CCAMLR

A major mid-1980s shift in ecological structure of significant portions of the Southern Ocean was partially due to the serial depletion of fish by intensive industrial fishing, rather than solely to climate factors as previously hypothesised. Over a brief period (1969-1973), several finfish stocks were on average reduced to <50%, and finally (mid-1980s) to <20%, of original size. Despite management actions, few stocks have recovered and some are still declining. Most affected species exhibit K-selected life-history patterns, and before exploitation presumably fluctuated in accordance with infrequent strong year classes, as is true of such fish elsewhere. A climate regime, the Southern Annular Mode, once oscillated between two states, but has remained in its “positive mode” since the time of the fish extraction. This may have increased finfish vulnerability to exploitation. As breeding stocks decreased, we hypothesize that availability of annually-produced juvenile fish fed upon by upper-level predators remained low. Correlations between predator populations and fish biomass in predator foraging areas indicate that southern elephant seal Mirounga leonina, Antarctic fur seal Arctocephalus gazella, gentoo penguin Pygoscelis papua, macaroni penguin Eudyptes chrysolphus and “imperial” shag Phalacrocorax spp. — all feeding extensively on these fish, and monitored at Marion, Crozet, Kerguelen, Heard, South Georgia, South Orkney and South Shetland islands, where fishing was concentrated — declined simultaneously during the two periods of heavy fishing. These patterns indicate the past importance of demersal fish as prey in Antarctic marine systems, but determining these interactions’ ecological mechanisms may now be impossible.

The Southern Ocean is a major component within the global ocean and climate system and potentially the location where the most rapid climate change is most likely to happen, particularly in the high-latitude polar regions. In these regions, even small temperature changes can potentially lead to major environmental perturbations. Climate change is likely to be regional and may be expressed in various ways, including alterations to climate and weather patterns across a variety of time-scales that include changes to the long interdecadal background signals such as the development of the El Niño–Southern Oscillation (ENSO). Oscillating climate signals such as ENSO potentially provide a unique opportunity to explore how biological communities respond to change. This approach is based on the premise that biological responses to shorter-term sub-decadal climate variability signals are potentially the best predictor of biological responses over longer time-scales. Around the Southern Ocean, marine predator populations show periodicity in breeding performance and productivity, with relationships with the environment driven by physical forcing from the ENSO region in the Pacific. Wherever examined, these relationships are congruent with mid-trophic-level processes that are also correlated with environmental variability. The short-term changes to ecosystem structure and function observed during ENSO events herald potential long-term changes that may ensue following regional climate change. For example, in the South Atlantic, failure of Antarctic krill recruitment will inevitably foreshadow recruitment failures in a range of higher trophic-level marine predators. Where predator species are not able to accommodate by switching to other prey species, population-level changes will follow. The Southern Ocean, though oceanographically interconnected, is not a single ecosystem and different areas are dominated by different food webs. Where species occupy different positions in different regional food webs, there is the potential to make predictions about future change scenarios.

The consequences of warming for Antarctic long-lived organisms depend on their ability to survive changing patterns of climate and environmental variation. Among birds and mammals of different Antarctic regions, including emperor penguins, snow petrels, southern fulmars, Antarctic fur seals and Weddell seals, we found strong support for selection of life history traits that reduce interannual variation in fitness. These species maximise fitness by keeping a low inter-annual variance in the survival of adults and in their propensity to breed annually, which are the vital rates that influence most the variability in population growth rate (λ). All these species have been able to buffer these rates against the effects of recent climate-driven habitat changes except for Antarctic fur seals, in the Southwest Atlantic. In this region of the Southern Ocean, the rapid increase in ecosystem fluctuation, associated with climate variability observed since 1990, has limited and rendered less predictable the main fur seal food supply, Antarctic krill. This has increased the fitness costs of breeding for females, causing significant short-term changes in population structure through mortality and low breeding output. Changes occur now with a frequency higher than the mean female fur seal generation time, and therefore are likely to limit their adaptive response. Fur seals are more likely to rely on phenotypic plasticity to cope with short-term changes in order to maximize individual fitness. With more frequent extreme climatic events driving more frequent ecosystem fluctuation, the repercussions for life histories in many Antarctic birds and mammals are likely to increase, particularly at regional scales. In species with less flexible life histories that are more constrained by fluctuation in their critical habitats, like sea-ice, this may cause demographic changes, population compensation and changes in distribution, as already observed in penguin species living in the Antarctic Peninsula and adjacent islands.

