CCAMLR

Antarctic krill Euphausia superba has a central role in the ecosystem of the Southern Ocean and knowledge of its growth rate is central to determining the factors influencing population dynamics. The length of Antarctic krill in the diet of Antarctic fur seals Arctocephalus gazella at South Georgia revealed a consistent increase in size between c 42 - c 54 mm over the period October-March, indicating growth rates much higher than predicted by existing models. Geographical variation in growth rate may result in 2 year-old krill at South Georgia attaining the same size as 3 year old krill in the Antarctic Peninsula region. The effect of geographical variation in growth rate on the population structure of krill has important implication for comparing the fate of individual cohorts over large scales and in the interpretation of krill life cycles.

The WG-EMM sub-group on methods is potentially in a position to change its emphasis from one that considers methods for collecting data as part of CEMP to one that considers and develops methods for analysing and using those data to provide management advice. Within this expanded role there are 4 key areas; 1.to maintain the process of validation and checking of the existing CEMP database, 2. to evaluate appropriate methods for analysis of existing parameters, 3. to develop methods to combine parameters at appropriate spatial and temporal scales and 4. to develop methods to take CEMP data forward as management advice.

Standardisation of methods to measure krill and estimate maturity and feeding status is essential if studies involving inter-comparison of datasets are planned. The methods used by workers from different institutes are outlined in the hope that standardisation by WG-EMM can be established and thus improve the quality of data from research surveys as well as through the CCAMLR International Observer Programme.

Three separate acoustic surveys of Antarctic krill (Euphausia superba) were conducted around South Georgia in the 2000/2001 season: in October 2000 (early-season); during late December / early January 2000/2001 (mid-season), and in March 2001 (late-season). The surveys were the first in a newly-planned five-year series of observations designed to complement and extend an existing time series maintained by the British Antarctic Survey regularly since 1996 (and on a more ad hoc basis since the early1980s). We hoped that conducting several surveys in one season would provide information on short-term variability that could be used to set data from more restricted “snap-shot” cruises in a broader context. The early- and late-season surveys were associated with logistic support voyages to South Georgia and were restricted to four transects within a box to the north-west of South Georgia. The dedicated mid-season survey covered that box in more detail (twice as many transects) and, in addition, examined boxes to the north, north-east and south-west of the island. Together these surveys provided temporally and spatially extensive coverage around South Georgia. Krill density in the western box in the early-season survey was very low (3.5 g m-2 ) but rose significantly (P = 0.048) by mid-season (to 34.7 g m-2). In a pattern that is consistent with observations from previous years, krill density in the western box mid-season was less than that in the eastern box (80.4 g m-2). Analysis of mid-season western survey box transect data revealed no significant difference between the mean krill density derived from only those 4 transects surveyed early- and late-season or from the full 8 transects. Our first occupation of a survey box off the central north coast of South Georgia mid-season revealed a krill density of 47.2 g m-2 that was intermediate between the eastern and western areas. The size structure of the krill in the central region also reflected a mix of those to the east (generally small) and west (generally large). Krill density to the south-west of South Georgia was 32.1 g m-2 mid-season. By March, krill density in the western survey area had fallen significantly (P = 0.04) from the mid-season high to 7.8 g m-2. Our multiple surveys at South Georgia have revealed major intra-annual changes in krill density at the island and have shown that the timing of the acoustic survey can significantly effect the estimate of krill density. The multiple estimates of krill density will allow indices of reproductive performance of top level predators to be compared to prey availability at time scales more appropriate than have previous single “snap-shot” acoustic survey data. This is a crucial step in the elucidation of response functions of dependent species to changes in krill abundance, and could be a useful contribution to ecosystem management.

The autonomous underwater vehicle (AUV) Autosub-2 was deployed on eight missions ahead of RRS James Clark Ross in the northern Weddell Sea and in the Bransfield Strait to assess avoidance of the research vessel by Antarctic krill Euphausia superba. The AUV was equipped with the same type of scientific echosounder as the research vessel (Simrad EK500 operating at 38 kHz and 120 kHz) and measured the density of krill along transect acoustically (g m-2 wet mass) prior to the ship’s arrival. We hypothesised that if krill avoided the ship, perhaps in response to radiated noise, then the ship should detect less krill than the AUV (which is very quiet). We were unable to detect any significant difference between the amount of krill detected by the ship or the AUV, either at the transect level or at finer scales within transects. We conclude, therefore, that avoidance by krill of RRS James Clark Ross will not significantly bias krill abundance estimates by this vessel.

