Impacts of Fishing Low-Trophic Level Species on Marine Ecosystems

Smith et al. (2011) explored the effects of fishing on low-trophic level (LTL) species.  They concluded that fishing these LTL species at conventional maximum sustainable yield (MSY) levels can have large impacts on the ecosystem, especially when they constituted a high proportion of the biomass in the ecosystem.  They also concluded that halving exploitation rates would result in lower impacts on the marine ecosystem while maintaining 80% of MSY. 
Smith, A., Brown, C., Bulman, C., Fulton, E., Johnson, P., Kaplan, I., Lozano-Montes, H., Mackinson, S., Marzloff, M., Shannon, L., Shin Y.J., Tam, J. 2011.  Impacts of Fishing Low-Trophic Level Species on Marine Ecosystems. Science 6046, 1147–1150

          Concern has risen over the effects of fishing on the structure and function of marine ecosystems, particularly LTL species because a majority of them are plankton feeders.  LTL species, which include anchovies, sardines, herrings, mackerel, krill, and capelin, are found in high abundance in schools or aggregations and account for 30% of global fisheries production. 
           LTL species are the principle means of transferring energy from plankton to larger predatory fish and upwards to marine mammals and seabirds.  Indirectly, seabirds, whales, and high- trophic level (HTL) species are affected by the maximum yield of LTL species. 
          To examine and summarize the broader effects of fishing LTL species, five-well studied ecosystem regions were modeled.   These regions included the California current, northern Humboldt, North Sea, southern Benguela, and southeast Australia.  For each ecosystem and model, five LTL species or groups were subjected to a range of fishing pressures which resulted in depletion levels relative to unfished biomass from zero to 100%.  Impacts on other species within the ecosystem were measured relative to biomass levels of unfished focal LTL populations and all other groups that were fished at current levels. 
          Widespread impacts of harvesting LTL species were found across the ecosystems and the LTL species that were selected.  The percentage of affected species increased with the level of depletion of the LTL species, but the exact extent of the impacts varied across LTL species.  Impacts were both positive and negative, and at times, counter-intuitive considering that there were severe impacts with low depletion levels.  Negative impacts were felt by marine mammals, seabirds, and commercial species, although the majority of these impacts were very small. 
          Overall, harvesting LTL species was found to have high impacts, although the species with high impacts were not consistent across all ecosystems.  Management implications then vary geographically; large impacts may require a change in overall harvest levels whereas LTL species with small impacts could be harvested at conventional levels.  The range of impacts could be explained by the relative abundance of the group in the ecosystem, the trophic level of the group, and the connectivity of the group in the food web. 
          Wider implications of exploitation of LTL species include the tension between global food security and the protection of biodiversity.  Lower exploitation rates can cause smaller impacts on the ecosystem but also sustain lower yield rates.  Lower impacts can be achieved by lowering the MSY exploitation levels to a target of 75% unexploited biomass for an LTL species.  This will cost less than 20% of long-term yield, implying lower fishing effort but long-term economic optimum levels.  This study supports the ongoing substantial yields of LTL species while achieving ecological objectives in the face of feeding the global population.  

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