Harvest management

Harvesting of populations by people is clearly in the realm of predator-prey interactions and harvest management relies on the theory of predator-prey dynamics (see Chapters 10 and 14). When a natural population is exploited by culling or harvesting - whether this involves the removal of whales or fish from the sea, the capture of 'bushmeat' in the African savanna or the removal of timber from a forest -it is much easier to say what we want to avoid than precisely what we might wish to achieve. On the one hand, we want to avoid overexploitation, where too many individuals are removed and the population is driven into biological jeopardy, or economic insignificance or perhaps even to extinction. But harvest managers also want to avoid underexploitation, where far fewer individuals are removed than the population can bear, and a crop of food, for example, is produced which is smaller than necessary, threatening both the health of potential consumers and the livelihood of all those employed in the harvesting operation. However, as we shall see, the best position to occupy between these two extremes is not easy to determine, since it needs to combine considerations that are not only biological (the well-being of the exploited population) and economic (the profits being made when a new pest invades...

... early control is best harvesting aims to avoid over- and underexploitation

Figure 15.7 Fixed quota harvesting. The figure shows a single recruitment curve and three fixed quota harvesting curves: high quota (hh), medium quota (hm) and low quota (hl). Arrows in the figure refer to changes to be expected in abundance under the influence of the harvesting rate to which the arrows are closest.

equilibria. At hh the only 'equilibrium' is when the population is driven to extinction. At hl there is a stable equilibrium at a relatively high density, and also an unstable breakpoint at a relatively low density. The MSY is obtained at hm because it just touches the peak of the recruitment curve (at a density Nm): populations greater than Nm are reduced to Nm, but populations smaller than Nm are driven to extinction.

N

from the operation), but also social (local levels of employment and the maintenance of traditional lifestyles and human communities) (Hilborn & Walters, 1992; Milner-Gulland & Mace, 1998). We begin, though, with the biology.

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