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

fishes are desirable could be determined only from a study of the relative abu¡clance
of vegetarians ancl carnivores and the needs of particular populations of trout
in the particular parts of different streams.
Consideration of the need for food supply and the possibilities of adclitions by
the establish'rent of importecr species shourcl have a pråp., prace in any long-terÁ
plans for development of inland fisheries. Most of th"- r."iur components of the
faunas of trout streams in other countries have their counterparts in the faunas of
trout streams in l{ew Tealand. The environmental requirements of all components
are to some extent specialized. Inter-relationships are.Ír1ore often complex than
sirrple, and at'e still so little known that the effects oi introducting new species
cannot be foretold with a'y degree of certai'ty. rn order to minìmizu d^ir"p_
pointment, waste of money, and possibly unwarranted efforts, the introduction
pf t'out foods shoutd be left to the direction of biologists, who
might favour the introduction of particular forms when it is demonstrated
as a reasonable probability that_
(1) Additional food is needed.
(2) Lile forms usefur as food and suited to the envir,onments offering
can be introduced.
(3) Food and other environmental essentiars will be avairable for such
animals without the displacement of varuabre indigenous forms.
(a) r,ife forms introduced will not have harmful characteristics.
(5) Their introduction will be so controlled that noxious'forms of life_
parasitic or otherwise-will not be accidentaily introduced with
' them.
PREDATIO\I AND COMPETITION
Eels
The presence in nlost trottt r,vaters of very great cluantities of eels is evicle'ce
that the foocl resources of different waters procltrce a great weight of fish flesh
adclitional to the trout flesh procluced. Recorcls of captrrrãs n-racle by the Southlancl
Acclimatization Society in the Fledgehope Rivel show that in 1g37-g quantities
of long-finnecl eels (1. rtieffenbachü) weighing approxirnatery 2,uJolb. per mile
of strea'r (approxi'ratery 15ft. to 30it. "tct.) *... ,..,rro,,rã¿. rn. progressive
cl_ecline i' the e'ergetic exploitation of eers by Maoris. especiaily in parts of trre
North Islan