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3.1 Generalised fishery development model

The process of development of a fishery (i.e. the exploitation of a given species with a given gear in a given area) as described by changes in landings with time, often with a “boom and bust” character, has been described by many authors (e.g. Caddy (1984), Rapport et al. (1985), Welcomme (1995)). This process is schematically represented in Figure 23 with a generalised model which is based loosely on Caddy’s (1984) idealised fisheries cycle for an open access resource. In the course of its development, a fishery can be considered to pass through four phases: (I) undeveloped, (II) developing, (III) mature, and (IV) senescent.

Figure 23. Generalized fishery development model

When a long-enough time series is available and marked changes in landings have occurred, this simple model may allow a diagnostic of the present state of the fishery, based on the development phase reached. Similar models have been implicitly used by many authors making inferences about resource potential based on historical landings. Table 2 is an example of such implicit use of the fishery development model. To our knowledge, however, such a model has not been used in a systematic way for quantitative analysis of the state of resources. For individual fisheries, the reason may be that the natural variability is too high and the possible causes of variation in landings too numerous to allow any safe interpretation and extrapolation of observed trends. Gulland (1971, page 247) also noted its potential application to global fisheries stating that “the world situation has been comparable to the expansion phase of a fishery on an individual stock” stressed the difficulty of extrapolating trends to estimate future yields. At the global or ocean levels, sufficiently long time series of landings data were not easily available before the preparation by FAO of the extended time series. In the following sections, attempts are made to apply the simple model to the extended time series for global and regional assessments of the state of fisheries development and fisheries potential.

Figure 23 shows the theoretical change in yield (C) and rate of increase of yield, (Ct+1-Ct)/Ct, during the development process. The rate of increase, which varies significantly as the maximum long-term yield is approached, reached and “overshot”, is of particular interest and has been used below to provide a rough assessment of the state of world resources. This rate is nil for a stable non-developing fishery (Phase I), increases rapidly (phases I-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. This simple description could be made more complex to reflect “overshooting” in long-lived resources where a period of high landings could only be transient before stabilisation at lower levels. It could also be made to reflect superimposed (“decadal”) oscillations due to climatic influence.

Implicit in this model, and underlying it, is the concept that fishing capacity and fishing effort (or extraction rate) increase with time and drive the fishery from one phase to the next. The length and slope of the different phases will result from both the rate of increase in fishing intensity (and mortality) and biological carrying capacity of the resource. While this model was conceptualised for a single fishery, it remains intuitively correct to describe the long-term development process of a “meta-fishery”, i.e. the exploitation of a species assemblage or a mix of available resources, by a mixture of interacting gears in a given area and, by extension, of the world resources, in the world ocean, by the world fleet.

The major difference between the basic individual fishery model and its meta-fishery equivalent is that the heterogeneity of the latter complicates interpretation. The total landings may continue to increase despite local overfishing situations as long as the process of increase through expansion to new areas and resource elements overshadows the process of decrease through overfishing. An important implication is that the highest landings observed in a complete process represent a sort of a composite average long-term yield (ALTY) which is different from the sum of the theoretical MSYs for the various resource elements, whether these elements represent different species in a given area or even different areas with their multispecies resources components in a given region or ocean. It must be recalled that it is not advisable to attempt to extract the MSY of any aquatic resource and that it is impossible to extract simultaneously the MSYs of all the components of a species assemblage in a given area. However, when a “meta-fishery” covers many areas, it might be possible to improve the overall ALTY by optimising the fisheries in each area.

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