| Pollution: Horokiwi Stream | |||
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 Where have all the trout gone? In the late 1940s K.R. Allen undertook a survey of Wellington's Horokiwi River. This study, which was published in 1951, focused on what was then a dynamic and thriving trout population. However, Allen's study has long been regarded as more than an investigation into one particular stream: it stands as a benchmark against which the deterioration of many New Zealand freshwaterways can be measured.  The Horokiwi Stream (located near Porirua and now known as the Horokiri) is just one of many streams and rivers that have lost what were once flourishing fish populations. When NIWA personnel returned to survey the eel population in the Horokiwi Stream in 1996, they noted the virtual absence of trout (in 1951 the trout population was estimated at over 16,000 fish; in 1996 only 6 trout were recorded). A study of the physical characteristics of the stream showed that no single event had led to the disappearance, but that it appeared to be due to general habitat decline.Bank erosion  Bank erosion caused primarily by stock trampling, seems innocuous, but can destroy stream habitat by silting up holding pools. Protection of riparian (stream-side) vegetation prevents erosion and restores stream habitat. Native galaxids (kokopu) are virtually absent from streams without overhanging streamside vegetation, while trout populations cannot survive without deeper holding pools, and silt buildup smothers stream insect life, vital food for the trout. Rivers such as the Selwyn and the Ashley in Canterbury now hold only limited trout populations in their upper and lower reaches, mostly due to extensive irrigation use. However, irrigation is only one of a number of reasons why so many rivers have lost their fish populations in recent decades. Change tends to be insidious and it often happens quite slowly. Causes can be many, including changes in land use, farming practices, new housing subdivisions or even changes in riverbank vegetation. Earlier last century swamps were drained and tussock and forest converted into grassland, resulting in faster water runoff, more floods, and the destruction of a stable habitat for fish. More recently we have seen the conversion of land into pine forests with a consequent reduction in water flow and fish numbers. Then we have had the expansion of dairying, with its extra demand for water and the associated problems caused by sediment and nutrient runoff into streams.The principal lesson to be learnt from the decline or loss of many lowland trout fisheries is the insidious nature of the change. Although the instantaneous rate of change may be small, the collective rate over time can be substantial. Reference: NIWA Technical Report 83 The Horokiwi Stream 50 years on: a study of the loss of a productive trout fishery. |
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