Are trout a threat to native fish species?

By Rex N. Gibson

This is a situation where you have to see the forest, and not just the individual trees; or more correctly see the ecosystem and not just individual species interactions.

Brown trout (three in total) were first successfully introduced to the Avon River in 1867 from the U.K., via Tasmania. Further liberations from imported fertilized ova followed in many locations around the country. By the 1890s they were well established in many waterways. Parallel liberations of rainbow trout from California occurred over the 1880s to 1890s. The rainbows did better in the main North Island lakes and the browns throughout the South Island.

They filled, and more, the niche of the Australasian grayling which had already begun its rapid disappearance from the country’s rivers before the introduction of trout. An extinction significantly hastened by changes in surrounding land use.

By the early 20th Century trout numbers had largely stabilised indicating a balanced food web existed in their habitats. “Balance” is used here but ecosystems are always in a state of dynamic equilibrium as they respond to varying environmental factors.

The first significant study of what trout in New Zealand eat, was done by Dr K. Radway Allen, in the 1940s. He worked for the Marine Department who oversaw freshwater fisheries in those days. He was the New Zealand government’s first professionally trained fisheries scientist. The subsequent publication of his studies on the Horokiwi stream (New Zealand Marine Department – Fisheries Bulletin No.10), which flows into Porirua harbour, became a primer for production biology studies in universities across the world.

By the 1960s, Allen had become research director at the Fisheries Laboratory of the Marine Department.

During his New Zealand career, he was a founder of the New Zealand Ecological Society and its first president, and was elected Fellow of the Royal Society of New Zealand in 1961. He left NZ in 1964, frustrated with unhelpful (more) senior bureaucrats and aspects of the public service work ethic.

Allen, assisted by his wife, examined every organism in that stream, down to microscopic level, that was a part of the trout’s food chain. In his words it was “to study a single New Zealand [brown] trout stock as comprehensively as possible”. He examined all the limiting factors affecting that trout population which had been established there for over 30 years.

Records showed that the Horokiwi trout numbers were stable over the period of 1938-1949. His study showed that from 900,000 ova, 500,000 fry resulted annually. By October of the first subsequent year there were just 3,700 (0.74%) left. The second year had 1,200 survivors; the third had 300, the fourth 85 and by the fifth it was 15 (= 1 in 60,000).

The trout needed to eat just 1.23% of their weight daily to survive. Each pound in weight increase needed 4.2 lb of food. This figure is much more efficient than that of farming livestock.

He found that the main sources of food were threefold:

  1. Bottom fauna; mainly stream insects.
  2. Surface food; mainly insects, etc., which fell, were washed in, or blown into, the stream.
  3. Small fish of various kinds; but only for trout at least two years old.

Bottom fauna formed 95%, by weight, of the food eaten in their first two years and 75% thereafter. Allen stated:

“When we realise that between 80 and 90% of the total food the trout eat is taken in their first two years (i.e. before they have begun to eat small fish) we see that small fish only make a very small contribution to the total amount consumed”

The main diet of bottom fauna was primarily caddis fly larvae (several species), mayfly nymphs (3 main species), beetles (various), freshwater snails, and earthworms (including those living in the stream bed). Of these, caddis, mayflies, midges, one snail species and adult parnid beetles dominated. The high dietary reliance upon insect larvae, pupae, nymphs and adults, allows trout to be a favoured target for fly fishing.

The trout in this stream supported about 4-500 lb of (long-finned) eels per acre; around twice the density of trout; but only 20% of these eels were large enough to eat small fish. This 20% thus dined well. Various fish-eating birds also benefitted from the young trout at Horokiwi. They included the Black Shag, White-fronted and Caspian Terns, the Kingfisher, Blue Heron and the Bittern.

Most trout population fluctuations in this study were related to flood events. The significance to different parts of the stream was related to adjacent land use and bush cover. Angler catch was distorted by the war years and petrol rationing, but returned to “normal” after 1946.

In the lower paddock section of the stream the total weight of trout flesh formed during the life of a year group was 3,750 lb. Of this 58% was lost by natural deaths whilst too small to be harvested. A further 35% died naturally as adults and only 250 lb (7%) were caught by anglers. In the bush-fringed upper stream area the catch was just 1%. The rest of that trout flesh ends up being consumed by predators, primarily native, or decomposing to return the protein to the waterways from whence it came.

Allen compared the trout meat production in a typical trout stream favourably with farming practices of that time stating that the stream was “actually producing just as much meat [per acre] as land under good farming management”. Farmers however are more efficient in harvesting than anglers.

A flurry of post-war studies in Europe duplicated Allen’s findings right through the 1950s and it became completely legitimised by J.M. Elliot’s (Exeter University) studies from the 1960s. His work from Dartmoor in England to Turkey all gave similar results, in terms of trout diet, to that of Allen. Electric fishing techniques had replaced Allen’s mesh nets but the results were parallel to his.

In recent years print materials from DOC, academia, and various popular magazines have pushed the case that trout are a major threat to some of our native fish species.

Why so, after 150 years? The real issue is that since 1990 there has been a major shift in agricultural land use, and in both river and ground-water quality and quantity. This has impacted dramatically on trout. Anglers were amongst the first people to be agitated by these changes. Numerous angling writers have used the phrase that “trout are the canary in the mine” (Re. water issues) for good reason.

The wide varieties of issues that adversely affect brown trout throughout its range do not affect them exclusively. In New Zealand’s waterways, brown trout are important consumers of macroinvertebrates, and declining brown trout populations in these specific areas affect the entire aquatic food web. Similarly the chemical warfare tactics of DOC and Regional Councils, in plant and animal pest control, have often also significantly reduced macroinvertebrate numbers. The “knock on effects” include lowered trout food sources.

This relatively rapid environmental shift, due primarily to agricultural intensification, has upset the balanced ecosystem of which trout was an inter-dependant part. Wetland drainage has also aggravated the habitat destruction. In Canterbury, where dairy now rules, there is 70% of N.Z’s total irrigation. This removal of river water continues to devastate the habitats of native fish, trout and salmon.

Image result for photos irrigators

Several recent studies have shown that trout have become localised predators and competitors of species which were relatively “safe” in the past. Just as Europeans turned to eating rats and pets in times of famine and sieges, so have trout been forced to change their diet; or starve. To blame the trout for this is akin to blaming the starving population of Leningrad/St Petersburg (1941-44) for their starvation when besieged by Hitler’s Nazi troops, or blaming the individual soldiers for the disastrous battles on the Western Front in WW 1. The expression of “shooting the messenger” also comes to mind.

The action when trout suddenly become a problem should be to look at the reason. Ask is it trout-made or man-made? Has it been caused by trout behavioural changes or a perceived human economic imperative? Will it be solved by demonising trout or by solving the man-made environmental or economic issues behind the problem?

It is not brown trout that sometimes threaten native fish species but the unsustainable environmental degradation inherent in much of the intensive agriculture that now dominates Canterbury.

Brown trout proved for a hundred years that they had acquired a legitimate balanced place in our New Zealand environment prior to corporate ownership and intensive farming; a legitimate place in Kiwis’ outdoor recreation, and a legitimate place in the well-being of the 100,000 fishing licence holders and their families.

The giant [grotesque] cannibalistic trout in the hydro canals are a classic example of what happens when human activity creates a highly modified environment. But that will need to be the theme of another article; along with the mice-eating trout in the beech forest mast-years.

Footnote: Rex N. Gibson is the Freshwater Spokesman for the NZ Federation of Freshwater Anglers. He is an ecologist and scientist and with a deep personal interest in rivers and outdoor recreation.

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