Beloved brown trout damage native fish, insects and waterways
They’re much favoured by anglers but science shows they damage natives fish and waterways.
In 2008 brown trout were recorded for the first time in the upper reaches of Coach Stream near Arthur’s Pass, Canterbury.
Over the previous 11 years, surveys of the upper stream hadn’t detected brown trout.
But suddenly they were present.
They had probably swam through a previously inaccessible road culvert – although it’s possible somebody physically moved the trout over the road.
In any event, the browns subsequently eliminated native Canterbury galaxias fish above the culvert.
In microcosm, that might be the the story of brown trout in New Zealand, as revealed in a New Zealand chapter of a new 808-page international textbook called Brown Trout: Biology, Ecology and Management.
In the chapter, Dr Phillip Jellyman, a freshwater fish ecologist at Niwa, and four others survey the state of knowledge of brown trout in this country. They acknowledge the importance of brown trout to recreational anglers and the NZ economy.
They mention the fierce debates swirling about river flows, irrigation, water quality and the like. But their focus is science.
Trout anglers already know brown trout are mostly daytime drift feeders. In steams and rivers, they eat what comes to them on currents. They identify prey with their eyes, rather than by smell. They don’t eat plants, they eat insects and other fish. In Lake Taupo, they mostly eat smelt.
They were introduced from Europe in 1867 and these days are found from Southland (but not Stewart Island) to Northland, although numbers dwindle further north. They prefer colder water.
They are the most common exotic freshwater fish here. They live in stream, rivers, lakes and estuaries. As salmonids, brown trout can live in salt water and some appear to live offshore for several years.
They can weigh up to 10kg although a fish over 5kg is considered a trophy, Fish and Game says. Fish reportedly as heavy as 19kg have been pulled from hydro canals in Otago.
Anglers say browns are more wary than rainbow trout and therefore harder to catch. They challenge anglers, especially fly fishers, which is why so many overseas anglers come to NZ to catch them.
Recent national data on the economic value of brown trout isn’t available.
In 2011, the Department of Conservation estimated that the Taupo area trout fishery, mostly rainbow trout, was worth $70 million a year.
A 2014 master’s thesis by Liang Jiang estimated the value of the Otago freshwater fishery was $88m-$130 annually.
It’s probably less known that at least two insect species have adapted their behaviour to become more nocturnal feeders, probably to avoid being eaten by trout. Similar patterns have been observed overseas, Jellyman and colleagues write.
They also report that small insects survive better in steams with brown trout than streams without. Browns “preferentially select larger individuals” because the energy return is better.
It’s brown predation on native fish that occupies much of the 19-page chapter.
Jellyman is quick to stress he is a trout angler as well as a fish scientist. He does not advocate eliminating trout from NZ waters and says it would be impossible anyway, both technically and politically.
But the science shows that brown trout can eat their way through native fish at an amazing rate.
In a classic study reported in 1991 and summarised by Jellyman, researchers looked at streams in Otago where waterfalls of at least 3 metres high were present. Waterfalls this size are barriers that browns can’t get past.
Above the waterfalls, native galaxias fish were common and often thriving. Below the waterfalls, galaxias were “repeatedly absent”. This was true regardless of the land-use beside the streams.
Citing more than 100 scientific papers, Jellyman and colleagues detail the “significant deleterious effects on native biodiversity” by brown trout.
At Lake Taupo, in the early 1900s, trout destroyed much of the Māori kōaro fishery. It has never recovered. At one point, trout were starving because they’d eaten everything.
Brown trout are linked, although not conclusively, to the extinction of NZ grayling, the only native fish species known to have gone extinct since European arrival.
Large brown trout eat vast quantities of īnanga-whitebait, probably because their distributions overlap in estuaries and lower reaches of suitable rivers, although it’s not clear this impacts īnanga numbers overall.
In contrast, juvenile kōaro hunting is “particularly intense” because they can penetrate several hundred kilometres inland and brown trout are likely to be present along much of the way.
In wider rivers, however, some galaxiids co-exist with browns probably because they are better adapted to New Zealand’s “globally distinctive” river conditions – floods and low flows are common all year.
It’s complex because so many variables are present: different habitats (estuaries to inland lakes), the age of the brown trout and the age or stage of the natives, whether the natives migrate, flow conditions and a host of other factors.
But the conclusion is stark: “Galaxias biomass is consistently higher in streams when trout are absent”.
Native fish, says Jellyman in an interview, are not well known to New Zealanders. This is probably because they are mostly nocturnal, mostly small and not sold as food. The exceptions are īnanga and eels.
Eels have a complicated relationship with brown trout. It’s long been known that eels eat young brown trout – to the extent that historically there were “intensive campaign[s] of eel destruction in some parts of New Zealand”.
But it another classic study summarised by Jellyman and colleagues, eels were removed from a stream and the condition of brown trout monitored over several years.
The result was a population 250 per cent boom in brown population but growth rates dropped by half – that is, the time needed to reach 280mm increased from two to four years. Their condition was also reduced.
In other words, “predation by eels can facilitate a trout fishery of greater value by suppressing juvenile trout abundance and indirectly enhancing growth of larger adult trout,” the authors write.
Jellyman and colleagues endorse a plea by other researchers for “increased restoration efforts to address impacts of trout on native fish and the seriously endangered galaxiid fauna”.
They also make a plea of their own.
“Angling advocates have generally been at the forefront of efforts to maintain water quality, and have been justifiably quick to point out the positive effects of their efforts,” they write. “Nevertheless, farmers, city‐dwellers and anglers must all accept responsibility and deal with all the problems they have created.
“As part of that, anglers need to accept that the brown trout‐based sports fishing resources, which are highly valued and woven into the fabric of New Zealand life, have had significant deleterious effects on native biodiversity.”
Brown Trout: Biology, Ecology and Management, edited by Javier Lobon-Cervia,