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

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Table 10. Shrimp biomass estimates (MT), 95%
confidence intervals and the effect of
conversion factors for diel variability (0+1),
1979.
Upper Mean Lower
Unadjusted 157,829 40,076 -77,678
Adjusted 130,289 57,523 -15,242
(Parsons 1979)
Adjusted 161,207 55,659 -49,889
(Carlsson et
al. 1978)
During a survey off Labrador in 1978, a
seasonal decline in shrimp abundance was noted
(Parsons et al. 1980). Biomass estimates were
not considered representative but it is
interesting to note that estimates using night
sets only were higher than those using data from
hours of daylight.
I
In 1979, a definite pattern of variability
was determined from 24-hour sampling off
Labrador (Figure 11). When applying conversion
factors to the data, some problems became
obvious. In many cases, the observed pattern
did not fit the data from within individual
strata. This resulted in artificially high
biomass estimates with increased variance. In
addition, elimination of data collected during
hours of darkness did not increase biomass or
reduce variability (Parsons et al. 1980).
In 1980, the Labrador survey was conducted
at the same time of the year as in 1979. The
period of maximal density was observed between
1200 and 1400 hours (NST) in 1979 in Hopedale
Channel and can be associated with the high sun
period. One year later, catches were optimal
around 1600 hours (Figure 12) and up-to-noon
catches remained consistent and relatively low.
This pattern was also noted by observers onboard
commercial vessels.
These observations would indicate that
high and low sun periods may not necessarily be
the controlling factors in the vertical
migration of shrimp off Labrador.
The indexing and averaging of data from
various areas using two different gears in the
Gulf of St. Lawrence in 1979 are presented in
Figure 13. The highest catch for each area and
gear is assigned the value 1 and other data are
prorated accordingly. If this pattern (observed
over 10 days) is reasonably consistent in the
Gulf, then surveys conducted during daylight
hours in this area should yield relatively good
estimates of biomass without the utilization of
conversion factors. This has received support
from data presented in this paper and through
personal communication with S. Labonte and
J. Frechette, both of whom have conducted
daylight surveys for shrimp in areas of the
Gulf. (Note- A 75% availability factor has
been used in estimating shrimp biomass in the
Gulf of St. Lawrence when utilizing trawls with
a vertical opening of around 3 metres.)
Statistics relevant to the analysis of cyclic
data from the Gulf of St. Lawrence are currently
being considered (Misra, pers. comm.).
An interesting approach to take into
account the vertical distribution has been
attempted which eliminates the necessity to deal
with highly variable catch data (Anonymous
1978b). Ovigerous animals carry the clutch
under the abdomen which obviously hinders free
movement of the pleopods used for swimming. By
noting the proportion of ovigerous animals at
various times of the day (a parameter supposedly
subject to considerably less variation),
patterns of vertical migration were
interpreted. The basic assumption is that as
movement of non-ovigerous animals off bottom
increases, the proportion of ovigerous animals
in the catch of bottom trawls also increases.
The method has interesting possibilities
but with one major logistic flaw: the survey
must be planned during the ovigerous period,
well after spawning and well before hatching.
This period, in some areas at least, is not the
most advantageous time to conduct a survey due
to seasonal fluctuations in abundance and/or
inclement weather.
SEASONAL DIFFERENCES IN DISTRIBUTION
Seasonal differences in distribution and
abundance are a common occurrence in many
Pandalid shrimp stocks and, though major changes
and times of high concentration are often
related to reproductive biology, other
behavioural changes are perhaps more related to
factors such as temperature or the availability
of food (Anonymous 1978a).
Northeast Gulf of St. Lawrence
Sufficient sampling has now taken place in
the Northeast Gulf of St. Lawrence to establish
a pattern whereby during the period when the
shrimp are ovigerous, and when temperatures are
more suitable in shallower depths because of the
breakdown of thermal stratification and mixing
of the upper water layers, best concentrations
are found in the shallower depths, approximately
140-180 m (February). As the cold surface layer
becomes more established, the shrimps tend to
move slightly deeper (and possibly also
northwards in the Esquiman Channel) so that when
the ice departs and vessels are able to start
fishing dense concentrations of these large
ovigerous shrimps occur at depths of 200-220 m.
Once the hatching of the eggs starts, catch
rates which were previously extremely high (as
much as 10 or more times the prevailing rate
during July-August) drop off, and the shrimp