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

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GROUNDFISH SURVEY TECHNIQUES AS APPLIED TO
ABUNDANCE SURVEYS FOR SHRIMP
by
D.G. Parsons and E.J. Sandeman
Department of Fisheries and Oceans
P.O. Box 5667
St. John's, Newfoundland
A1C 5X1
ABSTRACT
A brief history of trawl surveys for
shrimp and their use to extrapolate biomass are
reviewed. Methodology of stratification by
depth in survey areas in the Northwest Atlantic
is discussed and results are interpreted in
relation to accuracy and statistical
acceptability. Areas outside the Gulf of St.
Lawrence show considerable within-stratum
variation and produce lower limits of confidence
(g5%) which are often less than zero. Patterns
of diel variability are presented and their
interpretation applied to the survey results.
The effects of varying net parameters and towing
procedures are also examined and a tidal model
is presented which closely describes patterns of
variability in catches over a 24-hour period.
Consideration is also given to concentrations of
shrimp in areas which are not fishable but which
may contribute to the exploitable stock.
Key words: diel variability, shrimp,
stratification, trawl surveys.
L'histoire de 1 'echantillonnage de la
crevette au chalut et l'utilisation de ces
releves pour extrapoler la biomasse sont
passees en revue. On examine la .methode de
stratification par profondeur dans les zones de
l'Atlantique nord-ouest ayant fait l'objet
d'echantillonnage et on interprete les resultats
du point de vue de l'exactitude et de
l'acceptabilite statistique. Les zones en
dehors du golfe Saint-Laurent presentent des
variations considerables a l'interieur des
strates de sorte que la limite de confiance
inferieure (g5%) englobe souvent la valeur
zero. On presente des types de variabilite
journaliere et on les interprete relativement
aux resultats des echantillonnages. Les effets
de la variation des parametres du filet et des
methodes de remorquage sont egalement examines
et on presente un modele tidal qui decrit
fidelement les types de variabilite des prises
sur une periode de 24 heures. On examine
egalement les concentrations de crevettes dans
les endroits non exploitables, mais qui peuvent
contribuer aux stocks exploitables.
Mots cles: crevette, releves au chalut,
stratification, variabilite
nycthemerale.
INTRODUCTION
Fisheries for pandalid shrimps in boreal
oceans often employ trawls and trawlers not
unlike those used for groundfish fisheries. The
mesh size is, naturally, somewhat smaller for
shrimp but the techniques of trawling and the
designs of the otter trawls are essentially the
same. Most of our commercial shrimps are found
reasonably close to the bottom in concentrations
sufficient to make a groundfish-type fishery
viable. Despite a considerable movement off
bottom, especially during hours of darkness
(e.g., Smidt 1978), the bottom trawl has been
preferred in most shrimp fisheries of northern
areas.
Research programs tend to use gears which
efficiently catch the species being
investigated. Not surprisingly, and often
consequently, these gears are similar to those
used in the industry. (One notable difference
is that, in research, small-meshed liners are
commonly employed in an attempt to prevent the
escape of small individuals which are otherwise
selectively avoided by the commercial gears.)
Research efforts for shrimp have followed
similar lines and in most centres where such
fisheries are investigated, research vessel
trawl surveys have been long-standing. In many
cases, the surveys have been designed similar to
those used for groundfish species, and in others
both groundfish and shrimp have been
investigated concurrently.
Trawl surveys to locate concentrations of
pandalid shrimps began on the west coast of
North America around 1950, but the use of such
data to produce biomass estimates was not
attempted until the 1g7os (Balsiger 1g79).
Since 196g, Norwegian research has focused on
shrimp distribution in the Barents Sea and
Spitsbergen area and attempts have been made to
produce preliminary estimates of stock size by
the swept area method. Future plans include
stratified random design (Ulltang 1979). On the
east coast of Canada, shrimp biomass
extrapolated from trawl survey data were first
produced in the early 1970s (Sandeman 1978b;
Frechette 1979) and have been used in the
assessment and management of stocks in the Gulf
of St. Lawrence.
Shrimp investigations at West Greenland
have, since 1976, used estimates of biomass from
research and commercial CPUE data as a major
contributor to the management of the resource
(Carlsson 1979). Photographic techniques have
also been developed in this area as an
alternative method for estimating biomass
(Kanneworff 1979).
In more recent years, areas off Labrador
have been surveyed (Sandeman 1978a; Parsons et
al. 197g, 1980) and have produced stock size
estimates which have been plagued by extreme
variability in catches. This problem is
widespread and supports the suggestion by Dow
(1g79) that there is need for an increase in
research on abundance-meteorological
relationships, with less emphasis on cruise