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ST. JOHN'S, NEWfOUNOLANO.
This series Includes unpublished preliminary reports
and data records nol Intended for general dlstrlbullon.
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without clear Indication of their manuscript status.
FISHERIES RESEARCH BOARD
OF CANADA
MANUSCRIPT REPORT SERIES
No. 1009
Techniques for Sampling Zooplankton
and Underyearling Sockeye Salmon
in the Midwater of Large Lakes
by
Bruce C. Andersen and David W. Narver
Biological Station, Nanaimo, B.C.
December 1968
This series Includes unpublished prellmlnrny reports
and data records not Intended for general distrlbullon.
They should not be referred to In publications withoulciearancefrom Ihe Issuing Board establishment and
without clear Indication of their manuscript status.
FISHERIES RESEARCH BOARD
OF CANADA
MANUSCRIPT REPORT SERIES
No. 1009
Techniques for Sampling Zooplankton
and Underyearling Sockeye Salmon
in the Midwater of Large Lakes
by
Bruce C. Andersen and David W. Narver
Biological Station, Nanllimo, B.C.
December 1968
Midwater fishing and zooplankton gear is described
that was successfully used in an investigation of the die 1
vertical movements and feeding habits of underyearling sockeye
salmon in a large lake.
The gear was mounted on a 8 x 3 m,
3-pontoon barge powered by two 65 hp outboard motors.
Sampling gear was handled by use ofa davit and hydraulic
winchpoweredbyalOhpgasolineengine.
A 1m Isaacs Kidd
midwater trawl was used for young sockeye and Miller samplers
were used for zooplankton.
The equipment provided rapid
conversion from one type of sampling to the other.
Operational methods are described.
A study of the diel vertical movenents and feeding habits of juvenile
sockeye salmon in the pelagial zone of Babine Lake, British Columbia was
initiated in 1967 (Narver, MS). Since in any 24-hour period from early July
to October underyearling sockeye appear to move from considerable depths (2140 m) in midday to the lake surface at dusk, fishing gear was necessary that
would sample small fish throughout this depth range. Furthermore, since
zooplankton usually constitute the largest portion of the diet 0 fyoungsockeye,
gear was required that would efficiently sample eight species of zooplankton
from the surface to at least 40m. Since the investigation was based on
frequent sampling through a 24-hr period about every two weeks, both types of
gear had to be relatively easy and rapid to handle. Quick conversion from
trawling to zooplankton sampling was required.
DESCRIPTION OF EQUIPMENT
Al;x3m,3-pontoonbargepoweredbytwo65hpoutboardmotorswas
used to mount equipment as shown in Fig. 1. Each pontoon is 8 m long and 53 cm
indiameter. The steel pontoons were attached together by channel iron and
decked over with 2 cmplywood. Normally, this barge is powered by two 50hp
outboard motors, but trawling required use of two 65hp motors. The additional
weight and power of the 65 hp motors necessitated a planing board to give needed
lift to the stern at full power. The motor control station was positioned 3m
from the stern and .5 m from the port side to make room for handling gear. The
tiller and control cables were aligned under the decking from the control
station to the motors. A flood light was mounted at the motor control station
for use during night work.
~
Sampling gear was handled by the use of a davit and hydraulic winch
(Fig. 1).
The davit, mounted on the starboard side 3.7 m from the stern, was made
in two parts: abaseplateonwhichthewinchwasalsomounted,andaswinging
davit of 10-cm iron pipe measuring 2 mvertically and 46 cmhorizontally. A
wire rope block was attached at the base of the davit and was used a safair
lead so that the winch could be used for any davit position. Ameteringblock
was used at the top end of the davit.
AportablehydraulicpumpingunitpoweredbyalOhpair-cooledgas
engine was used to supply power to the winch. Winch speed could be controlled
by a hydraulic control valve mounted on the pumping unit or by adjusting the
engine speed. The winch drum held 244 m of .5 cm galvanized aircraft cable
(breaking strength 18,140 kg). An eye was spliced in the end of the cable for
attaching equipment.
