CCAMLR

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.

In this paper the krill spatial distribution in the in coastal and pelagic SSMUs of the Subarea 48.2 depending on oceanological factors is considered. Estimates of krill biomass and aggregation characteristics, and krill transport factors according to different modifications of Antarctic water mass are presented based on the CCAMLR 2000 Survey data. It is shown, that despite the high biomass concentrated in the South Orkney Pelagic Area (SOPA) during the CCAMLR 2000 Survey, the krill aggregation patterns did not meet the requirements of the present day fishery. Krill distribution and water mass circulation according to CCAMLR 2000 Survey (January-February 2000) are compared with those obtained from the data of long-term observations and fishery in Subarea 48.2. The types of geostrophic current fields and correspondent fishing ground allocations in the South Orkney Islands area revealed from 1962 to 1997 are presented. It is concluded that the development of options for krill stock management call for actual materials, describing annual and seasonal changes in biomass and characteristics of krill distribution in the SSMUs areas.

We validate the acoustic target classification protocols developed for the Stochastic Distorted-Wave Born Approximation (SDWBA) model using three frequency acoustic data and concurrent net hauls that were collected during two cruises to the South Georgia region in 1996. For each krill aggregation sampled by net we calculated the difference between acoustic backscatter at 120 and 38 kHz (Sv120-38) and at 200 and 120 kHz (Sv200-120). We considered the performance of 4 different acoustic target identification algorithms for krill: (i) ‘3 freq model’ - using the SDWBA to set the acceptance windows for both Sv120-38 and Sv200-120, (ii) ‘2 freq model’ - using the SDWBA to set the acceptance window for just Sv120-38, (iii) ‘2 freq 2-16’ - where the Sv120-38 window was fixed at 2-16 dB and (iv) ‘2 freq 2-12’ - where the Sv120-38 window was fixed at 2-12 dB. The overall aggregation dB difference for 120 – 38 kHz for every net fell within the SDWBA model derived target id window, however, for 200 - 120 kHz the SDWBA model derived target id window only identified krill in 6 of the 16 nets correctly. The ‘2 freq 2-16’ algorithm attributed more than 90 % of the total backscatter to krill in all but 1 aggregation with the ‘2 freq model’ using a smaller window but still attributing more than 90 % of the total backscatter to krill in 12 out of the 16 nets. The ‘2 freq 2 – 12’ window only attributed more than 90 % of the total backscatter to krill in 6 nets while the ‘3 freq model’ attributed only just greater than 50 % of the backscatter to krill in only 2 aggregations, and in 6 aggregations attributed less than 10 % of the total backscatter to krill. Therefore the SDWBA(11,4) using 38, 120 and 200 kHz in the present configuration to set variably sized windows is likely to substantially underestimate krill. In contrast the SDWBA(11,4) used at 38 and 120 kHz identifies very well the krill detected during these net hauls and because it uses a window substantially smaller than the fixed 2-16 window, will at the same time reduce the amount of bycatch that may occur when targets other than Antarctic krill are present in the water column.

One recommendation from the Predator Survey workshop was the undertaking of some immediate inter-sessional work to be reported to WG-EMM-08. The work to be undertaken included preliminary estimation of SSMU-specific gentoo and Adélie breeding abundance in Area 48, and similar analyses of east Antarctic data for Adélie penguins. It was recognised that the abundance estimates would not be corrected for availability (e.g. nest failure) and hence would to some extent be biased estimates of the breeding population, and would only account for uncertainty in the accuracy of the count data. However, the work was seen as useful in illustrating both the database of penguin count data compiled for the workshop, and the underlying basis of new estimation procedures presented to the workshop. At an SSMU level, uncertainty associated with accuracy or repeatability of the counts varied substantially between SSMUs (95% C.I. as a percent of the count ranged from 1.5% to 37.1%).