| Popis: |
Many world fisheries display a declining mean trophic level of catches. This "fishing down the food web" is often attributed to reduced densities of high-trophic-level species, reflecting changes in the structure of harvested food webs. However, this pattern can also result from a restructuring of the fishery, that shifts from a focus on depleted upper trophic levels to abundant lower trophic levels. In fact, we show here that fishing down the food web can emerge from the adaptive harvesting of a predator-prey community, where changes in fishing patterns are driven by the relative profitabilities of the harvested species. The shift from a predator- to a prey-focused fishing pattern can yield abrupt changes in the system, and cause sudden losses in species densities. Such regime shifts occur when the predator species is highly valuable relative to the prey, and if it exerts a strong top-down control on the lower trophic level. Moreover, we find that when the two species are jointly harvested, high adaptation speeds can reduce the resilience of fisheries. Our results therefore suggest that flexibility in harvesting strategies will not necessarily benefit fisheries but may actually harm their sustainability. |
