin, he tested whether any life history traits of Chinook salmon had diverged since their introduction to the South Island approximately 90 years previously. Among the rivers tested, he found differences substantial enough to be considered “highly significant in a biological as well as a statistical sense”.

In fact, as Unwin later wrote, “virtually every trait we looked at seemed to differ from one [river] population to another” indicating that salmon had diverged not only from their parent stocks, but also from one another according to the catchment they were inhabiting. This had occurred over only around 30 generations since introduction.

Subsequent research has only consolidated the thesis that substantial post-introduction genetic changes have occurred in introduced salmon. These changes are understood to be a consequence of adaptation to different spawning environments, not to genetic drift or founder effects, and have occurred even where discrete populations maintain some level of gene flow. Such rapid divergence prompted Unwin and colleagues to hypothesise in a 2000 paper in the Journal of Fish Biology that Chinook salmon populations in New Zealand were in an “intermediate stage on the evolutionary pathway towards establishment of genetically distinct… life histories”. In other words, the precursors of speciation were already in evidence.

Although these results relate only to introduced Chinook salmon it is instructive to note that this species is in the same genus (Oncorhynchus) as the more familiar rainbow trout, implying that some level of genetic change might also be expected in other New Zealand salmonids.

Evidence, however, relies on research being undertaken. To date, few studies have investigated the potential for evolutionary change in introduced trout in New Zealand. What the studies on Chinook salmon demonstrate, nevertheless, is that meaningful evolutionary changes can occur over relatively short time scales. They are not limited to geological time periods.

Introduced salmon are evolving into uniquely Kiwi varieties and sub-species which may come to be seen as treasured forms of New Zealand biodiversity in their own right.

Continued in part 2…

This article was originally published in Fish & Game New Zealand magazine. The second post in this two-part series tomorrow is available here.

Featured image credit: Les Williams / Flickr.

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