The open ocean – defined legally by the UN Law of the Sea Convention as waters beyond national jurisdiction – makes up 60% of the world ocean. This includes areas of all the major regions of the ocean, and a diverse collection of fisheries. To achieve sustainability of fisheries in the open ocean more nees to be known about the biology and dynamics of fish populations.

The ecological zones of the open ocean can be defined according to different sets of criteria. The ocean could be divided by depth zones and distance from the coast, or into oceanic provinces. Alternately, the world ocean can be divided into an enormous body of cold and nutrient-rich water beneath a lens of nutrient-poor warmer water.

Massive primary production occurs when mixing processes bring deeper, nutrient-rich water to the sun-lit surface. In low-latitude areas, the warm surface waters are much lighter than the waters in the deeper layer and hence it is harder for the water layers to mix. There, massive production occurs only when a mechanism stronger than the occasional storm breaks the thermocline dividing the layers and pumps a regular supply of nutrient-rich, deep water to the surface, a phenomenon known as ‘upwelling’.

When warm, nutrient-poor waters seals off a deep basin (for example, in the central gyres of the ocean), primary and secondary ecosystem production remains low. Several adaptations are used by organisms that live in such areas, including recycling nutrients quickly within the sunlit euphotic layer, or (for larger organisms) feeding opportunistically over a large area. Thus, large predators in the open ocean undertake feeding migrations of various lengths. The best migrators of all are the warm-blooded tropical tunas, e.g., yellowfin and skipjack tuna. These animals spend almost their entire lives hopping frenetically from one food patch to the next, and suffering a quick death by starvation if they fail to reach a new patch of life-sustaining, high-density food.

As the biomass transfer efficiency (i.e., the ratio of food consumed to flesh produced) from one trophic level of a food chain to the next is low (about 10%), tuna biomass per unit area still remain low even in areas with high primary productivity. Seasonal and year to year variability in the open ocean forces the vertical and lateral migration of marine organisms. This variability is combined with the trophic mechanisms to help understand the biology and dynamics of fish populations, and thus provide a basis for presenting some of the requirements for sustained fishing from the Open Ocean.

The governance regime for the Open Ocean is radically different from that prevailing within Exclusive Economic Zones in that anyone is allowed to fish in Open Ocean areas. The current system of Regional Fisheries Management Organizations (RFMOs), which attempts to regulate fishing in the Open Ocean, cannot legally compel any country from exploiting any of the Open Ocean fish resources more or less as it pleases. In the meantime, there is a debate about the sustainability of open-ocean tuna fisheries among tuna industry representatives, RFMO and NGO staff and fisheries scientists about the status of tuna fisheries, and their ecosystem impacts. Although the major stocks of yellowfin and skipjack are doing relatively well, it is unlikely that their catch will continue to increase. Given the increasing fishing pressure and the uncertainty of global warming and ocean acidification, the open ocean fisheries for major tuna species may not be as sustainable as their appear to be.