7118

• UCREFRP Catalog Number: 7118
• Author: Gatz, J. A., J. M. Loar and G. F. Cada
• Title: Effects of repeated electroshocking on instantaneous growth of trout
• USFW Year: 1986
• USFW - Doc Type: North American Journal of Fisheries Management
• Copyright Material: YES

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178

TABlE I.-Number of rainbow and brown trout with
instantaneous growth rates reduced by electroshocking
as a function of number of shockings. Expected values
are in parentheses..

Number of trout
Nll11Iberof with lower than
ekan>- average growth
shoctings rates

Number of trout
with greater tban
or equal to
average growth
rates

Total
number
of trout

,,;3 29 (32.1) 16 (12.9) 45
<<4 16(12.9) 2(5.1) ]8

All 45 ] 8 i 63

. xl - 3.613; df = 1; P < 0.10 for the n\ll hypothesis that the
number of eIectroshockings has no effect on instantaneous growth
rate.

nilicant. The time elapsed since the most recent
previous electroshocking (i.e., recovery time) did
matter (Table 2). Trout recaptured 1.5-2.5 months
after an earlier electroshocking were significantly
more likely to show below-average growth .'fates
over their entire capture histories than repiatedly
electrosbocked trout that had not been electro-
shocked so recently. Finally, the age of the trout
affected the reduction in growth rate (Table 3).
Age-3+ trout that had been repeatedly e]ectro-
shocked showed no tendency for reduced growth
rates, whereas age-] and age-2 trout had be]ow-
average growth rates 84 and 89% of the time, re-
spectively.
Decreases in growth rates were severe. Of the
'17 trout that showed below average instantaneous
growth rates from among those electrosbocked less
". than 3 months earlier, ] 5 (55%) actually lost weight
"over the 1.5-2.5-month interval between captures.
.Instantaneous growth rates of the other 12 fish in
this category averaged only 51% (range, 3-87%)
oftbe growth rates for all trout of the same species
and age at their respective sites. Although less fre-
quent, reductions in growth could also be severe
among trout with 3 or more months to recover
from electroshocking. Weight losses over the en-
tire time interval between captures occurred in
only three trout with this capture history (17% of
those with reduced growth rates). However, in-
stantaneous growth rates for the other 83% of the
trout with reduced growth rates averaged only 4 ] %
(range, 7-87%) of the growth rates for all trout of
the same species and age at their respective sites.

Discussion

Most investigations'on the adverse effects of
e1ectro&bocking on fish have dealt with acute re-
sponses, such as death or physical injuries, caused

GATZ ET AL.

TABLE 2.-Number of rainbow and brown trout with
instantaneous growth rates reduced by electroshocking
as a function of time since last capture by shocking.
Expected values are in parentheses..

Months since Number of trout
previous Number oftrout with greater than
capture by with lower than or equal 10 Total
electro. average growth average growth number
shocking rates rates of trout
<3 21 (22.1) 4 (8.9) 31
<<3 18 (22.9) 14(9.1) 32
All 45 18 63

. x2 = 1.411; df = I; P < 0.01 for the null hypothesis that
recency of last shocking has no effect on instantaneous growth
rate.

by single electroshocking events (e.g., Hauck 1949;
Vibert 1963; McCrimmon and Bidgood 1965;
Spencer 1967; Adams et al. 1972; Whaley et al.
1978), although Saul (1980) investigated the ]ethal
effects of repetitive electroshocking. Two studies
have dealt with the effects ofa single electroshock-
ing on reproduction. Maxfield et al. (]971) found
no effect of a single e]ectroshocking on subsequent
fecundity of shocked fish or the survival and growth
of their young, whereas Marriott (1973) reported
a higher mortality in eggs from shocked ripe fe-
males than from unshocked ripe females. The few
earlier studies that have pertained to growth (Hals-
band 1967; Egglishaw 1970; Maxfie]d et a!. 197 I;
Ellis 1974; Kynard and Lonsda]e 1975) found no
effects of e]ectroshocking. Because the results we
report here differ from those of these earlier stud-
ies, a detailed comparison of procedures is desir-
able.
The experimental procedures employed by
Maxfie]d et al. (197]), Ellis (1974), and Kynard
and Lonsdale (1975) were similar to each other
and differed from our study in three major re-

TABLE 3.-Number of rainbow and brown trout with
instantaneous growth rates reduced by eJectroshocking
as a function of age ofthe trout. Expected values are in
parentheses..

Number of trout
Number of trout with greater
with lower than than or equal to Total
average growth average growth number
Age rateS rates oftrout
1 16(13.6) 3 (5.4) 19
2 16 (12.9) 2 (5.1) 18
3+ 13 (]8.6) 13 (1.4) 26
All 45 18 63

.)(2 = ]0.043; df = 2; P < 0.01 for the null hypothesis that
age of the trout has no effect on instantaneous growth rate.