Sampling gear
Trawling was carried out using aIm Isaacs Kidd midwater trawl which
was scaled down from the standard 3.6 m model. The forebody of the net was made
of 210/10 thread l.3-cm stretch measure knotless nylon and the codend of 210/6
thread .6-cm stretch measure knotless nylon. A zipper in the codend facilitated
removal of fish.
Zooplankton sampling was done with multiple Miller samplers (Miller,
1961) with nets of 223 micron mesh. Miller samplers are efficient (LeBrasseur
etal.,MS,1967) and are easy to use compared to other types of nets.
Midwater trawling
Withthebargeproceedingslowlyandthedavitswingingfreely,the
trawland18mofcablewerepayedout;thenthebargewasswungsothat the
cablewentoverthesternofthebargewherethecablewasputinasnatch
block that was centered on a steel frame built over the outboard motors (Fig.
1). With the barge at half speed the cable was payed out until the desired
depth could be attained; then fUll power was applied and the tow mad eforthe
desired time. To retrieve the gear, the barge was slowed to half speed and the
cableretrievedtoapointwherethelengthofcableoutwaslessthan the
water depth. The barge was then stopped, the cable taken out of the snatch
block and retrieval of gear completed.
All trawling was monitored with a depth sounder that was mounted in
an outboard cruiser and equipped with a paper recorder. When trawling at fUll
power with two 65 hp motors the barge travelled at 11-13 kmph, depending on
wind and water conditions. Trawl depths of over 61 m were easily attained;
the ratio of cable out to trawl depth was roughly 3 : 1. It was not possible
to tow a 1.8-m Isaacs Kidd trawl with this barge.
Zooplankton
For zooplankton tows the trawl was replacedbya 227-kgcable
depressor, a flow meter was attached near the depressor and up to five Miller
samplers were attached at desired intervals on the cable. The davit was locked
in the outward position (Fig. 1) and the required cable was payed out. The
barge was accelerated until the cable angle was 30° tothevertica 1; this angle
was maintained for the duration of the tow, requiring speed of about 4.ll kmph.
If the water depth precluded letting out all therequiredcablebefore
commencing the tow, the additional cable was payed out or retrieved while the
barge was moving. The position of samplers on the cable was predetermined by
trials using a depth sounder as for traWling. A vertical profile of
zooplankton standing crop was obtained by towing samplers at 1, 3, 6, 9 and 15
m in one tow and at 21, 26 and 32 m in a second tow. Since the Miller sampler
is not a closing device, one sampler hauled vertically from the bottom sampling
depth provides information on possible contamination related to setting and
retrieving the gear.
The l-m Isaacs Kidd midwater trawl caught up to 98 young sockeye in a
l2-min tow. However, the gear was apparently selective to size, catching
smaller fish than were captured by night purse seine sets. While fish samples
collected by this gear may not be usable for studies of growth and abundance,
they are useful for a study of food habits, distribution and movements. The
use of this gear at Babine Lake, to our knOWledge, marks the first time
underyearling sockeye have been sampled successfully in deepwat erduringthe
day through an entire growing season.
The barge and gear proved to be quite versatile, but operations had to
be curtailed if waves on the lake became higher than about .6 m. Two men are
required to operate the gear. The cost of the barge, including all gear, was
approximately $6,000.
LeBrasseur, R. J., C. D. McAllister, J. D. Fulton, and O. D. Kennedy. MS 1967.
Selectionofa zooplankton net for coastal observations. Fish.Res.Bd.
Canada, Nanaimo, B. C. Tech. Rept. No. 37, 13 p.
Miller, D. 1961. A modification of the small Hardy Plankton Indicator for
simultaneous high speed plankton hauls. BUll. Mar. Ecol. 5(45): 165-172.
Narver, D. W. MS. Diel activity of underyearling sockeye salmon in the
pelagialzoneofBabineLake,B.C. J.Fish.Res. Bd.Canada(inpress).
davit inmidwater trawl position
pontoon
Fig. 1.
A drawing of the 3-pontoon barge used to mount midwater trawling and
zooplankton sampling gear. The lower sketch shows approximate
placement of the planing board.