The first point to grasp about harvesting theory is that high yields are obtained from populations held below, often well below, the carrying capacity. This fundamental pattern is captured by the model population in Figure 15.7. There, the natural net recruitment (or net productivity) of the population is described by an n-shaped curve (see Section 5.4.2). Recruitment rate is low when there are few individuals and low when there is intense intraspecific competition. It is zero at the carrying capacity (K). The density giving the highest net recruitment rate depends on the exact form of intraspecific competition. This density is K/2 in the logistic equation (see Section 5.9) but, for example, is only slightly less than K in many large mammals (see Figure 5.10d). Always, though, the rate of net recruitment is highest at an 'intermediate' density, less than K.
Figure 15.7 also illustrates three possible harvesting 'strategies', although in each case there is a fixed harvesting rate, i.e. a fixed number of individuals removed during a given period of time, or 'fixed quota'. When the harvesting and recruitment lines cross, the harvesting and recruitment rates are equal and opposite; the number removed per unit time by the harvester equals the number recruited per unit time by the population. Of particular interest is the harvesting rate hm, the line that crosses (or, in fact, just touches) the recruitment rate curve at its peak. This is the highest harvesting rate that the population can match with its own recruitment. It is known as the maximum sustainable yield (MSY), and as the name implies, it is the largest harvest that can be removed from the population on a regular and repeated (indeed indefinite) basis. It is equal to the maximum rate of recruitment, and it is obtained from the population by depressing it to the density at which the recruitment rate curve peaks.
The MSY concept is central to much of the theory and practice of harvesting. This makes the recognition of the following shortcomings in the concept all the more essential.
1 By treating the population as a number of similar individuals, or as an undifferentiated biomass, it ignores all aspects of population structure such as size or age classes and their differential rates of growth, survival and reproduction. The alternatives that incorporate structure are considered below.
2 By being based on a single recruitment curve it treats the environment as unvarying.
3 In practice, it may be impossible to obtain a reliable estimate of the MSY.
4 Achieving an MSY is by no means the only, nor necessarily the best, criterion by which success in the management of a harvesting operation should be judged (see, for example, Section 15.3.9).
Despite all these difficulties, the MSY concept dominated resource management for many years in fisheries, forestry and wildlife exploitation. Prior to 1980, for example, there were 39 agencies for the management of marine fisheries, every
MSY: the peak of the net recruitment curve
... but has been frequently used one of which was required by its establishing convention to manage on the basis of an MSY objective (Clark, 1981). In many other areas, the MSY concept is still the guiding principle. Moreover, by assuming that MSYs are both desirable and attainable, a number of the basic principles of harvesting can be explained. Therefore, we begin here by exploring what can be learnt from analyses based on the MSY, but then look more deeply at management strategies for exploited populations by examining the various shortcomings of MSY in more detail.
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