Migration of fishery resources in the World’s oceans
All fish, and other motile fishery resources, make migrations. These range from tens or hundreds of metres to large-scale oceanic migrations over thousands of kilometres. Migrations are usually cyclical over time scales from diurnal to annual or longer. Diurnal migrations are driven by the light/dark cycle and seem to have evolved to balance the requirement to feed against the risk of predation. The major seasonal migrations have evolved because of the differing habitat requirements for breeding and feeding, and in the oceans these migrations are linked to prevailing current patterns. There is considerable concern in world fisheries about the top-down effects of exploitation of marine resources because approximately one third of the world’s fishery resources are either fully or over exploited. However, there are also bottom-up effects, driven by pollution and manifestations of global climate change, which are increasingly recognized to be important drivers of change in the world’s fisheries. Changing ocean current systems at the surface which might be caused by global climate change have the potential to cause disruption of migration patterns and prevent the successful completion of life cycles. Bottom-up effects of variable environments on populations of migratory species such as Atlantic cod, Antarctic krill and shortfm squid have been identified and provide different kinds of examples of how changing oceanographic conditions can drive change in exploited stocks. Examples of this kind of variability provide valuable insights into the likely effects of large-scale ecological change on the world’s fisheries.