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TROUT FISHING WITH MAURICE RODWAY - Weekly Column: 31-March-00
 Southland, New Zealand

UnderstandingThe Real Causes

A superficial understanding of interactions in complex ecosystems such as forests and streams leads one to believe that the obvious animals have an important influence over the lives of others in it.  When anglers complain about lack low numbers of fish they often blame over fishing by other greedy anglers, or on the presence of shags. The presence of mammalian predators in the bush leads some to believe that they are doing a good job by catching predators. Indeed there are large amounts of money spent of killing the large and obvious predators while equally, or possibly more important factors are ignored.

Fisheries managers used to shoot shags and kill eels in the hope that this would provide more trout and better fishing. Enthusiastic conservationists still kill ferrets, cats and stoats and believe they are saving bush birds from extinction. We discovered many years ago that trout are well able to look after themselves provided they have suitable habitat. In the past the ammunition reserved for shags would have been better spent on saving a wetland, or protecting the headwaters of the streams trout live in.

There are a number of bush birds that cannot survive the attention of predators such as stoats, cats and rats. But there are many species  of native birds are not likely to be extinguished by these obvious causes. A 100 year study in the bush near Wellington revealed that fantails, bellbirds, and tuis remained relatively abundant throughout and even fragile tomtits and grey warblers showed no long term decline and were more common than several introduced birds such as sparrows and starlings.

Like trout these birds have evolved behaviour patterns which enable them to survive the attentions of predators. In the stream trout are produced in vast numbers each year and provided there are habitat types available for each stage of their life cycle no amount of eels or shags will reduce their overall abundance. Other factors such as drought, or other causes of lack of water, drainage of tributary streams, and pollution (especially combined with drought) will.  But even these factors do operate in a straightforward way.  A little bit of enrichment in a stream will increase trout growth and abundance. A pristine river is pretty but will be troutless. Trout move from place to place in a river and need different types of habitat so good trout streams must provide all of these features. In a forest predators may eat each other more than birds and changes in the amount or type of bush might result in competition between native birds, leading to the decline of one. The only thing we can be sure of is uncertainty in these matters.

It seems that our confidence in knowing the cause of something we notice, such as a decline in fish or bird numbers, is proportional to our lack of understanding of the real causes of it.

Maurice Rodway
Southland, New Zealand                           E-mail: information@southlandfishgame.co.nz

Article © 2000 Maurice Rodway, All Rights Reserved.

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