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

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ABUNDANCE ESTIMATES OF THE TRAWLABLE RESOURCES
AROUND THE ISLANDS OF SAINT-PIERRE AND MIQUELON
(NAFO SUBDIV. 3Ps): METHODS USED DURING
THE FRENCH RESEARCH SURVEYS AND DISCUSSION
OF SOME RESULTS
by
A. Forest and J.P. Minet
Institut Scientifique et Technique
des Peches Maritimes
Centre de Recherches de Saint-Pierre et Miquelon
97500 Saint-Pierre et Miquelon
ABSTRACT
Since 1971 the "Centre de Recherches de
l'Institut Scientifique et Technique des Peches
maritimes" has carried out surveys on groundfish
stocks of N.A.F.O. subdivision 3Ps.
Since March 1977, two surveys have been
carried out each year (spring and fall) on board
the R/V CRYOS (stern trawler). These consisted
of 30-minute hauls, using a standard bottom
trawl, randomly distributed over the stratified
study area.
Observations on catch data (yield, length
and age composition, maturity stages ... ) and on
environmental conditions were systematically
made and abundance indices were thus obtained
for each sampled stock.
The analysis of data shows that the
coefficients of variation of the stratified mean
yields have an average value close to 0.30 and
that the variance increases faster than the
mean; this is due to the aggregation of
individuals on the bottom.
The different factors which can affect the
accuracy of the coefficients found are discussed
and it appears that in practice, a decrease in
the variability can be obtained only by better
control of fishing gear performance.
Key words: biological surveys, trawl surveys.
RESUME
Des 1971, le Centre de Recherches de
l'Institut Scientifique et Technique des Peches
maritimes a Saint-Pierre et Miquelon a entrepris
des recherches sur les stocks de poissons de
fond de la subdivision 3Ps de la N.A.F.O.
Depuis mars 1977, deux campagnes sont
realisees chaque annee (Printemps et Automne) a
bord du N/0 CRYOS (chalutier peche arriere).
Elles consistent en des chalutages de 30 minutes
effectues a des positions reparties au hasard
dans la zone d'etude selon un processus
aleatoire-stratifie en utilisant un chalut de
fond standard. Des observations sur les captures
(rendements, compositions en taille et en age,
stades de maturite ... ) ainsi que sur les ·
conditions de milieu sont systematiquement
effectuees et des indices d'abondance sont ainsi
obtenus pour chaque stock echantillonne.
L'analyse des resultats montre que les
coefficients de variation des rendements moyens
stratifies sont voisins de 0.30, et que la
variance augmente beaucoup plus vite que la
moyenne; ceci est dQ au type de repartition des
individus sur le fond (repartition agregative).
Les differents facteurs qui peuvent
affecter la precision des indices obtenus sont
discutes et il appara1t qu'en pratique une
diminution de la variabilite ne pourrait etre
obtenue que par un meilleur controle des
performances de l'engin de peche.
Mots-cles: releves biologiques, releves au
chalut.
INTRODUCTION
Among the research activities carried out
by France in the Northwest Atlantic since the
establishment of the "Centre de Recherches
ISTPM" in the department of St. Pierre et
Miquelon in 1969, one important program is
devoted to the evaluation of the trawlable
resources around the archipelago. This area
consists mainly of the St. Pierre Bank, the
Burgeo Bank and a part of the Green Bank as well
as the channels between these banks and the
Laurentian Channel, i.e. the major part of the
actual NAFO Subdivision 3Ps.
In 1971 and 1972, four research trawl
surveys were carried out in this area (Minet,
1975) using standard transect and trawling
stations since, at that time, no stratification
scheme was yet available for the region.
Once the stratification scheme in Subdiv.
3Ps was set up (Pinhorn, 1972; Pitt, 1976) and
adopted by ICNAF, the random-stratified method
described by Grosslein (1969) was used during
the French research surveys. Since 1977, two
annual research surveys have been systematically
carried out, one in the spring and another in
the autumn.
The methods used during these surveys are
described in this paper and some of our results
are discussed, particularly with respect to some
possible sources of bias.
METHODS
The standardization of the research vessel
and of the fishing gear is the basis of the
methods.
1. RESEARCH VESSEL
These surveys are always carried out on
board the R/V "CRYOS", a stern-trawler of 1400
HP and 48.70 m long based in St. Pierre.
Detailed specifications concerning the
capacities of this vessel, laboratories and
working space, propulsion, fishing gear,
winches, radio, navigation and scientific
equipments are given in Annex I.