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

occurred fairly generally, may be noted. There has been reduction in the accessibility
of water to anglers involving, at the best, the provision of a reasonable
amount of sanctuary water and, at the u'orst, so great a loss of access as sttbstantially
to prevent stocks from being exploited. There has been an increase in
harbourage for tree-frequenting insects, although there is doubt as to the significance
of such insects as trout food.
There has been an increase in the essential environmental factor of cover for
trout and, possibly very much more significantly, of cover for eels. The cover
requirements of trout, in many waters of suffrcient size, may be met by depth
of wate¡ alone. The nocturnal eel, however, requires either soft banks or detritus
to burrow in or large boulders or logs under which to spend the daylight hours.
In many waters there would appear to have been a tremendous increase through
the introduction of willows in the amount of cover for eels, with possibilities o{
a consequent change in the relative abundance of eels and trout.
AQUATIC PLANTS
Conspicuous amongst introduced aquatic plants is watercréss, which Thomson
(1922) states was introduced soon after 1850 to Canterbury, where it developecl
with extraordinary rapidity and strength in the Avon. It has since almost
clisappeared from that stfeam, except as a margin-frequenting plant. It is nou'
found very generally in spring streams in both islands, not only as a marginal
plant, but in open, fairly fast-flowing water, and is often rooted at depths of over
4 ft. Commonly it harbours great quantities of pond-snails, chironomid larvae, and
water-fleas (Poracattioþe-,sþþ), ancl, where the growth is dense enough, crayfish
.and eels. It can, however, so obstruct the flow as to intensify precipitation of flne
defritus and thus leacl to the loss of spawning ground for trout (Hobbs, 1937).
Canadian water-weed (Elod,ea canad,ensis), Thomson states, was introducecl in
1868. Although now very widely spreacl, its development in streams has not been
so general as that of watercress. It is, however, a conrûlon plant, and is especirlly
abundant in association with a species of Nitetta in silt beds in the Avor .ir-r
Canterbury, where it was first established. The usually rapicl character ol Nerv
Zealand. rivers over beds of boulders and gravels does not favour the developmer-rt
of equatic plants, but Elodea is found on silt deposits in tl-re lower reaches of many
rivers, ancl l-ras developed strongly among boulclers in pools in the Kakanui River
in North Otago, where weed-bed provides harbourage for eels.
DIVERSION OF WATERS
The diversion of waters for irrigation purposes' the clraining of swanrps, anrl
the straightening of streams have altered the flow of many streams. Diversion of
.lvaters to irrigate lands and to r,vater stock occttrs on a moderate scale only as yet
in the canterbur¡, ancl North ancl Central Otago districts of the South Islancl.
In these relatively lo.¡'rainfall areas, operations on the present scale would not
infltrence fisheries adversely if lvater were taken only from rnajor rivers, or if
the practice of impounding floocl lr,'aters for subsequent reticulation were adopted.
In areas where drought conclitions are sometimes encountered ancl flolr'' of
surface water almost ceases in sorne streams, fisheries and agricultural interests
rnay conflict sharply. In the Opihi River, a farnous South Canterbury angling
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Fisheries bulletin (N.Z. Marine Dept.) no. 9 (1948)
lr7