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

Sample density varies considerably from area to
area and usually is dependent on priority,
logistics and the overall success of the
cruise. In 1980, the year of greatest coverage,
the densities were one set per 10.7, 30.4 and
71.7 sq n miles for the Cartwright, Hopedale and
Hawke channels, respectively. Coverage was as
low as one set per 142.5 sq n miles in the Davis
Strait in 1979. These sampling intensities,
however, exceed minimum requirements (ICNAF) for
groundfish surveys (one set per 350 sq n miles).
EFFECTS OF NUMBERS OF STRATA AND DENSITY OF
SAMPLING WITHIN EACH STRATUM IN THE GULF OF
ST. LAWRENCE
When the first sampling plan was designed
in 1972 for the Esquiman Channel area it seemed
clear that depth would probably be the most
useful variable on which to base a
stratification scheme. The size of the strata
within the depth zones was picked so that the
borders conformed to natural boundaries or lines
of position and yet contained areas in which,
from our relatively scanty knowledge, the
distribution and abundance of shrimps in the
area would be relatively homogenous.
In this it was also recognized that there
appeared to be a general decline in abundance
north to south and from east to west within the
Esquiman Channel and that the size of vessels
being used, as well as the higher shrimp
abundance, generally limited most of the fishing
to the northern part of the area.
These relatively small stratum areas were
used for trips in 1972, 1973 and 1975 when
research vessel trips were mounted which
specifically aimed at obtaining a better
understanding of shrimp distribution, abundance
and biology. In 1976, and during subsequent
surveys in the summer, the major target species
was juvenile redfish though, because small
redfish appear to have similar preferences to
shrimps for depth and temperature, these species
often occur together and can be conveniently
sampled with the same gear. Furthermore,
because the intention of the new surveys was to
cover the complete deep water area of the Gulf
of St. Lawrence, it became necessary to reduce
the density of stations in the Esquiman Channel
area. Thus, the stratification scheme was
modified, reducing strata from 24 to 10 and
using the 50"N parallel as the division between
north and south.
To examine the implications of this change
on the estimates of biomass obtained, the data
obtained in the 1975 survey have been ·
re-analyzed in some detail. Some of the results
of this analysis are summarized in Tables 6 to
8.
In the area north of 50"N, five separate
biomass estimates are available (Table 7): (a}
Estimates using all available small strata are
shown in Figure 1. {b) Catch rates from strata
within the same depth zone were then subjected
135
to an analysis of variance which showed that
{with the exception of one stratum {4))
differences did not exist (P=0.05) between
strata and that it was reasonable to combine
strata within each depth zone. This was done by
maintaining Stratum 4 as a separate stratum, and
an estimate of biomass was obtained using the
strata combined as follows: (1, 2, 3}, (17, 18,
22, 23), (9, 10, 14, 15, 16), (5, 6, 7, 8), and
( 4 )·. (c) The new strata (based on the metric
depths) were used and estimates of biomass
obtained based on a fixed distance of tow of 1.5
miles (assuming a good concordance between
towing speed used and distance covered by the
trawl). (d) The new strata were used but the
distance towed as calculated from the positions
obtained for the beginning and end of each set
was used to adjust the catches to that
equivalent to a tow of 1.5 miles. (e) The area
was treated as a single stratum and the estimate
of biomass derived as the mean of 44 sets made
within the area.
8 10 12 14 16 18 20 22 24
CARAPACE lENGTH (MM)
SEPTEMBER 1978
(78,595)
OCTOBER 1978
(177,169)
Fig. 9. Landings in number/hours fishing in
fall of 1978 - Esquiman Channel, Gulf of St.
Lawrence. (Total number caught.)