Bottom Trawl Surveys - Proceedings of a Workshop Held at Ottawa ...

2. TRAWL GEAR
The fishing gear used during the research
surveys on board the "CRYOS" is the standard
trawl "Lofoten" with 31.20 m headline and 17.70
m footrope, 140 mm stretched mesh in the wings
and body and 50 mm stretched mesh in the
codend. A detailed diagram of this gear is
provided in Fig. 1.
The design of the rigging as well as
details of floats, sweepline, boards and opening
of the trawl are given in Fig. 2. More precise
details of the groundrope and steel-bobbins used
can be obtained from Fig. 3.
3. SAMPLING DESIGN
STRATIFICATION - The scheme used is
described by Pinhorn (1g72) and Pitt (1g76) and
-shown in Fig. 4.
The stratified portion of NAFD Subdiv. 3Ps
represents 17.575 square nautical miles divided
into 33 strata.
SELECTION PROCEDURE - Positions of the
trawling stations are determined by selecting,
for each stratum, the sampling units of the
master chart using a random number table. In
case of rough bottom, as observed on the shelf
of St. Pierre Bank at depths inferior to 30
fathoms in strata No. 314 and 320, the location
of the tow is chosen to be as close as possible
to the originally selected position.
Two to five sets are chosen for each
stratum, depending on its surface.
TRAWLING OPERATIONS - Stations are visited
on a twelve-hour basis, during daylight.
Duration of the tow is 30 minutes on the bottom
(measured between the application and the
release of the winch brakes), at a speed of 4
knots. The length of wire varies with depth by
a ratio of 3 to 4 in these shallow waters, as
illustrated by Fig. 5.
To gain information on the nature of the
bottom, a special device is fixed on the
groundrope to collect a sample of sediments
during the tow. Also a systematic XBT cast is
made during or after trawling.
When the gear grips on the bottom, the tow
may be interrupted before the 30 minutes have
elapsed. If the tow lasts at least 15 minutes,
the results are considered valid and weighted
for a 3D-minute duration. When it lasts for
less than 15 minutes, the results of the tow are
not taken into account.
In cases of serious damage to the gear or
rigging, after a 30-minute duration tow, the
results are systematically discarded.
All this field information as well as that
on meteorology, gear, catches and water
temperature are recorded on the form shown in
Fig. 6A.
SAMPLING OF CATCHES- All catches are
sorted by species. Each species of fish is
weighed; the totality of each species sampled
(or a random sub-sample in the case of large
catches) is numbered. Thus, for each species,
the number and weight per 30-minute tow is
obtained. Among the invertebrates (mainly
crustaceans and molluscs) the non-commercial
species are noted, their abundance estimated and
observations on communities and other pertinent
ecological factors recorded. The commercial
species of invertebrates are weighted and
numbered using the same method as for fish.
The main commercial species (cod, redfish,
American plaice, witch, yellowtail, haddock,
halibut, anglerfish, squid, scallops, ... ) are
measured in totality, or after sampling (random
or stratified according to the size of
species). All measurements are made, after sex
determination, in order to obtain length
frequencies of sexes separately and sex-ratio.
Measurements are made on standard boards (or
with a calliper-square) and recorded on the data
sheet (Fig. 6B). For fish, total length is
measured to the nearest centimeter (or
half-centimeter) below. Squid mantles are
measured to the nearest half-centimeter below
and height of shells to the nearest millimeter
below. For crustaceans, carapace lengths are
measured from the hiatus of the eye to the
posterior edge.
For each commercial fish (mostly gadoids,
redfish and flatfishes) a stratified sample of
otoliths is progressively obtained at each
station during the whole survey. For cod, 4
pairs are sampled per 1 em class for fish
measuring less than 60 em, or 8 pairs for fish
larger than 60 em. For redfish, 10 pairs are
sampled per 1 em class and for flatfishes, 5
pairs are sampled per 1 em class.
For length-weight keys, individual weights
are obtained on small precise scales.
Also, when sexing fish, maturity stages are
observed using a standard maturity scale for
each species and recorded on the forms.
Observations on repletion and stomach
contents are also roughly noted for the main
commercial fish, unless a special feeding study
justifies collection of frozen stomachs.
Miscellaneous observations on catches are
also made and reported on forms (necrosis,
parasites, ... ). Samples of 6 kg of each species
are also collected during each survey, for
pollution studies (hydrocarbons, radio-activity,
etc ... ).
ABUNDANCE ESTIMATES
The following formulae (Pennington and
Grosslein, 1g78) are_used for the calculation of
the stratified mean Yst_(weight or number per
tow and its variance V{Ystl:
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