BOTTOM TRAWL SURVEYS DESIGN, OPERATION AND ANALYSIS
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BOTTOM TRAWL SURVEYS DESIGN, OPERATION AND ANALYSIS |
| INTERREGIONAL FISHERIES DEVELOPMENT AND MANAGEMENT PROGRAMME - INT/79/019 FISHERY COMMITTEE FOR THE EASTERN CENTRAL ATLANTIC | CECAF/ECAF SERIES 81/22 (En) |
by
M.D. Grosslein
National Marine Fisheries Service
Northeast Fisheries Center
Woods Hole, Mass., USA
and
A. Laurec
Centre national pour l'exploitation des océans
Centre océanologique de Bretagne
Brest, France
This document has been prepared as a result of the Training Course on Acoustic and Trawling Surveys, held at the ISPM Casablanca, on 2–14 June 1980
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
UNITED NATIONS DEVELOPMENT PROGRAMME Rome, 1982
| ACKNOWLEDGEMENTS |
| FAO acknowledges its indebtedness to the Director of the Institut scientifique des pêches maritimes, Casablanca and to the Project Manager and staff of the supporting Project MOR/78/018 - Estimation and continued monitoring of marine resources - for their generous contributions and facilities (mini-computer) which helped to a great extent to make the seminar successful. Additional basic assistance was provided through the Interregional Fisheries Development and Management Programme (INT/79/019) particularly from CECAF's field project based in Dakar, Senegal. |
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| 1. | Sampling strata used in US bottom trawl surveys in the New England region |
| 2. | Strata and area of survey for Octopus vulgaris |
| 3. | Trawl survey strata in Ivory Coast |
The Committee for Eastern Central Atlantic Fisheries (CECAF) during its Sixth Session in Agadir, 11–14 December 1979, stressed the necessity of intensifying the work leading to the assessment of the resources of the region by using acoustic and trawling surveys, and to accelerate the training of national expertise.
The optimization and standardization of the survey methods and techniques to be utilized by all the laboratories are necessary to assure, on the one hand, the rational utilization of available funds and, on the other hand, the comparability of the results between one region and another, between one year and another.
A seminar was organized for that purpose from 2 to 14 June 1981 at Casablanca (Morocco) by the Interregional Fisheries Development and Management Programme (CECAF component) jointly with the MOR/78/018 project in order to assist in the training of the CECAF scientists in the design and operation of surveys for the assessment of fishery resources in the region.
This seminar was a follow-up of the two previous ones on the same subject (Acoustic methods for fish detection and abundance estimation, TF/RAF/106 (NOR), June-July 1978, Casablanca, and the Joint ICCAT/ICSEAF/CECAF course on statistics and sampling, April-May 1978, Tenerife) and offered a deeper knowledge and application to the very important question of surveys, both by demersal trawling and by the utilization of acoustic techniques. These questions are presently even more important because a new phase is starting whereby all the laboratories in the region will carry out this kind of research.
This document represents a summary of the materials and discussions for Part 1 of the course which focused on bottom trawl surveys designed to monitor changes in the distribution, abundance, and structure of multispecies fish populations over a period of years. The emphasis of the lectures and discussion was on design and operational characteristics of surveys which affect the accuracy of results and efficiency of operations. Only a summary of these topics is presented here, but examples of detailed survey manuals were provided to participants in the course, and examples are cited in the references. It is hoped that this outline, together with the examples and available literature, will be sufficient for CECAF countries to write their own manuals, tailored to the specific characteristics of their fisheries and need of each country.
In addition to lectures and discussions on survey design and operations, practical class exercises in calculating survey abundance indices were carried out, using actual survey data from CECAF countries. The class was divided into three subgroups each of which calculated stratified mean catch per haul indices for selected species. The statistical precision of these indices relative to sample size and other aspects of survey design were discussed; and a brief summary of the results of these exercises is included here.
This report was written largely by M. Grosslein with major contributions to sections 2.3, 2.4, and 5.2 by A. Laurec. Laurec, Grosslein and also G. Bazigos (Fisheries Department, FAO, Rome) presented lectures on various aspects of trawl surveys.