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

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The area bounded by the 100-fath contour
on the east and north, 'by the 4goN 1 at itude 1 ine
on the south, and by the 60°00'W longitude line
on the west was divided into a series of 24
strata following the depth zones 100-120 fath,
121-140 fath, 141-160 fath, 161-180 fath and a
small zone of depth> 180 fath. This
stratification scheme has been described
previously (Sandeman 1g78b) but is described
here also to clarify discussion. Figure 1 shows
the stratification scheme used.
Prior to any sampling cruise, a series of
random starting positions for each set was
generated throughout the survey area by a
computer, and these positions were plotted until
each stratum received an allocation of sets
approximately proportional to its area. As
might be expected from such a random selection
process, a few sets occurred very close to other
sets and some areas were devoid of sets.
Arbitrary adjustments were made such that,
except where strata were narrow because of rapid
depth change, no sets were nearer to each other
than about three miles and a few extra sets were
included where obvious bare patches were noted.
These adjustments and extra sets (also taken
from the randomly generated series) totalled
less than 10% of the total sets. Figure 1 (from
Sandeman 1g78b) shows the stratification scheme
used.
Until 1g76, the same stratification scheme
was used in other surveys (though of course with
different randomly generated set positions).
Subsequent to 1g76, when most surveys were
targeted at both shrimps and small redfish and
when metric charts generally became available,
the stratification scheme was changed. Though
it still used depth as the stratification
variable, the intervals were changed to metric
ones and the number of strata within each depth
zone was reduced. The depth intervals used in
this new stratification scheme are as follows:
160-180 m, 181-220 m, 221-260 m, 261-300 m and
> 300m. It will be noted that the stratum
depth boundaries are quite similar to the
previous boundaries except that the deeper
strata have been combined and an extra level of
shallower strata incorporated. The general
position of these strata has been described by
Parsons (1g78). Subsequently, the shallowest
depth level was further expanded to take in a 40
m depth interval, in keeping with the remainder
of the scheme, so that this stratum level now
contains the depth interval 141-180 m.
Though no figure is included to show the
boundaries of these new strata, their
description, which follows, is relatively
simple:
Using the general boundaries: defined by
the 140 m contour to the east and north,
the 4goOO'N line of latitude to the south
and the 60°00'W meridian to the west (total
area 4225 sq n miles). The new area is
first divided into two by the 50°N line of
latitude. North of this line five depth
strata are designated by the numbers 111 to
115 with the units digit referring to the
five depth zones (listed above) with one
being shallow and five being deep. The
area south of the 50th parallel is
designated by numbers 121 to 125 in a
similar manner. Thus, the 24 strata
previously used have been now reduced to 10
though, in effect, because of the extension
to cover shallower depth zones than
previously the effective shrimp strata are
now really only six or seven.
Because shrimp catches usually vary widely
on a diurnal basis, and because most of the
surveys were conducted using a small vessel
whose work hours were restricted, fishing in
these surveys was normally confined to daylight
hours. Care was always taken to ensure that all
sets were exactly of 30 minutes' duration and
were as similar as possible with respect to net
and towing parameters.
Surveys have been carried out during 1g72,
1g73, 1g75, 1g76, 1g78, 1g7g and 1g80, though in
some years work was limited to the area to the
north of the 50th parallel.
LABRADOR SHELF AND DAVIS STRAIT
Over most of this area, detailed and
accurate bathymetric charts are lacking. The
only exception is the Hawke Channel off southern
Labrador where sufficient information is
available (Warren 1g76) to produce a reliable
stratification chart (Figure 2). The area is,
however, the least important to the commercial
fleet. The Cartwright and Hopedale Channels
have been mapped in a very general way using
soundings obtained from research vessels and
relating these to the existing information on
more general charts (Figures 3 and 4). The
stratification is based on 40 m depth intervals
and areas for each stratum are calculated using
an 'A. OTT planimeter (Table 1). The offshore
grounds on the Store Hellefiske Bank (Figure 5)
are not mapped with sufficient detail and,
consequently, the stratification is based on
100m intervals.
Inaccuracy in the stratification charts
makes the reality of random station selection
rather difficult to achieve. Any such selection
must be completely flexible since the assignment
of random sets to depth strata is prone to
considerable error, requiring adjustment at sea
and resulting in violation of the theory of
randomization. In addition, valuable vessel
time is lost. Areas of abundance off Labrador
and West Greenland are definable in general
terms and, in past surveys, lines have been
selected to cover the entire range for each
stock (Figures 2 to 5). Sets in each of the
depth zones are designated and during the actual
survey are fished depending on the pattern of
shrimp distribution at that time. If
distribution varies between years, adjustments
of set positions are made accordingly. Fishing
is usually conducted on a 24-hour basis.