The objectives of trawl surveys range from exploration of an area in terms of the distribution and relative abundance of fishes and commercial feasibility of various gear and fishing methods, to stock monitoring and resource appraisal. The various types of surveys and their respective methods have already been described in general in several FAO manuals (Alverson, 1971; Mackett, 1973; Gulland, 1975; Saville, 1977; Ulltang, 1977; and Troadec, 1980). Emphasis at this workshop was on trawl surveys designed to monitor changes in the distribution, abundance, and structure of multispecies fish populations over a period of years. Essential characteristics of such surveys include the use of standardized trawl gear and regular annual or seasonal sampling over the entire region inhabited by the priority fish stocks taking into account their migration patterns. The principal objective of monitoring surveys is to help assess the effects of fishing on the stocks by providing quantitative measures of the fluctuations in the relative abundance and structure (age and length) of major stocks, which are free of the many sources of bias inherent in commercial fishery statistics (Grosslein, 1969c). For example, commercial fishing practices change in response to market demand as well as fish availability (degree of aggregation in many cases), and therefore commercial catch-per-unit-effort data seldom provide reliable indices of absolute population abundance for any extended period of years. In addition, changes in commercial gear and fish detection methods occur from time to time, resulting in changes in catch-per-unit-effort which are very difficult to measure. Properly conducted trawl surveys on research vessels are free of such biases because they use strictly standardized trawls and fishing methods, and because trawling is done at random (or systematic) preselected locations.
Other important advantages of monitoring surveys are synoptic coverage and completeness of catch records. Commercial vessels often discard many species and especially small fish (pre-exploited sizes), whereas research vessel surveys can provide information on the total species composition and size range available to the trawl. Predictions of recruitment to the major commercial stocks can often be made from trawl surveys, as well as a much more complete evaluation of production potential and composition of the total fish biomass (Grosslein, Brown and Hennemuth, 1980). The synoptic coverage of surveys can also provide valuable information on the relation between fish distribution and environmental factors such as annual and seasonal changes in hydrography. Finally, trawl surveys can provide a wide variety of important biological data (e.g., size at maturity, spawning times and areas, scales or otoliths for age and growth studies, stomach contents) which may not be available from commercial fish landings.
While there are many advantages to trawl surveys it is important to keep their role in proper perspective. They are in many cases a necessary supplement to commercial statistics but they cannot provide information on the size and composition of the commerical catch which is critical to any assessment of the effects of fishing. Care must be taken to make the best use of available resources in a proper mix of sampling commercial catches and conducting trawl surveys. In order to obtain the full benefits of trawl surveys it is necessary that they be conducted regularly every year, and this represents a large investment in terms of research vessel time and scientific manpower. To ensure that the results justify the cost of a long-time series of surveys it is important that the design of the survey is appropriate to meet the major objectives of the survey. This report summarizes the principal elements of trawl surveys, beginning with initial planning and setting objectives, including sampling design and survey operations, and concluding with data processing and analysis. The emphasis is on principles of survey design and operation, and it is not intended as a detailed survey manual. Examples of other survey manuals are referenced in this report, but each country or region essentially has to produce its own survey manual, tailored to its own specific characteristics and unique needs. This report, together with the referenced material, and the handouts and discussions at the training course, should provide an adequate basis for developing trawl survey programmes (and manuals) in the CECAF area.
