Mr. Fontana submitted the findings of his work on the Congolese trawl fishery to the Working Group (Fontana, in publication, Appendix 6). In order to study the impact of modifications in the trawl mesh sizes used by the Ponte Negro fleet on the stock and on the catch, he applied a Ricker model to six species (Pseudotolithus senegalensis, P. typus, Galeoides decadactylus, Pteroscion peli, Pentanemus quinquarius and Brachydeuterus auritus). The study took into account the respective importance of their average levels of recruitment (six years). He next studied fluctuations in production and yield, including economic fluctuations, for this group of species under various combinations of fishing efforts and mesh sizes.
Apparently, these findings can be extrapolated to all catches shelf-wide. In fact the first five species on the list represent 80-85 percent of the catch from the shallower section (0-50 m) and about 65 percent of the total catch for all of the continental shelf (0-120 m). In addition, the specific makeup of this group of species (three large-size target species, three smaller species of lesser commercial value) should be able to give a good account of the conflict between large and small species with respect to mesh size regulation.
The main conclusions of this study are (Table 10 of this report and Figures 1, 2 and 3 of Appendix 6):
(a) With respect to current fishing effort, any increase in mesh size (now 40-45 mm), would, in the long run, amount to a rather low increase in production (+ 9 percent).The Working Group felt that these findings were of considerable importance for the definition of a mesh size appropriate for the exploitation of demersal resources in the Gulf of Guinea. For want of the relevant analyses, this definition is still in abeyance. The analysis of the state of stocks (Section 7) showed that inshore stocks were being fished at a level close to but below maximum balanced yield for all sectors on which sufficient data is available. There can be no increase in the rates of exploitation for the economic yields of the various fisheries are now close to zero. Any increase in the level of exploitation or in total captures would therefore involve improved yields and hence, as Fontana's work has shown, the use of a larger mesh size.(b) If the fishing effort were doubled, production increase would be near 50 percent assuming that mesh size were changed at the same time to 77 mm.
(c) The result of the adoption of a 77 mm mesh size would be an immediate drop in captures of nearly 50 percent. The drop would be only ten percent if a 60 mm mesh size were adopted as an intermediate stage of the process.
(d) These findings are valid, leaving out Brachydeuterus auritus, which is now only partially marketable in Congo but which could be marketed rather easily (at least large-size specimens), as its potential market value is not that different from that of species, even top quality species, now being marketed. If the marketing of this abundant species were to coincide with the adoption of a new mesh size, the immediate drop in catch would not be over 15 percent for a mesh size of 77 mm. Indeed there would be an immediate increase in the catch of about 10 percent if a transitional mesh size of 60 mm were adopted.
(e) Lastly, the changeover to combined doubling of present efforts and the adoption of the 77 mm mesh size would involve no change in the spawning capacity of the stocks. Of course, this capacity would increase if the mesh size were enlarged without any modification of the fishing effort.
The data gathered on mesh sizes actually used in the various fisheries (Table 11) show that in most cases, with the undoubted exception of the Nigerian fishing industry, mesh sizes are comparable to those used in Congo.
Keeping in mind this ensemble of similar situations, the Working Group strongly recommended an increase in mesh size. In addition to the gains in production and yield which are to be expected, a mesh size increase would also increase trawler efficiency and facilitate the sorting of the catch.
In order to minimize immediate losses, the Working Group recommended that the legal minimum be temporarily set at 60 mm. To further limit the immediate consequences, it also recommended that a promotional campaign be launched at the same time to publicize the consumption of species such as Brachydeuterus auritus, at least in those countries where this species is still underutilized.
Lastly, the Working Group listed the selectivity studies published on commercial species of the region (Table 12). It drew attention to the findings of work done in Senegal on trawl selectivity vs. shrimp (Lhomme, 1978). These studies showed no virtual modification of yield when mesh size was increased to about 60 mm (Figure 9). The Working Group therefore decided that the adoption of a 60 mm mesh size should apply at the same time to shrimpers and to trawlers specializing in fish. The adoption of a uniform mesh size would make enforcement much easier and would at the same time protect the undersized specimens of many species of economic importance from being taken by shrimpers, as is now the case.
It was also noted that in certain countries, such as Ghana where the beach seine was widely used, this gear, mounted with overly small mesh (Table 11) may well be responsible for catching appreciable quantities of demersal juveniles. In any case, the lack of data on the nature of these catches prevented the Group from putting forth any recommendation on mesh size. It therefore formally hoped that sampling programmes would be undertaken very soon in those countries where beach seining is important (such as Benin, Ghana, Togo), so as to get even a rough idea of the weight and number of the quantities fished, as well as their breakdown by species and size frequency.
