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

there are fundamental biological reasoirs why increased stocking gives increasingly
poor results. This can be established in several ways. The work of White (1930),
Slretter and}J.aizard (19,+O), and Needham and Slater (1944) showed that, after
stocking, the greater the density of stock was per unit area, the poorer was the
percentage of survival. Again Nicholson (1937) has shown that predation becomes
increasingly effective as the prey species increases because, with an increased
clensity of stock, the probability of encounter of predator and prey increases.
A further effect of change of density of a stock was shown by the writer (Hobbs,
I94O). Spawning areas are limited in extent. As a population develops beyond the
point where additional spawning ground is available, destruction of eggs at an
increasing rate takes place because later spawners disturb older redcls in depositing
cheir own eggs. Pritchard (1939), in a study of the pink salmon, shows how
survival may be conditioned by density of population. Deposition of eggs in a
spawning area and the percentage of resultant migrants which left it in different
years were stated thus:-8.5 million eggs gave a survival, as migratory fish, of
24.0 per cent.; 13.3 millions, 16.7 per cent'; 50.9 millions, 10.6 per cent.;
53.3 millions, 6.9 per cent.; and 139.0 millions, 9.1 per cent.
The basic physical and chemical limitations of environments, which compel
populations to control their densities through competition, bring into play a system
of natural regulation. This system so operates that, on the one hand, it opPoses
with increasing force attempts of a population to develop beyond a certain level.
On the other hand, it permits survival at an increasing rate when, through exploitation
or natural causes, the density of a population drops. The rule that a stock may
be considered as the product of a like number of eggs to that which it produces
is òound only as regards a wild population at íts mean density. \Mhen, through
exploitation, a stock is brought belorv its mean level, the expectation of survival
of the individual rises, and a pair of fish produces more thau one pair. Foerster
(19362) has shown, in the case of an exploited stock of sockeye salmon, that in
some years the spawning of the average pair resulted in the procluction of as many
as five or six pairs of fish.
Elsewhere (Hobbs, 1936) attention was clrawn to a fundamental rveakness of a
system of stocking under which the rate of artificial stocking iucreases only
proportionately to the increase of angling effort. Where natural sparvning occurs
a¡¿ artificial stocking also takes place, stocking must be increased more than
proportionately to the angling elÏort, if the earlier standard of fishing is to be
maintained. Allen (1945) shows that the number of fry present in the Horokiwi
River in October, 1940, was about 250,000. This small stream in that yeaf was
stocked with 10,000 hatchery fry" the same quantity as it receivecl in each of the
previous two years. Thus a twenti-fifth of the fry may have come from the
-hatchery and the rest from natural spawning. If licence sales and angling effort
were dóubled and the a¡tificial stocking were doul¡lecl, there rvould be 260,000 fry,
as against 250,000 when there were half as lnany anglers. To provide double the
amount of angling, the artificial stocking rate would need to be not doubled'
but increased in this case by over 25 times. That is to say, at least 500,000 fry
would l¡e needed; of 'these, not 2O,0OO, but 260,000 lvoulcl have to cotne from the
lratcherv. Even then it is guestionable .rvhether 26o,aæ' hatcher¡' fry rvoultl
Fisheries bulletin (N.Z. Marine Dept.) no. 9 (1948)
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