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Generalized fishery model

Although any analysis of a time series of landings usually shows large variations along the time axis, some trend patterns have been identified and described. Caddy (FAO, 1984) presented a generalized model of a fishery (Figure 1) which, in the course of its development, can be considered to pass through four phases: I) undeveloped, II) developing, III) mature, and IV) senescent. Implicit in this model, and underlying it, is the concept that fishing capacity and fishing effort (or extraction rate) increase over time and drive the fishery from one phase to the next (FAO, 1996).

The undeveloped phase is characterized by the exploitation of fishery resources at well below their maximum potential, usually with less efficient (e.g. artisanal) fishing gears. In the development phase there is rapid growth of catches concomitant with increased numbers of fishers and boats together with the modernization of fishing gears and methods. The revenues obtained during this phase are often reinvested to increase fishing effort.

Although during the mature phase there is still an increase in catches, in most cases they are already very close to the maximum sustainable yield (MSY) and the rate of increase, which starts to decrease during the developing phase, now drops very rapidly. However, the momentum of the investment process means that the fishing effort continues to grow, and the fishery soon enters into the senescent phase as catches begin to diminish. This process can be accelerated by unfavourable changes in environmental conditions.

The duration and slope of the different phases are results of both the rate of increase of fishing intensity (and mortality) and the biological carrying capacity of the resource (FAO, 1996).

Figure 1 showns the theoretical change in yield (C) and the rate of increase of yield (Ct+1 - Ct)/Ct, where t is the time in years, of the development process. The rate of increase, which varies significantly as the maximum long-term yield is approached, reached and surpassed, is of particular interest and has been used to provide a rough assessment of the state of Cuban marine fishery resources. The rate is nil for a stable non-developing fishery (Phase I) and increases rapidly (Phases I to II) as the fishery starts to develop. It then decreases during the phase of steady growth of the fishery (Phase II) and drops to zero when the fishery reaches its maximum production (Phase III). Following Phase III, fishing capacity may also develop, further aggravating depletion, and the relative rate of increase may become negative as overfishing progresses.

FIGURE 1: Generalized model showing the different phases of a fishery

Figure 2 shows the development sequence of Cuban fisheries, considered as a whole, from 1935 to 1995; the four phases of the generalized fishery model can be clearly distinguished. According to Baisre (1987a), the undeveloped phase lasted from 1935 to 1954. In the mid-1950s, the developing phase began with landings continuing to grow up until the mid-1970s, when fisheries reached the mature phase. In the 1980s, most of the fishery resources were fully exploited and some cases of overfishing were already evident (Baisre, 1987b). A more detailed analysis of each fishery resource is presented in the following sections.

FIGURE 2: The phases of Cuban total landings, shrimp fisheries by-catches and non-identified fishes


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