It is very difficult to achieve this goal. Typically, fisheries undergo stocks permanently at different temporal and spatial scales of exploitation that exceeds the biological capacity and ecological recovery of the species. On the other hand, the fishery always begins without further study of the population which will be exploted. When these studies exist, and these are more intensive, the population has already changed to another phase, ie, the response is delayed (Figure 1).
Figure 1. Evolution of changes in a fishery.
One factor that clearly shows these effects are average sizes of the populations subjected to exploitation. Body sizes of fish as well as the quality and quantity of their eggs, are subject to biological changes. Drastic natural events, and fishing pressure, can severely affect the exploited stocks (Figure 2).
Figure 2. Some changes in a fish population during its inappropiate exploitation.
Generally, these changes can be observed after several years of inappropiate fishing pressure, but in some cases may occur over short periods due to a combination of several factors. Fisheries also have problems due to these changes, yields may decrease while the exploited population undergoes various changes (Figure 3). It is necessary to apply the ecosystem approach to fisheries management, through a comprehensive and multidisciplinary study taking in account all parts involved.