Etude bio-économique de l'exploitation des ressources sous diverses hypothèses de gestion

The model developed is a multi-species, multi-fleet and multi-sector model that consists of a dynamic allocation of the fishing effort. It can therefore measure the amplitude of the effort's generated outputs by strict, implemented and honoured regulatory measures. The management measures likely to be simulated give an account of the main trends of the regulations in effect in Community waters. Their effects on both the resource and the fleets are evaluated. Each stock, confined or not to a sector, has its own dynamic described by a model structured by ages: recruitment is assumed to be constant. Biological interactions (intra- and inter-species contact) as well as the problems of exchanges between sectors are not taken into account. In order to treat the fishery in its entirety, species that are caught but not described are integrated arbitrarily in the form of yields assumed constant in time. The fishing fleet is treated as homogeneous fleets at the economic and technical level. Each fleet is likely to practise several trades successively and is equipped with a relative efficiency for each species sought depending upon the trade practised. The potential effort appropriate to each fleet is given in sea days. Fleets that are not modelled and sectors outside of the area under consideration are treated, in the same way as for species, as external variables. Thus the impact of a fleet's activity on the results of the other fleets is modelled; this is what is called technical interactions. At the beginning of each interval of time, a decision rule determines, for each fleet, the trade or trades that it is going to practise. The user sets the time interval: this can be by quarter or by half-month to come close to the idea of tides. The decision rule incorporates the capacity of the skippers to choose the trades that are the most attractive financially (opportunism), balanced by a certain stasis that renews for a period a part of the effort delegated to various trades at the same period of the preceding year (principle known as adhesion). The share of potential sea time devoted to each trade is converted into fishing time after deducting travel time between the home port and the fishing area, then translated into mortality rate per fish, by means of the catchability associated with the fish and efficiencies associated with the ships. The biological model is based on classic catch equations. From the economic point of view, the passage from weight catch to turnover occurs simply by applying an average price to the ages. Each fleet is also equipped with cost structures that are appropriate for it. In addition to the fixed costs, the practising of a trade involves the formation of costs known as production costs, to which must be added costs associated with income (salaries, taxes ...). At the end of each year, we have the age structure of the main stocks, the landings in weight and values for each species and per fleet, as well as the operating expenses and therefore Gross Operating Surplus for each fleet.