Figure 1. (a) The relationships between exploitation effort, equilibrium population abundance and equilibrium yield in the logistic single-species model. For too high effort, population goes extinct. The dependence between effort and yield is dome-shaped, with the maximum sustainable yield (MSY) obtained in the middle. Sustainability declines with effort, but threshold beyond which exploitation is ‘unsustainable’ is a matter of debate. Similarly, the threshold for ‘overexploitation’ is a matter of debate. In this example, it is assumed that obtaining MSY is the management objective, and that effort levels that are higher than one giving MSY are considered biological overexploitation. Economic yield is usually maximized at a lower effort than biological yield. Consequently, economic overexploitation may be scored even when biological overexploitation is not yet occurring. While the detailed relationships depicted here depend on the model, the overall picture is model-independent. (b) Dynamics of unexploited and exploited population in the logistic model. Unexploited population reaches carrying capacity (thick black curve). If exploitation is started, abundance declines. In the logistic model, intermediate effort (thick grey curve) gives the highest sustainable yield, but this also results in substantial reduction in equilibrium abundance. Notice that overexploitation (thin black curve) gives higher initial yield than exploiting with effort giving MSY.

Figure 2. Number of species extinct or threatened by harvesting according to IUCN Red List. This includes hunting and gathering for food, medicine, fuel, material and cultural, scientific and leisure activities. No marine or freshwater plant species were listed. Data from the International Union for Conservation of Nature and Natural Resources (IUCN; http://www.iucnredlist.org/, accessed in March 2008).

Figure 3. State of the world marine fish stocks (FAO, 2007). FAO started monitoring the world's fish stocks in 1974, and categorizes them in different groups based on the level they have been exploited (see figure legend). The percentage of stocks ‘underexploited’ or ‘moderately harvested’ has decreased while the percentage of stocks ‘fully’ or ‘overexploited’ has increased. Underexploited and moderately exploited stocks could produce higher sustainable yields if exploited more, whereas fully exploited stocks are producing catches that are at or close to their maximum sustainable limits. Overexploited, depleted and recovering stocks are yielding less than their maximum potential owing to excess fishing pressure, with no possibilities in the short or medium term of further expansion. Reproduced from The State of World Fisheries and Aquaculture (2006), with permission from the Food and Agriculture Organization of the United Nations.