A safer alternative fixed harvesting effort

The risk associated with fixed quotas can be reduced if instead there is regulation of the harvesting effort. The yield from a harvest (H) can be thought of, simply, as being dependent on three things:

Yield, H, increases with the size of the . .

regulating harvesting harvested population, N; it increases r r effort is less risky -

with the level of harvesting effort, E

but leads to a more (e.g. the number of 'trawler-days' in a ...

variable catch fishery or the number of 'gun-days'

with a hunted population); and it increases with harvesting efficiency, q. On the assumption that this efficiency remains constant, Figure 15.9a depicts an exploited population subjected to three potential harvesting strategies differing in harvesting effort. Figure 15.9b then illustrates the overall relationship to be expected, in a simple case like this, between effort and average yield: there is an apparently 'optimum' effort giving rise to the MSY, Em, with efforts both greater and less than this giving rise to smaller yields.

Adopting Em is a much safer strategy than fixing an MSY quota. Now, in contrast to Figure 15.7, if density drops below Nm (Figure 15.9a), recruitment exceeds the harvesting rate and the population recovers. In fact, there needs to be a considerable overestimate of Em before the population is driven to extinction (E0 in Figure 15.9a). However, because there is a fixed effort, the yield varies with population size. In particular, the yield will be less than the MSY whenever the population size, as a result of natural fluctuations, drops below Nm. The appropriate reaction would be to reduce effort slightly or at least hold it steady whilst the population recovers. But an understandable (albeit misguided) reaction

Figure 15.9 Fixed effort harvesting. (a) Curves, arrows and dots are as in Figure 15.7. The maximum sustainable yield (MSY) is obtained with an effort of Em, leading to a stable equilibrium at a density of Nm with a yield of hm. At a somewhat higher effort (Eh), the equilibrium density and the yield are both lower than with Em but the equilibrium is still stable. Only at a much higher effort (E0) is the population driven to extinction. (b) The overall relationship between the level of the fixed effort and average yield.

Figure 15.9 Fixed effort harvesting. (a) Curves, arrows and dots are as in Figure 15.7. The maximum sustainable yield (MSY) is obtained with an effort of Em, leading to a stable equilibrium at a density of Nm with a yield of hm. At a somewhat higher effort (Eh), the equilibrium density and the yield are both lower than with Em but the equilibrium is still stable. Only at a much higher effort (E0) is the population driven to extinction. (b) The overall relationship between the level of the fixed effort and average yield.

might be to compensate by increasing the effort. This, however, might depress population size further (Eh in Figure 15.9a); and it is therefore easy to imagine the population being driven to extinction as very gradual increases in effort chase an ever-diminishing yield.

There are many examples of harvests being managed by legislative regulation of effort, and this occurs in spite of the fact that effort usually defies precise measurement and control. For instance, issuing a number of gun licenses leaves the accuracy of the hunters uncontrolled; and regulating the size and composition of a fishing fleet leaves the weather to chance. Nevertheless, the harvesting of mule deer, pronghorn antelope and elks in Colorado was controlled by issuing a limited but varying number of hunting permits (Pojar, 1981). In the management of the important Pacific halibut stock, effort was limited by seasonal closures and sanctuary zones - although a heavy investment in fishery protection vessels was needed to make this work (Pitcher & Hart, 1982).

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