N.Z. MARINE DEPARTMENT-FISHERIE,S BULLETIN No. .į

stream, fully one-half of the surface-flowing water has been diverted for
irrigation purposes during drought periods. This diversion race serves a number
of farms. A substantial portion of the lowJevel flow was diverted from the Kakanui
in North Otago to benefit a single landowner. No routine procedure has existed
for according prior consideration to the conflict of interests which nray arise in
such cases.
In particular cases, where knowledge of ihe project and its possible effects
have come to the notice of others concerned in tirne, concessions have sometimes
been made. For instance, an electric barrier \Mas erected on the
opihi diversio' race; elsewhere fish passes (of questionable value) have
been incorporated in dams. Substantial losses of smolts, such as occur in unscreened
diversion races elsewhere, have rarely been reported in New zealand.
The draining of swamþs to increase the extent of p-asture lancls, the straightening
of streams to reduce flooding, the ditching and dráining of fields have pioceedeã
steadily ever since trout were first established here. Such changes tend usually to
reduce the total area of water capable of supporting life and tã increase the rate
of run-off. Generalization as to effects on stocks of trout are unsafe. As regards
large streams, drainage works have usually affected marginal lowlands towards
the river mouths, outside those parts of the catchment arãas where precipitation
is lheaviest. \4/hère this is so, clrainage works may contribute but little to the
instability of the bed of the river.
The clraining of extensive marginal swamp lands along the lower Manawattr
and lower waikato in the North Island "nd, to a less .-*terrt, along the lower
reaches of many other rivers has had the effect of reducing the extent of environment
very suitable for inanga and eels. Any effects on trout have probably been
indirect, and seem most likely to have arisen from changes in the quaniitative
relationship of stocks of trout, inanga, eels, and other forms of life. Ín general,
shallow swamps do not provide ä suitable environment for trout, but mù be of
ìmportance to trout when they produce food or predators which find théir way
out of the swamps into streams frequented by trout.
The collection of surface water and its clirection into channels may be beneficial
to trout where the effect is to concentrate a suffi.cient body of flowing water to
support trout. In the vicinities of Halswell and Springston, north from Lake
Ellesmere in canterbury, a network of drains picks up wãter from low-lying farm
lands. At places such drpins have been dug ihrough soft grognd to undãrtying
shingle. Where this is so, spawning ground is providãd for trout and is extensiel|
used.
The straightening of sections of Waituna Creek in Southland accomplished
much besides a substantial reduction in total length of the stream. A very sluggish
and sinuous stream, with a silty bed $rowing much vegetation, became r *or.
rapid stream with an increased extent of gravel bed, as drertging and more rapid
flow removed fine matter off underlying gravels. Such incieased shingle areas
carne, in a short time, to be used extensively by spawning trout. The work on
Waituna Creek either destroyed or displacecl quantities of eels. Thus a heavily
eel-infested stream became for a time comparatively free of them, and chances of
survival of trout fry were thereby enhanced.
In the Auckland Province, the removal of willows from streams to give improved
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Fisheries bulletin (N.Z. Marine Dept.) no. 9 (1948)