7.1 Catch quotas or effort limitation
7.2 Changes in mesh size of shrimp trawl
7.3 Regulation of artisanal fishery to protect juvenile shrimps
7.4 Closed seasons at sea
7.5 Priorities in management
A study on management possibilities of P. duorarun fisheries of the West African coast was made by Garcia and Lhomme (1977). The most general conclusions will be repeated here. The main management measures that can be envisaged include:
- overall control of level of exploitation (catch quotas or effort limitation);
- changes in mesh size of shrimp trawls;
- monitoring of the fishery for juveniles in lagoons;
- closed seasons.
Annual catch quotas are not possible for short-lived animals. This fishing effort will tend to be concentrated on the months following recruitment and fishing mortality and consequently the yield will be affected. Besides, changing of mortality by age class, depending on the success of recruitment, increases the risk of overexploitation if a wrong quota is established. A seasonal quota applicable over a period of 2 to 3 months will be more logical, but its implementation will be more difficult.
A limitation of fishing effort is unquestionably advantageous in terms of simplicity of implementation. It also allows in principle to stabilize the exploitation rate when annual recruitment varies appreciably. To apply it, a system of licences should be established and the development of the efficiency of the boats should be followed up so as to be in a position to maintain the total fishing power at a level corresponding to the desirable exploitation rate. Such a regulation would allow the best use of year to year variations of abundance due to variations in recruitment. On the other hand, in case the effort corresponding to the maximum catch should be overestimated, this regulation would not bring about damages that would result from a regulation based on catch quota in case the average maximum catch should have been overestimated.
Such a regulation is, a priori, interesting for several reasons:
- shrimps being short-lived and fast growing, the (possible) annual gain would be obtained as soon as such a measure is applied, before the end of the first annual cycle;An optimum exploitation of the two types of resources would necessitate a study of possibilities of utilizing selective shrimp nets, allowing the escapement of juvenile fishes, because mesh sizes for shrimps are smaller than those suitable for most marketed fish species.- the increase of the size of the mesh results in an increase in the age and mean weight of the individual and the price per kilogram depending on the individual weight, the gain in value would be proportionally more important than the gain in tonnage;
- the mesh now used by all shrimp boats off the African coast (40 to 50 mm opening) results in the catch of large quantities of unmarketable small fish which are actually thrown back (several tons per day and per boat). These fishes are most often juveniles and their intensive destruction seriously affects the recruitment of species exploited by other fisheries (e.g. Brachydeuterus auritus, Pseudotolithus spp., Galeoides sp., etc.). An even limited increase of the mesh (the benefit in terms of shrimp yield should be assessed) should therefore be beneficial to coastal trawlers searching for fish.
Whatever the fishing methods now in use in the various countries, artisanal fishing for shrimps in lagoons causes a significant reduction in recruitment at sea. One can assume that at the age when these shrimps are caught, the more important mortality has already taken place (at the larval stage in particular), and that the reduction of potential recruitment (in November) will cause a similar reduction (in percentage) of the total potential catch at sea (in weight). It is clear therefore, that the average maximum catch at sea would be obtained by totally reducing the artisanal fishery. Due to the social importance of the artisanal fishery, it is not possible to eliminate it completely. From a purely economic point of view, it should be observed that the increase in fishing pressure on juveniles results in a diminution of the overall value of catches (then made up of small shrimps) and that the small-sized shrimps caught in lagoons in general put them in direct competition with the carids of colder areas whose price makes them more competitive on the world market.
Several possibilities of regulating lagoon catches exist:
- increase of the mesh size of stakenets, enabling a greater number of shrimps to escape to the sea;- limitation or ban of dragnets which catch small shrimps of low market value and juvenile fishes;
- limitation of the number of setnets, together with a minimal interval between them in the narrower sections of the migratory routes (bridges, dikes, etc.);
- adoption of closed seasons and areas to protect small-sized individuals during the periods of peak migration.
The fishery could be closed at the time when recruitment is most intensive. This would avoid the exploitation of concentrations of fast-growing immature juveniles, which takes place from 6 to 7 months in the Ivory Coast (Garcia, 1976). Such closure could be coordinated with that of the artisanal fishery. The advantages to expect from such a regulation depends much on the seasonal catchability patterns.
Taking into account the present state of development of the fisheries, it is important to take measures without delay that will ensure a good future. These measures will depend on options made by each country for the development of the fishery. An expansion of the artisanal fishery allows, at least in the short term, a greater use of labour - notably in the rural areas - and probably better distribution of incomes. The necessary investments - notably in hard currencies - are small compared to those required for the development of the fishery at sea. This type of exploitation causes, on the other hand, a diminution of the overall value of catches, resulting in low value on foreign markets, and could theoretically jeopardize the very survival of the stock if this development is not strictly monitored (often difficult to carry out). Whatever the options made, some measures are necessary, concerning in particular:
- the monitoring of the artisanal fishery in order to maintain it at its present level as long as its exploitation rate has not been assessed and the management objectives have not been set;These recommendations may seem elementary but they are the only ones possible for the time being, taking into account the level of statistics available and the evaluations made.- the immediate monitoring of the industrial fishery, so as to take advantage of the fact that the maximum yield has not been reached or exceeded, in order to set up a system of licences on the sea fishery, restricting in a first stage the size of fleets to the present level. For the few sectors where expansion is possible, the effort should only be increased carefully, after showing that the expansion envisaged is compatible with the productivity of the stock.
