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

herring were set in comparable depths on both
sides of the channnel and left in position
overnight. Results are not sufficient for
analysis but it is worth noting that more shrimp
were caught in the unfishable grounds (45 vs.
7).
These untrawlable areas are not included
in the estimates of biomass and it is not known
to what degree shrimp in these areas contribute
to the fishable stock. Therefore, biomass is
further underestimated to some extent.
CONCLUSIONS
Research surveys to estimate shrimp
abundance are questionable in terms of their
usefulness. In areas such as the Gulf of St.
Lawrence, it appears that reasonable estimates
of stock size can be obtained from surveys
conducted during daylight hours. In areas such
as Labrador and the Davis Strait variation
associated with the data used to extrapolate
biomass is unacceptable. Bottom photography
used in the latter area also involves extreme
variability.
Attempts to solve the problem, or at least
reduce it to acceptable levels, have met with
limited success. Increased sample size in areas
off Labrador has resulted in some improvement
but indicates as well that high sampling density
is necessary in relatively small areas, and in
larger areas such coverage cannot possibly be
achieved.
Accounting for the pattern of diel
vert i ca 1 migration pr.esents addition a 1
problems. The desired effects are not always
attained. It also appears that the patterns
themselves are variable perhaps on a daily basis
and influencing factors such as light intensity,
currents, fishing pressure, season, depth, gear
and area cannot be ruled out.
The question of stratified random versus
line surveys is academic considering the state
of the art, and, since some of the areas being
studied are not well defined topographically,
the line surveys becomes more attractive if only
for convenience. Mean biomass estimates from
two surveys in the Davis Strait in 1979 were
similar; one used the line method and the other
the stratified random method (Parsons 1979;
Dupouy et al. 1979).
Alternatives are challenging. Acoustic
methods may have potential but will require
considerable research to interpret data which
are masked in bottom echos or echos from
associated fish species. Underwater photography
may be of some assistance in solving these
problems. Abundance-meteorological
relationships should be given high priority (Dow
1979), not necessarily to replace the trawl
survey but perhaps to refine data which they
produce. Association with other species which
are more "well-behaved" provides another
possible alternative. The patterns of feeding
by a major shrimp predator such as Greenland
halibut may give some insight into relative
shrimp abundance.
While new ideas are investigated and the
commercial data base increases, it appears that
the surveys must continue. There are a number
of suggestions which can be forwarded to
ameliorate the accumulating data. Horizontal
openings of various gears used can and must be
measured with considerable accuracy. Settling
and retrieval times for gear under different
circumstances should be monitored and used in
conjunction with accurate records of distance
covered by the net to determine area swept
during each set. Sample size should be
145
maximized and data on vertical migrations should
be collected with each survey. (Depending on
the area it may be more appropriate to increase
sample size than spend considerable time
monitoring diel effects.) Some idea of the
seasonal distribution is mandatory so that
surveys can be timed to coincide with periods of
maximum density. Areas which appear unfishable
must be considered in relation to potential
shrimp distribution and, more importantly, their
contribution (if any) to the fishable stock.
And, finally, attention must be paid to
understand the effects of winds and currents in
affecting the speed of tow and behaviour of the
gear, behaviour of the shrimps, or both.
REFERENCES
Anonymous. 1978a. Report on literature review
of pink shrimp fishery potential in
northeastern Canadian waters for Supply and
Services Canada and Fisheries and Environment
Canada. Maclaren Marex, Inc.
Anonymous. 1978b. Report on exploratory
fishing assessment of pink shrimp fishery
potential in northeastern Canadian waters for
Supply and Services Canada and Fisheries and
Environment Canada. Maclaren Marex, Inc.
Balsiger, J.W. 1979. A review of pandalid
shrimp fisheries in the Northern Hemisphere.
Pandalid Shrimp Workshop, Kodiak, Alaska,
Feb. 13-15, 1979. ·
Carlsson, D.M. 1979. Review and discussion of
current stock assessment techniques used in
the management of Greenland shrimp resources.
Pandalid Shrimp Workshop, Kodiak, Alaska,
Feb. 13-15, 1979.
Carlsson, D.M., Sv. Aa. Horsted and P.
Kanneworff. 1978. Danish trawl surveys on
the offshore West Greenland shrimp grounds in
1977 and previous years. ICNAF Sel. Papers 4:
67-74.
Dow, R.L. 1979. Management strategy for
Pandalus borealis in the Gulf of Maine.
Pandal1d Shr1mp Workshop, Kodiak, Alaska,
Feb. 13-15, 1979.