In 1867 New Zealand introduced brown trout into its rivers for sport fishing: decades later chaos erupted among local species

A plan to promote recreational fishing ultimately resulted in one of the most damaging biological invasions in the history of New Zealand’s freshwater ecosystems. What appeared to be a harmless act – importing thousands of brown trout eggs from British colonies – eventually transformed entire ecosystems across the country.

It began in 1867, and more than 60 million brown trout were released into New Zealand’s rivers and streams over the following five decades. The objective was to recreate the recreational fishing the settlers had enjoyed in Europe. But the consequences were significant: according to a study published in Biological Conservation, the rapid spread of Salmo trutta, the species’ scientific name, caused native fish populations to decline.

It did more than that. The trout fragmented native fish populations and contributed to the extinction of other local species on both the North and South Islands. Yet the introduction was initially celebrated as a successful example of acclimatization. Early societies built hatcheries and released trout fry into the headwaters of almost every major river system.

Forced into isolation

What the British settlers did not know was that New Zealand’s native fish had evolved in isolation for millions of years, with few predators inhabiting their waters. That became a serious problem. Galaxiids, a group of small freshwater fish, were unprepared for the arrival of such a formidable predator. Everything changed.

Brown trout soon took control of the main rivers, leaving the small galaxiids confined to isolated tributaries. The trout not only preyed on them but also competed with them for other food. A Ministry for the Environment report found that about 76% of native freshwater fish species were threatened or at risk of extinction.

Changes in movement

Besides transforming the native species’ traditional habitats, brown trout have also changed how they move through New Zealand’s river systems. Galaxiids once traveled freely throughout entire river networks. They are now restricted to small headwater areas where waterfalls, shallow channels and other natural barriers prevent trout from reaching them.

The trout’s presence alone has even changed the behavior of insects and other organisms living in the streams. The Biological Conservation study found that in some trout-free streams, native herbivorous insects feed during the day on algae growing on rocks. In streams containing trout, however, the insects hide beneath rocks during daylight hours and emerge to feed at night to avoid being eaten.

The management of brown trout is now a major part of New Zealand’s environmental debate. The fish is an invasive species, but it also supports the recreational fishing industry. Although it is one of the leading causes of the decline in native fish populations, recreational fishing is protected and managed by regional Fish & Game councils.

Strong measures have been introduced in some conservation areas to protect native species. These include building artificial barriers to keep trout out of certain streams and using targeted removal methods such as electrofishing.

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