We describe our advances in applying acoustic techniques to the study of Antarctic zooplankton. First we show how the use of three-frequency data enables us to identify a greater number of common species of macro-zooplankton than is possible when using only two frequencies. For practical reasons we are not able to utilize three frequency techniques on routine krill surveys so, secondly we illustrate how dual-frequency acoustic techniques are providing detailed information on the biomass and size of Antarctic krill.
Accurate estimation of krill biomass by acoustic techniques is dependent on a number of factors of which one of the most fundamental is accurate echosounder calibration. The Simrad EK500 scientific echosounder used aboard RRS James Clark Ross is calibrated regularly at South Georgia before and after krill surveys there, and exhibits temporal stability in system gain settings. Between Antarctic seasons this echosounder has also been calibrated in temperate European waters. Under these warmer conditions, calibrated gain settings differ markedly from those applied in the Antarctic, even after adjustments have been made to account for differences in sound speed between locations. Here we present results from multiple Antarctic and European calibrations which suggest that echosounder transducer performance is dependent on ambient water temperature. Highly significant differences in volume backscattering (Sv) and target strength (TS) transducer gains were detected at both 38 and 120 kHz between calibrations conducted at the two locations. At 120 kHz, the required Sv transducer gains at South Georgia (sea temperature at depth of transducer = 2.3°C) were on average 1.4 dB less than in European waters (16.6°C), and a similar trend was detected at 38 kHz. If European calibration parameters were to be employed during surveys around South Georgia, and no account were taken of the differences in gain settings, then integrated 120 kHz echo signals would be under-reported by 2.8 dB, leading in turn to an under-estimation of krill biomass by 52.5%. Every effort should therefore be made to ensure that echosounders are calibrated at temperatures as close as possible to those prevailing within the area in which surveys are conducted. In addition, the implications for biomass estimation of temperature variation across a survey area should be considered carefully.
Interannual variability is a characteristic feature of the Southern Ocean ecosystem, yet the relative roles of biological and physical processes in generating these fluctuations are unknown. There is now extensive evidence that there are years when there is a very low abundance of Antarctic krill ( Euphausia superba ) in the South Georgia area, and that this variation affects much of the ecosystem, with the most obvious impacts on survival and breeding success of some of the major predators on krill. The open nature of the South Georgia ecosystem means this variability has large‐scale relevance, but even though there are unique time series of data available, information on some key processes is limited. Fluctuations in year‐class success in parts, or all, of the krill population across the Scotia Sea can generate large changes in the available biomass. The ocean transport pathways maintain the large‐scale ecosystem structure by moving krill over large distances to areas where they are available to predator colonies. This large‐scale physical system shows strong spatial and temporal coherence in the patterns of the interannual and subdecadal variability. This physical variability affects both the population dynamics of krill and the transport pathways, emphasizing that both the causes and the consequences of events at South Georgia are part of much larger‐scale processes.
Abstract Cox, M. J., Watkins, J. L., Reid, K., and Brierley, A. S. 2011. Spatial and temporal variability in the structure of aggregations of Antarctic krill (Euphausia superba) around South Georgia, 1997–1999. – ICES Journal of Marine Science, 68: . Antarctic krill are important in the South Georgia (54°S 35°W) marine ecosystem. They form aggregations that vary widely in packing density (<1 to 1000 s of individuals m−3), length (tens to thousands of metres), and height (tens of metres). Acoustic surveys are often used to estimate krill biomass and provide data that give insight into aggregation structure. Using dual-frequency (38 and 120 kHz) acoustic data collected during six surveys conducted around South Georgia during the 1997, 1998, and 1999 austral summers, we isolated 2990 aggregations by applying the Shoal Analysis and Patch Estimation System algorithm in Echoview and a krill-length-dependent acoustic identifier (ΔSv120–38). Multivariate cluster (partition) analysis was applied to metrics from each of the aggregations, resulting in three aggregation types with an overall proportional split of 0.28:0.28:0.44. Types 1 and 3 had low mean densities (<2 g m−3), whereas Type 2 had a mean density of 94 g m−3. Intersurvey differences were found between the effort-corrected numbers of aggregation types (p = 2.5e−6), and between on- and off-continental shelf areas (p = 1.5e−7), with a greater number of Type 2 aggregations being found on-shelf. The findings suggest intersurvey variation in krill catchability, with krill being more likely to be caught on-shelf.
Fifty-six Angus and 56 Charolais steers were evaluated for the effects of breed, slaughter weight and diet energy density on feedlot and carcass traits. Eight calves of each breed were slaughtered at the start of the experiment. Forty-eight steers of each breed were assigned for slaughter at 86 (light), 100 (middle) or 114% (heavy) of mean mature cow weight (Angus = 476 kg, Charolais = 612 kg). Diets of corn silage, corn and soybean meal were fed ad libitum; the diets contained 12.5% crude protein and either (1) 2.72 or (2) 2.96 Meal metabolizable energy (ME)/kg dry matter. Mean off-test weights were 267, 409, 472 and 5 34 kg for Angus and 270, 516, 602 and 681 kg for Charolais (initial, light, middle and heavy groups, respectively). Average daily gain (ADG) for both breeds decreased (P<.01) as weight increased, but ADG of Angus and Charolais did not differ from each other. Feed conversions (kilograms diet per unit weight gain) for the breeds were similar. Although light Angus had higher (P<.01) quality grades than light Charolais (∼average versus ∼low Choice), grades did not differ between needs at other weights. Breeds did not differ in longissimus muscle fat content at any assigned weight class. Fat thickness increased (P<.01) for both breeds as weight increased and was greater (P<.01) for Angus within each weight class. Although the higher ME diet (2) did not improve ADG or marbling for either breed, Angus fed diet 2 had greater (P<.05) mean fat thickness and lower (P<.05) carcass cutability than Angus fed diet 1. Angus fed either diet had lower (P<.01) cutability than Charolais. Steaks from Angus carcasses had higher (P<.01) average sensory panel flavor and juiciness ratings than did those from Charolais at the light weight, but sensory panel means did not differ at other weights. Low Choice longissimus muscle fat content (∼4.3%) was attained at shrunk body weights of 368 kg for Angus and 464 kg for Charolais.
Much of the distribution range of Antarctic krill, Euphausia superba, is covered by permanent or seasonal sea ice. Sea ice extent has been implicated as a major factor affecting reproductive success of krill and krill dispersal, but little is known of the way in which ice cover may influence krill behaviour. This is largely because the under-ice environment is difficult to study. Ship-borne echosounders have, however, detected krill aggregations in midwater in ice-covered regions. We used 120-kHz echograms collected underway during three cruises that crossed ice-covered and adjacent open waters in the Bellingshausen, Weddell, and Scotia seas to compare morphological and next-neighbour characteristics of krill swarms within and without ice cover. No significant differences were detected between the horizontal and vertical extent of swarms or swarm next-neighbour distance in ice-covered or open waters. Distributions of swarm mid-depths did, however, differ significantly between ice-covered and open areas in all three seas, although the direction of difference was not the same in each instance: swarms in the Weddell and Scotia seas were generally shallower under ice than in open water, whereas in the Bellingshausen Sea the opposite prevailed.
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