Types of high seas stocks
FAO/Fisheries & Aquaculture Department
The term "straddling" stocks is not used in the 1982 UN Convention on the Law of the Sea (UNCLOS) which refers instead to a "stock or stocks of associated species occurring both within the exclusive economic zone and in an area beyond and adjacent to the zone." The FAO Glossary also specifies "Fish stocks that migrate between EEZs and the high seas", adding the notion of movement during the life cycle. A case not foreseen explicitly by UNCLOS, however, is that of a stock occurring within the EEZs of two or more coastal States and the adjacent high seas. As the straddling nature of a stock is a local (geographic) peculiarity related mostly to the width of the continental shelf, straddling stocks must be referred to not only by their species name (as with highly migratory species) but also by their specific location (for example, Grand Banks cod).
The degree to which a stock of bottom fish "straddles" usually depends on the extension of the continental shelf beyond the limit of the national jurisdiction and the seasonal or life cycle migrations of the species. The extent to which a stock of pelagic fish "straddles" depends on hydrographic structures, currents and related distribution of ocean productivity. The Chilean horse mackerel, for example, extends across the South Pacific reaching close to New Zealand. Straddling stocks can therefore be demersal or pelagic and live on the shelf (neritic stocks) or beyond it (oceanic stocks).
The straddling stocks most referred to though are bottom fish stocks with most of their biomass in one EEZ, straddling "out" in the high seas a few nautical miles and perhaps only seasonally. The "adjacency" required implies that there is probably some geographic continuity between the EEZ and straddling portions of the stock. Straddling stocks are particularly important for small island countries that have a very limited shelf (the reef and lagoon) in the middle of an immense oceanic ecosystem. Their reef resources are purely national but their oceanic resources, which represent a large proportion of the marine resources available to them for development, are straddling or highly migratory. The situation makes the small island resources very vulnerable to high seas fishing and probably underlies the particularly deep concern of these countries over uncontrolled high seas fishing.
The biological distinction between the "straddling" and highly migratory species is not always straightforward. The Chilean horse mackerel, for instance, which straddles about 1 500 miles off the EEZs of Chile and Peru is a particular case of straddling stock which might, from the biological standpoint, be as "highly migratory" as some of the smaller tunas mentioned in UNCLOS in which it is however not mentioned. It could be noted that some small tropical tuna species, considered as "highly migratory" indeed have limited migrations outside the EEZs, particularly when their life span is significantly reduced by fishing.
Importance of straddling stocks
The world number of straddling stocks is largely underestimated because they do not attract international attention if they are not exploited, particularly by foreign fleets in the adjacent high seas, and no conflicts of interest arise. In any case, only the target species tends to be mentioned when, in reality, the whole species assemblage is straddling and all the related bycatch species should also be included in any management plan.
A deficiency in data makes the economic dimension of the straddling stock problem difficult to assess. A crude estimate based on the reported landings of the species known as "straddling" or "likely to straddle" suggests that the total catches in EEZs and adjacent high seas increased from 5.8 million tonnes in 1970 to 12.4 million t in 1991, after a peak of 13.7 million t in 1988-89. In 1991, the total catches were about 11.4 million t of demersal fish and squids and 1 million t of tuna and tuna-like species including highly migratory species. The most important sources of straddling stocks (mainly demersal) seem to be the Northwest and Southeast Pacific areas followed by the Northeast Pacific and the Southwest Atlantic. However, there can be no reliable information in this respect until a proper reporting/collecting system is set in place.
Management of straddling stocks
The management issues related to straddling stocks depend on their biological characteristics and, in particular, on the degree of mixing between the EEZ and high seas components of the stock. In instances where mixing is considerable (probably in most cases) because of random dispersion and life cycle or seasonal migrations, the stock should be managed as one single unit and management measures must be harmonized over the entire range of distribution of the stock. This Principle of "compatibility" between management measures across the entire distribution of the stock has now been recognized in the UN Fish Stocks Agreement.