REPEATED ELECTROSHOCKlNG EFfECT ON GROWTH

179

spects. In their work, fish were (1) exposed to a
single electroshocking experience, (2) not handled
subsequent to electroshocking, and (3) maintained
and fed under hatchery-like conditions prior to
assessment of the effects of electroshocking on
growth. Al] three of these differences from our
experimental design could contribute to the dis-
similarity of results.
Consider these differences seriatim. First, our
repetition of shocking could be critical. Although
no one has previously studied the effects of re-
petitive shocking on growth, repetition of shocking
does increase the one effect, mortality, that has
been studied (Saul 1980). Second, a general stress
response may have contributed to our growth re-
sults, although previous work offers litt]e support
for this idea. Stress could have been caused by
handling, fin clipping, or MS-222. Pickering et al.
(1982) found no effect of handling on growth rate
but did find that handling decreased feeding by
brown trout for the next 3 d. Their results suggest
that part or all of the short-term 5% average weight
loss reported here may have been a response to
handling stress but not the long-term changes in
growth. Similarly, previous workers have not found
that fin-clipping decreased growth of either hatch-
ery or wild trout (Shetter 1952, 1967; Brynildson
and Brynildson ] 967) even when fin-c]ipping was
combined with the use of MS-222 (Stauffer and
Hansen 1969). This last result does not allow us
to rule out the possibility of an adverse growth
effect in response to multiple exposures to MS-
222. In our work, however, reductions in growth
were similar in 1982 when MS-222 exposure did
not accompany each electroshocking and in 1983
when each electroshocking was accompanied by
MS-222 exposure. Thus, repeated exposure to MS-
222 does not seem to be the cause of the decreased
growth rates we observed. Finally, the high avail-
ability of food under hatchery-like conditions
might mean that only severely weakened fish would
show any decrease in growth rate under the ex-
perimental design used by others, whereas in a
. field study such as ours, the ability to feed, and
I hence also growth, might decrease greatly for even
slightly weakened fish.
The remaining previous study (Egglishaw 1970)
bears more similarity to our own, although one
critical difference exists. Like our work, EggIi-
shaw's was a field study in which salmonids were
subjected to repeated (seven to eight times per
year) e]ectroshockings. The difference is in how
Egglishaw assessed an effect on fish growth. We
calculated growth rates for individual marked fish,

~

whereas Egglishaw compared age-specific mean
weights for fish collected in three study sections
with the corresponding weights for fish collected
from control areas (areas electro fished once a year),
upstream and downstream from the study sites.
This difference in whether or not marked fish were
used has considerable ramifications. Our proce-
dure ensures that individual fish were, in fact, re-
peatedly electroshocked, and that growth rates cal-
cu]ated are for fish with a known history of
electro shocking. In contrast, although Egg]ishaw
repeatedly sampled tbe same sections (27.5 to 30.8
m long) of the stream, neither natural nor exper-
imental barriers prevented movements either in
or out of the study areas after sampling was com-
pleted and the stop-nets were removed. Any fish
that emigrated from the study areas, after having
been sampled one or more times, not only would
have avoided the repeated electroshockings but
also would have been excluded from the growth-
rate calculations for the study section. Instead, pre-
viously unshocked fish could move into the r(TIi-
tively short study sections from elsewhere in the
stream. If such mixing occurred, the uniformity
of growth rates in the control and study areas that
Egglishaw reported is to be expected.
Our own observations suggested that such hy-
pothesized movements were likely. First, 40% of
the trout we captured four or more times moved
between subsections at our study sites. Our sub-
sections averaged nearly three times as long - 8].8
m versus 28.8 m-as EggIishaw's sites so that an
even higher rate of migration might well have oc-
curred there. Second, among all our sites, an av-
erage of 15% of the marked fish emigrated from
the entire study section during just the mark-re-
capture interval of 24-72 h. Again, higher emi-
gration rates might be expected either from smaller
sections or over longer periods of time. Oear]y,
migration of stream-dwelling salmonids out of
short study sections is a phenomenon to be taken
into account, especially in long-term studies. In
the present case, it may well explain why Egglishaw
(1970) found no effect of repeated electroshocking
on growth.
The exact cause of the decrease in growth rates
we observed for repeatedly electrosbocked age-l
and age-2 trout is not known, but it is highly un-
likely that the results were artifacts of our meth-
ods. A nonrandom sampling for slow-growing trout
is unlikely because repeatedly electroshocked trout
averaged the same initial length and weight as the
other trout of the same age at their respective sites.
Spurious results due to changes in the reproductive