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

BOTTOH TYPE
Another factor which appears to be
correlated with depth to a large extent is
bottom type. An examination of the surface
geology of the Scotian Shelf shows that there
is, in general, a distribution of sediments:
the shallows on the Banks are mainly sand and
gravel grading, through sandy mud and mud with
increasing depth, to silt in the deep water.
These are interspersed with areas of rough
bottom, clay, gravel and rocks. Each type of
bottom supports its own ecosystem of benthic
and epibenthic animals on which the various
species of groundfish prey according to dietary
preferences. Thus, it is reasonable to propose
that the bottom type may be a definitive factor
in determining groundfish distribution, and
changes in fauna related to bottom type would
explain the changes in fish size with depth
which are typical of so many groundfish species
- a general increase in size with increasing
depth.
CONCLUSIONS
Each one of the above factors - depth,
temperature and bottom type - indubitably plays
its part in determining groundfish distribu­
tion, but all are interrelated. On the Scotian
Shelf, bottom type appears to influence local
distribution; the decreasing bottom temperature
from southwest to northeast influences
distribution along the Shelf; and variation in
depth influences distribution across the Shelf.
In summer surveys, when the fish stocks
are most widely distributed in their feeding
phase, the depth-stratified system is probably
as adequate as any on which to base the
sampling design, and it is doubtful if a
temperature-related or bottom-type-related
design would be very different or any more
effective, besides being more difficult to
implement. At other seasons, however, such as
in spring when many species form spawning
concentrations in specific breeding areas, and
in winter when there is a tendency for most
species to leave the shallow water on the top
of the Banks and seek deeper, warmer water
between the Banks and along the edge of the
Shelf, there is a different situation. The
general survey pattern of the summer cruises
with stations fairly evenly distributed over
the whole area, although a slight emphasis may
be given to selected areas, would result in a
great deal of effort for little useful
information in the shallow water areas and
underexpenditure of effort in the areas of fish
concentration. It would seem rational,
therefore, to design surveys for assessment
purposes with a considerable degree of species
orientation in the autumn to spring period.
The design of such surveys would have to be
based on a sound knowledge of the distribution
of the various groundfish species in the
relevant seasons, which can only be gained by
general surveys supplemented by what can be
learned of fish concentrations from the
commercial fisheries. A program of general
autumn and winter-spring surveys has already
been initiated at St. Andrews and may lead,
after a few years, to the design of surveys
biased towards selected species or species
associations, although the need for attention
to minor species should not be neglected.
REFERENCES
Grosslein, M.D. 1969. Groundfish survey
program of BCF, 11oods Hole. Com. Fish.
Rev. 31: 22-30.
Halliday, R.G., and A. C. Kohler. 1971.
Groundfish survey programmes of the St.
Andrews Biological Station. Fisheries
Research Board of Canada - objectives
and characteristics. Int. Comm. Northw.
Atl. Fish. Res. Doc. 71/35.
Scott, J.S. 1971. Abundance of groundfishes
on the Scotian Shelf. Fish. Res. Board Can.
Tech. Rep. 260: 8 pp (2 figures, 8 tables).
193