Simple MSY models of harvesting fixed quotas

fixed-quota harvesting is extremely risky.

The MSY density (Nm) is an equilibrium (gains = losses), but when harvesting is based on the removal of a fixed quota, as it is in Figure 15.7, Nm is a very fragile equilibrium. If the density exceeds the MSY density, then hm exceeds the recruitment rate and the population declines towards Nm. This, in itself, is satisfactory. But if, by chance, the density is even slightly less than Nm, then hm will once again exceed the recruitment rate. Density will then decline even further, and if a fixed quota at the MSY level is maintained, the population will decline until it is extinct. Furthermore, if the MSY is even slightly overestimated, the harvesting rate will always exceed the recruitment rate (hh in Figure 15.7). Extinction will then follow, whatever the initial density. In short, a fixed quota at the MSY level might be desirable and reasonable in a wholly predictable world about which we had perfect knowledge. But in the real world of fluctuating environments and imperfect data sets, these fixed quotas are open invitations to disaster.

Nevertheless, a fixed-quota strategy ... whose dangers has frequently been used. On a speci-

are illustrated by the fied day in the year, the fishery (or Peruvian anchovy hunting season) is opened and the fishery cumulative catch logged. Then, when the quota (estimated MSY) has been taken, the fishery is closed for the rest of the year. An example of the use of fixed quotas is provided by the Peruvian anchovy (Engraulis ringens) fishery (Figure 15.8). From 1960 to 1972 this was the world's largest single fishery, and it constituted a major sector of the Peruvian economy. Fisheries experts advised that the MSY was around 10 million tonnes annually, and catches were limited accordingly. But the fishing capacity of the fleet expanded, and in 1972 the catch crashed. Overfishing seems at least to have been a major cause of the collapse, although its effects were compounded with the influences of profound climatic fluctuations. A moratorium on fishing would have been an ecologically sensible step, but this was not politically feasible: 20,000 people were dependent on the anchovy industry for employment. The stock took more than 20 years to recover (Figure 15.8).

Figure 15.8 Landings of the Peruvian anchovy since 1950. (After Jennings et al., 2001; data from FAO, 1995, 1998.)

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