7706

• UCREFRP Catalog Number: 7706
• Author: Mesa, M. G. and C. B. Schreck
• Title: Electrofishing mark-recapture and depletion methodologies evoke behavioral and physiological changes in cutthroat trout
• USFW Year: 1989
• USFW - Doc Type: Transactions of the American Fisheries Society
• Copyright Material: YES

646
MESA AND SCHRECK
each observation interval, the diver counted the
number of marked fish that seemingly behaved
normally (hereafter referred to as normal fish) and
marked fish that behaved abnormally (abnormal
fish), as determined by comparisons with the be-
havior of undisturbed cutthroat trout. We also
snorkeled outside the section boundary to assess
the extent of fish emigration. We plotted the per-
centage of marked cutthroat trout that behaved
normally or abnormally or that were not observed
over time to determine the length of time required
for half of the marked fish to return to the normal
preshock behavioral condition.
In 1988, six sections (some reused from the pre-
vious year; Table 1) were subjected to a multiple-
electroshock protocol. At each section, after block
seines were placed, two divers counted the num-
ber of cutthroat trout longer than 100 mm. One
diver swam upstream and counted the fish in cov-
er and out of cover; the second diver followed
after the first diver had snorkeled about 75% of
the length of the section. After completion of both
fish surveys, we made a single-pass electrofishing
run, as previously described. Captured fish were
held in buckets along the stream bank. Immedi-
ately after the first pass, the divers again snorkeled
along the section, in reversed order of entry, to
count fish located in and out of cover. This se-
quence continued for a maximum of three elec-
trofishing passes. (Equipment failed at section 10
midway through the protocol, and data from that
section could not be used.) The counts of fish after
each electrofishing pass were compared to note
any changes in the proportions of fish in and out
of cover in relation to those seen in the initial dive.
Artificial stream observations. -We conducted
laboratory tests with an oval stream aquarium that
had recirculating water (Reeves et al. 1983). The
stream was 4.3 x 4.9 m on the sides, 0.76 m wide,
and 0.61 m deep. It was filled to varying depths
with gravel and cobble substrates to produce a
stream with four pools (50 cm deep) and three
riffles (40 cm deep). A perforated feeding tube along
the stream bottom evenly distributed food to sim-
ulate insect drift. Each pool contained hollow tiles
or stacked bricks to provide additional shelter.
Lighting, provided by nine 60-W incandescent
bulbs spaced evenly above the stream channel,
was controlled by a timer (Everest and Rodgers
1982) that provided a graded-intensity photope-
riod of 12 h light : 12 h darkness. We maintained
water temperature with a cooling and heating unit
set at 12-13°C, and water velocity (0.0-10.0 cm/
s) with a rotating paddle wheel. Water was con-
tinually passed through a sand filter and ultravi-
olet-light sterilizer; make-up water was added to
the channel at 0.5 L/min. A curtain with screened
windows surrounded the inside perimeter of the
stream to permit observation of the fish without
disturbing them. Fish were fed frozen brine shrimp
thawed in collecting tanks and then passed through
the feeding tube to simulate drift. We conducted
six trials: three with hatchery-reared cutthroat trout
(average weight ± SE, 63 ± 2.7 g) and three with
wild fish (46 ± 5.7 g) captured by angling in local
streams with artificial flies and barbless hooks.
The stream was drained, sterilized, refilled, and
stocked with new fish between trials, which were
conducted from May to July 1988.
For each trial, seven cutthroat trout were al-
lowed to acclimate to the stream for up to 2 weeks
(wild fish often required the longer time). We
judged acclimation by the sustained presence of
active feeding and aggressive behavior that led to
the formation of a dominance hierarchy. During
this period, we fed the fish twice daily (at various
times from 0800 to 1000 hours and 1500 to 1700
hours) and observed their behavior.
After the acclimation period, we conducted fo-
cal-animal sampling (Altmann 1974) during feed-
ing sessions for three consecutive days. Each fish
was observed for 5 min during which the number
of feeding bites taken and the number of aggres-
sive acts elicited and received were recorded. The
number of aggressive acts consisted of a sum of
the individual agonistic elements of salmonids,
including nips, charges, chases, and lateral and
frontal displays, as described by Kalleberg (1958),
Keenleyside and Yamamoto (1962), and Hartman
(1965). The sequence in which fish were observed
was randomized for each feeding session. At 0800
hours on the fourth day, fish were captured by
electrofishing (Coffelt model II-A or Smith-Root
model VII backpack units set at 300 V DC) and
then marked as described for the natural stream
observations. At release, the location and behav-
ioral state of each fish was recorded. Feeding and
aggression data were recorded by use of focal-an-
imal sampling for 2 min each hour during hours
1-7 and once 24 h after release. Because the ob-
servations were made each hour, we reduced ob-
servation time to maximize the time interval be-
tween observations and minimize any effect of
satiation later in the day. All data were converted
to events per fish per minute for comparison.
We calculated mean feeding and aggression rates
for each fish from the pretreatment data. We then
calculated a grand mean for each trial from the