The distribution, density and length composition of krill in commercial concentrations were investigated in the regions of South Shetland, Elephant, South Georgia and South Orkneys in spring and summer 1997-1999. The concentration densities varied with area and season. The densest concentrations were found near South Shetland. Concentrations at night were several times less dense than those during the day. Between February and April the concentration density increased, and in May and June it decreased. Since during the period of February - June day illumination progressively becomes shorter it must be assumed that there are other factors besides illumination which have an impact on the degree of krill concentration. The internal structure of the concentration was very diverse and the mean density varied from 11 to 31,370 specimens * m-3. At night, the krill concentrations were scattered throughout the water, however, no clear evidence of vertical migrations was found. Between February and April, the night concentrations occurred in much shallower waters than during the day, while in May and June they occurred at the same depths as the day concentrations or sometimes even deeper. The day concentrations between February and April, ocuured in shallower waters, while between April and June they were found in deeper waters. The size structure of the krill was diverse during all periods and in all areas, however, a decrease in krill size was observed as the season progressed.

There is a long-term interest in the population genetics of Antarctic krill species because of their ecological end economic importance. The possibility that there are distinct genetic stocks of these species would affect the design of management strategies for conserving them. A recent resurgence of interest in identifying distinct stocks of swarming krill species has been driven by the development of genetic technologies that are more sensitive to subtle population structure than older methods. Previous studies of the population genetics of the Antarctic species Euphausia crystallorophias, E. superba and the boreal species Meganyctiphanes norvegica that used allozymes found no evidence for population structure. More recent investigations using sequence variation in mitochondrial DNA (mtDNA) did indicate genetic differentiation between samples taken from different parts of the each species' range. However, the underlying assumption of these studies that differentiation between samples is caused primarily by restricted gene flow between widely separated sampling sites may be incorrect. Our recent study of E. crystallorophias mtDNA variation shows that there is significant genetic differentiation between samples taken within one region. This has important implications for the design of future studies of krill population genetics, which must be able to accommodate this variance component as well as variance attributable to differences between regions. Genetic differentiation between stocks of krill in different regions can therefore not be adequately assessed unless multiple samples are taken from each region.

Monitoring of the sea surface temperature (SST) in Subarea 48.3 on satellite data (GOES-E, Meteosat), use of in situ data and the further analysis of the SST maps, maps of the SST anomalies and dynamics provide strictly special, but constant information on hydrological situation in the survey area. Analysis of this situation based on data obtained in expeditions, as well as literature and the CCAMLR materials could allow us to assess, with a certain degree of reliability, a potential situation of krill fishery in Subarea 48.3 for the entire coming year already by the summer season onset. Reliability of the krill fishery forecasts could be judged only by actual results of fishing activities under various hydrological conditions. This work is an attempt to perform such an analysis using a certain array of materials obtained in 1989-1991 and 1999-2001.

A net sampling survey was carried out for krill in an established standard station grid around Elephant Island during 27 January to 4 February 2001. Results show a spatial separation of the juvenile and the spawning stock. The station grid was extended to the south, where a large proportion of small size classes, one year old juvenile krill was found. Krill density was significantly higher than during the past years (between 198 and 230 krill 1000 m-3 , i.e. between 40 and 46 m-2 or 11.3 g m-2). The proportional recruitment index for the entire survey area for the 1999/2000 year class was R1 = 0.573, and the absolute index was RI1 = 131.4 * 1000 m-3 , which is among the highest values for the past 20 years. The high krill abundance and the high recruitment index reflect the end of a succession of years with poor recruitment success. The maturation index (G, according to gravid stages), which is discussed as an indicator for a successful spawning season, was G = 0.99, indicating an early initiation of the spawning season. This is thought to be the first step for a successful spawning event and a later potential recruitment success of the 2000/01 year-class. The spatial extent of the station grid is discussed in the light of a representative coverage of the stock and the estimated recruitment index.

In 2000, WG-EMM requested that the Secretariat review the historical development of CEMP indices and ecosystem assessments (Table 3 and paragraph 3.55). The following paper provides a starting point for such a review, as well as a conceptual approach which may provide a framework for a set of reference documents which could be held electronically on the CCAMLR website and/or on CDROM.