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

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APPENDIX C
Injuries to Trout in Stripping
In W'estland ]n 1934, an endeavour was macle to learn what losses of trout resu;ltecl fronr
stripping' All fish stripped were m¿rrkecl by fin clipping and released belorv traps. Of 301
fer¡ales and a number of males, seven fish, all females, were recoverecl clead froni pools inspectecl
regularly below traps. The fish were stripped b1' the writer, who rvas much less experienced
than men employed regularly in obtaining eggs for hatcheries. More than ,**al prersr.,.. *",
usecl in obtaining eggs' A dozen fish kílled for examination showecl that the extrusion of ova
hacl been 91.5 per cent complete, as against 88.6 per cent. at the selwyn, where the work of
the usual operative was checked in the same year. The extrusion of ova from the fish in
Westland which died was 94.5 complete. These data thus suggest an outside limit of 2.3 pet
ceut. for losses of mature frsh, which is probably not approachecl under usual conditio¡s.
Where ova are obtained almost 'wholly from wild stocks, it is difficult to lean whether
trout suffer injuries from which they s1ow1y succumb. It is eviclent from reports (Kenclall,
192O-subjoined cliscussion) that stripping of poncl stocks can be carried out without substantial
ensuing loss. Stokell (7936 and 1938) believecl that heavy losses of trout which took
place some miles above the trap-site in the Selwyn River were of fish which, although too
incapacitated by. stripping to return downstream to their usual feecling grou¡cls in Lake Ellesmere,
had got past the trap, below q,hich they were released, or waited until it was removecl
and have then travelled several miles upstream. He hacl accepted the views of Armistead
(1920) and Titcomb (Kendall, 1920-subjoinecl cliscussion) that strippecl trout jour¡ey
upstream and make redds if one or two eggs are left in them. He acceptecl the erroneous
view of Kendall (1920) that ova do trot naturally fall into the abdominal cavity and could not
be extruded if displaced into it. He i:nferrecl that eggs founcl there in clead fish in the Selwvn
had been displaced by handling at the trap.
The reaches of the Selwyn where heavy losses occurrecl are ones in which fish, which are
not stripped, have access before the trap is installed ancl to which further fish have access
after the trap is removed. Fish r:omprising the fraction of the run that is strippecl
were released below the trap, but cotLld have gainecl access to these reaches after its removal-
Observations in New Zealand' are entirely contrary to those of Ar¡risteacl a¡cl Titcon.n
on the behaviour of stripped fish. Percival (1937) quotes Mr. J. Digby, who was in charge
of the Selwyn trap for 14 ¡'ears, as saying that of strippecl females releasecl below, only a¡
occasional one returns to the trap, r:xcept when stripping is carriecl out on a rising river,
when up to 10 per cent. might return within a few hours. The river clid ¡ot rise i¡ the perio¿
in question in 7932 and 1933. In Appr:ndix B it lvas shown that in Westlancl in 1934, stripped
fish released above a trap would not journey up beyoncl the nearest pool offering cover.
There is thus nothing to suggest that any quantity of stripped fish woulcl have sought to swim
up the Selwyn to the areas where losses occurred. The reporte