The Southern Ocean—the seas off Antarctica south of approximately 50 to 60 degrees—is substantially isolated from other oceans by the Antarctic Convergence or South Polar Front (Kock 1992). It is noted for the presence of the world's heaviest seabirds (penguins), large populations of krill (planktonic crustaceans, mainly Euphausia superba ), formerly large populations of whales, and some commercially valuable fish species. Most of the fishes are relatively slow growing and thus cannot support high exploitation rates (Kock 1992). Indeed, Kock estimated that the total sustainable catch of all finfish from the Southern Ocean is no more than 100,000 t per year, about 0.1 percent of the world catch. As mentioned in Chapter 2, the marbled rockcod (Notothenia rossi) has been severely overexploited, with 400,000 t being taken in 1969–1970 alone. Recent production has been on the order of a few thousand metric tons per year. The Patagonian toothfish (Dissostichus eleginoides) also appears vulnerable to overexploitation, which has probably already occurred. It has become popular in restaurants, where it is known as Chilean sea bass. Catch rates and population estimates of this fish are poorly known (Kock 1992, Albemus 1997), but Kock reported that the some D. eleginoides populations were significantly overfished.
Although ecosystem effects of overexploitation have probably occurred in the Southern Ocean, they have unfortunately not been well studied (Kock 1992). Despite the general lack of information, it is known that the main prey species for larger marine vertebrates in the ecosystem are krill, and they have been studied extensively (Miller and Hampton 1989). They appear to be larger, longer lived, and slower growing than most marine plankton. Biomass estimates vary, but the standing stock is probably at least several hundred million metric tons in the summer (Miller and Hampton 1989). Calculations by Bengston (1984), Laws (1985), and Yamanaka (1983) and a detailed review by Miller and Hampton (1989) suggest that current consumption of krill by predators in the Southern Ocean exceeds 200 million t per year, of which perhaps 40 million t is due to baleen whales. Baleen whale populations in the Southern Ocean consumed perhaps 190 million t per year before exploitation. Thus, there is a "krill surplus" of about 150 million t per year (Miller and Hampton 1989). This is not currently being exploited by fisheries (krill fisheries are only around 100,000 t/year [David Agnew, Imperial College, London, personal communication, 1997] and finfish landings are very low, as mentioned above). It would be of great interest to know how the "surplus" is being consumed in the ecosystem and how recovery of whale populations would affect the ecosystem. As Miller and Hampton (1989) pointed out, it is not really a surplus; the term refers only to krill that is no longer consumed by whales. Some of the "surplus" may be recycled through decomposition pathways; most of it probably is eaten by other predators. Thus, the removal of whales has led to increases in other predator populations. It would