Surveys of Euphausia superba often target localised shelves and ice edges where their growth rates and predation losses are atypically high. Emphasis on these areas has led to the current view that krill require high food concentrations, with a distribution often linked to shelves. For a wider, circumpolar perspective we compiled all available net-based density data on postlarvae: 8137 mainly summer stations from 1926-2004. Unlike Antarctic zooplankton their distribution is highly uneven, with 70% of the total stock concentrated between 0o and 90oW. Within this Atlantic sector, krill are abundant over both shelf and ocean. At the Antarctic Peninsula, by contrast, they are found mainly over the inner shelf whereas in the Indian-Pacific sectors krill prevail in the ocean within 200-300 km of the shelf break. Overall, 87% of the total stock live over deep oceanic water (>2000 m) and krill occupy regions of moderate food (0.5-1.0 mg chl a m-3). Advection models suggest some loss northwards from these regions and into the low chlorophyll belts of the Antarctic Circumpolar Current (ACC). We found possible evidence for a compensating southwards migration, with an increasing proportion of krill found south of the ACC as the season progressed. The retention of krill in moderately productive oceanic habitats is a key factor in their high total production. While growth rates are lower than those over shelves, the ocean provides a refuge from shelf-based predators. The unusual circumpolar distribution of krill thus reflects a balance between advection, migration and top-down and bottom-up processes.

The Southern Ocean is known to have warmed considerably during the second half of the 20th century but there are few locations with data before the 1950s. In addition, assessments of change in this region are hampered by the strong seasonal bias in sampling, with the vast majority of data collected during the austral summer. However, oceanographic measurements near South Georgia span most of the last century, and we here consider almost year-round data from this location over an 81-year period (1925 to 2006). Based on these data, we observe significant warming between the early and late 20th century, with differential warming between summer and winter months and an indication that late 20th century summer temperatures peaked ~6 days earlier. To quantify the long-term warming trend in this highly variable data, a mixed model utilising a Residual Maximum Likelihood (REML) method was used. Over the 81-year period, a mean increase of ~0.9°C in January and ~2.3°C in August was evident in the top 100 m of the water column. Warming diminished below 100 m and approached zero at 200 m. Thus the long-term warming around South Georgia is substantial – more so than documented previously for the circumpolar warming of the Southern Ocean. We examine potential causal effects of this trend, including local atmospheric and cryospheric change, the influence of upstream waters and the role of coupled modes of climate variaibility. It is likely that all of these play a part in the observed temperature increase. However, the role of the Southern Annular Mode (SAM) is strongly indicated, via its likely role in the circumpolar warming trend in the Southern Ocean, and also due to the atypical response of the South Georgia region to changes in heat fluxes associated with the SAM. In addition, we consider the implications that long-term warming has for South Georgia’s lower trophic levels. For Euphausia superba, we find a significant negative relationship between summer South Georgia water temperatures and mean summer density of E. superba across the southwest Atlantic sector of the Southern Ocean. Simple abundance and growth rate relationships with our long-term temperature data appear to show declining habitat suitability for E. superba. In general, the warming trend is likely to favour other macro- and mesozooplankton species that occupy the more northerly parts of the Antarctic Circumpolar Current and it is likely to promote phytoplankton growth.

