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

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arhythmic in spring-early summer to regular diel
migrations in late summer-fall. Since the Shelf
survey is conducted during the former and the
Gulf survey during the latter period, the
regularity of migrations could well be an
important factor contributing to the Gulf
survey's enhanced results. The other factor to
be considered is horizontal dispersion, which
may also be related to vertical migration
patterns. Brunel (1972) observed that the
periods of most regular and intensive diel
vertical migrations of Gulf cod coincided with
periods of greatest horizontal migration (and
possibly less contagious horizontal
distribution).
In cone 1 usi on, it would appear that whi 1 e
some methodological changes, for example to the
stratification schemes, may have a considerable
effect on the precision of estimates for some
species, it is fish behavior which ultimately
determines the magnitude of the variances. The
success of the Gulf survey for cod is probably a
matter of being in the right place at the right
time of year. The survey is, more by chance
than intention, designed for cod. It
demonstrates that with readily obtainable
information on changes in horizontal and
vertical distribution patterns over time,
species-specific surveys could be designed which
give results that meet the demands of present
fisheries management techniques.
10
10
Plaice Mjt·
Gulf .. /
" ', /
Time (Houra}
' 1
20 24
Fig, 11. Mean number of cod and plaice caught per
tow during each hour of the Scotian Shelf
(July) and Gulf of St. Lawrence (September)
groundfish surveys, 1970-79. Solid lineadjusted
mean numbers; dashed line - 3-hour
running average.
400
300
200
BIBLIOGRAPHY
Beacham, T.D. 1980. 1980 assessment of cod in
Division 4T and 4Vn (Jan.-Apr.). CAFSAC
Res. Doc. No. 80/22.
Beamish, F.H.H. 1966. Vertical migration by
demersal fish in the Northwest Atlantic.
J. Fish. Res. Board Can. 23: 109-139.
Boerema, L.K. 1964. Some effects of diurnal
variation in the catches upon estimates of
abundance of plaice and sole. Rapp. Cons.
Explor. Mer 155: 52-57.
Brunel, P. 1964. Food as a factor or indicator
of vertical migrations of cod in the
western Gulf of st. Lawrence. ICNAF
Environmental Symposium, Rome 1964,
Contribution No. C-2, p. 1-16.
1972. The Gaspe cod ecosystem in the
--""'Gu"l.,f: of St. Lawrence. III. The daily and
seasonal vertical migrations of cod (Gadus
morhua) in 1960-62. Nat. Can. 99: --
287-357.
Cleary, L. 1979. Flatfish of the Scotian
Shelf. CAFSAC Res. Doc. No. 79/27.
deGroot, S.J. 1964. Diurnal activity and
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Explor. Mer 155: 48-51.
El-Sabh, M. I. 1973. Seasonal and 1 og-term
variations of the water properties in the
Gulf of St. Lawrence. In Proceedings of
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and Estuary of St. Lawrence, Rimouski,
Oct. 11-12, 1973.
Grey, D.F. 1979. 4VsW cod: background to the
1979 assessment. CAFSAC Res. Doc. No.
79/30.
Grosslein, M.D. 1969. Groundfish survey
program of BCF Hoods Hole. Comm. Fish.
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1971. Some observations on accuray of
--a"'b:-:-u-ndance indices derived from research
vessel surveys. ICNAF Res. Doc. 71/59.
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Halliday, R.G., and A.C. Kohler. 1971.
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Andrews Biological Station, Fisheries
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characteristics. ICNAF Res. Doc. 71/35.
Halliday, R.G., and P.A. Koeller. 1981. A
history of Canadian groundfish trawling
surveys and data usage in ICNAF Divisions
4TVWX. In W.G. Doubleday and D. Rivard
(Ed.). Bottom Trawl Surveys/Releves au
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