Proportion or allowing constant escapement

Two further management strategies are based on the simple idea of availability of a surplus yield. First, a constant proportion of the population can be harvested (this is equivalent to fixing a hunting mortality rate and should have the same effect as harvesting at constant effort) (Milner-Gulland & Mace, 1998). Thus, in the Northwest Territories of Canada, 3-5% of the caribou and muskox populations can be killed each year (Gunn, 1998), a strategy that involves the expense of preharvest censuses so that numbers to be harvested can be calculated.

Another strategy leaves a fixed number of breeding individuals at the end of each hunting season (constant escapement), an approach that involves the even greater expense of continuous monitoring through the hunting season. Constant escapement is a particularly safe option because it rules out the accidental removal of all the breeding individuals before breeding has occurred. Constant escapement is particularly useful for annual species because they lack the buffer provided by immature individuals in longer lived species (Milner-Gulland & Mace, 1998). The Falkland Islands government uses a constant escapement strategy for the annual Loligo squid. Stock sizes are assessed weekly from mid-season onwards and the fishery is closed when the ratio of stocks in the presence and absence of fishing falls to 0.3-0.4. After 10 years of this management regime the squid fishery shows good signs of sustainability (Figure 15.10).

Stephens et al. (2002) used simulation models to compare the outcomes for a population of alpine marmots (Marmota marmota) of fixed-quota, fixed-effort and threshold harvesting. In the latter case, other MSY approaches:... ... harvesting a constant proportion ...

... or leaving a constant 'escapement' of breeding individuals constant escapement seems to work best for alpine marmot hunting

Figure 15.10 Monthly Loligo squid catches by licensed vessels in the Falkland Islands where a constant escapement management strategy is used. Note that there are two fishing seasons each year (February-May and August-October). The dotted lines (1984-86) represent estimated rather than actual catches. (After des Clers, 1998.)

Figure 15.11 Multiple equilibria in harvesting. (a) When recruitment rate is particularly low at low densities, the harvesting effort giving the MSY (Em) has not only a stable equilibrium (S) but also an unstable breakpoint (U) at a density below which the population declines to extinction. The population can also be driven to extinction by harvesting efforts (E0) not much greater than Em. (b) When harvesting efficiency declines at high densities, comments similar to those in (a) are appropriate.

Figure 15.10 Monthly Loligo squid catches by licensed vessels in the Falkland Islands where a constant escapement management strategy is used. Note that there are two fishing seasons each year (February-May and August-October). The dotted lines (1984-86) represent estimated rather than actual catches. (After des Clers, 1998.)

Figure 15.11 Multiple equilibria in harvesting. (a) When recruitment rate is particularly low at low densities, the harvesting effort giving the MSY (Em) has not only a stable equilibrium (S) but also an unstable breakpoint (U) at a density below which the population declines to extinction. The population can also be driven to extinction by harvesting efforts (E0) not much greater than Em. (b) When harvesting efficiency declines at high densities, comments similar to those in (a) are appropriate.

Recruitment rate ---Harvesting rate harvesting only occurred during years in which the population exceeded a given threshold and exploitation continued until that threshold was reached (essentially a constant escapement approach). These social mammals are hunted in parts of Europe but the modeling was performed using extensive data available from a nonhunted population. They found that threshold harvesting provided the highest mean yields coupled with an acceptably low extinction risk. However, the introduction of error, associated with less frequent censuses (3-yearly rather than yearly), led to higher variance in yields and a much increased extinction probability (Stephens et al., 2002). This emphasizes the importance of frequent censuses for constant escapement strategies to succeed.

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