Determining how climate fluctuations affect ocean ecosystems requires an understanding of how biological and physical processes interact across a wide range of scales. Here we examine the role of physical and biological processes in generating fluctuations in the ecosystem around South Georgia in the South Atlantic sector of the Southern Ocean. Anomalies in sea surface temperature (SST) in the South Pacific sector of the Southern Ocean have previously been shown to be generated through atmospheric teleconnections with El Niño Southern Oscillation (ENSO)-related processes. These SST anomalies are propagated via the Antarctic Circumpolar Current into the South Atlantic (on time scales of more than 1 year), where ENSO and Southern Annular Mode-related atmospheric processes have a direct influence on short (less than six months) time scales. We find that across the South Atlantic sector, these changes in SST, and related fluctuations in winter sea ice extent, affect the recruitment and dispersal of Antarctic krill. This oceanographically driven variation in krill population dynamics and abundance in turn affects the breeding success of seabird and marine mammal predators that depend on krill as food. Such propagating anomalies, mediated through physical and trophic interactions, are likely to be an important component of variation in ocean ecosystems and affect responses to longer term change. Population models derived on the basis of these oceanic fluctuations indicate that plausible rates of regional warming of 1°C over the next 100 years could lead to more than a 95% reduction in the biomass and abundance of krill across the Scotia Sea by the end of the century.

Antarctic krill (Euphausia superba) is a large euphausiid, widely distributed within the Southern Ocean, and a key species in the Antarctic food web. The Discovery Investigations in the early 20th century, coupled with subsequent work with both nets and echosounders, indicated that the bulk of the population of postlarval krill is typically confined to the top 150 m of the water column. Here, we report for the first time the existence of significant numbers of Antarctic krill feeding actively at abyssal depths in the Southern Ocean. Biological observations from the deepwater remotely operated vehicle Isis in the austral summer of 2006/07 have revealed the presence of adult krill (Euphausia superba Dana), including gravid females, at unprecedented depths in Marguerite Bay, western Antarctic Peninsula. Adult krill were found close to the seabed at all depths but were absent from fjords close inshore. At all locations where krill were detected they were seen to be actively feeding, and at many locations there were exuviae (cast molts). These observations revise significantly our understanding of the depth distribution and ecology of Antarctic krill, a central organism in the Southern Ocean ecosystem.

Scientific observation on the species composition and abundance of fishes incidentally caught during Antarctic krill (Euphausia superba DANA) fisheries by F/V Niitaka Maru (5200t) were made from 6 August to 30 August to the north of South Georgia Is. Among 87 net hauls quantitatively examined, by-catch fish was recognized in 26 trawl catches (29.9%) and only one fish was found as by-catch in 21 hauls. Among a total of 7 fish species, Myctophidae 3, Zoarcidae 1, Nototheniidae 1 and Channichthyidae 2, recognized, Krefftichthys anderssoni of Myctophidae occurred most frequently (38.5% of net hauls containing by-catch fish). Owing to the small amount of by-catch, no clear relationships between krill CPUE and fish by-catch could be confirmed in the present study.

In order to calculate the target strength of Antarctic krill (Euphausia superba) using an acoustical scattering model, information on size, morphology, orientation, sound–speed and mass-density contrasts between the animal and the surrounding water are required. Sound-speed and mass-density of krill were measured during the Antarctic surveys conducted by the Japanese RV Kaiyo Maru in 1999/2000 and 2004/2005. Samples of krill were caught by a RMT(1+8). Mass-density of krill was measured by density bottle method. The mean total length and the mean mass-density contrast were 43.5mm and 1.028 near South Shetland Islands in February 2000. These were 21.7mm and 1.049, and 45.1mm and 1.043 in the Ross Sea in January and February 2005. Sound-speed was measured using the "time of flight" method. The corresponding sound-speed contrast of krill with mean total lengths of 44.2mm was 1.011 in the South Shetland Islands. These contrasts of krill with mean total lengths of 25.1mm and 48.6mm were 1.044 and 1.035 respectively in the Ross Sea. To examine the effect of these parameter differences, the target strength and its directivity of krill were calculated using the stochastic DWBA model.