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Title: Distribution and abundance of larval fishes collected in the western Bransfield Strait region, 1986-87,
Description: Larval fishes were obtained from 0-50 m Bongo net and stratified 0-200 m Nansen net samples collected in the western Bransfield Strait region during four RACER (Research on Antarctic Coastal Ecosystem Rates) program cruises, December 1986 to March 1987. Three nototheniid species, Nototheniops larseni, Notothenia gibberifrons, and a form tentatively identified as Tremtomus scotti numerically dominated the total catch. Two typically abundant species in this area, Pleuragramma antarcticum and Notothenia kempi, were relatively uncommon. Gear differences, the restricted geographical coverage, and between-year variability in hydrographic conditions may explain species dominance differences between the RACER samples and prior surveys in the Antarctic Peninsula area. Largest abundances occurred in the vicinity of Gerlache Strait and island shelf areas; larvae were infrequently collected in Drake Passage waters. Individual species demonstrated different distributional patterns, suggesting that water mass influence, larval depth distribution and behavioral differences may be involved. Estimated January to March growth rates for N. larseni (0.06 mm day-1) and T. scotti (0.05 mm day-1) are similar but low compared to that of N. gibberifrons (0.12 mm day-1). © 1991., Cited By (since 1996):12, Antarctica, Fish and Fisheries, ,
Author: Loeb
Date: 1991-01-01T00:00:00Z

Title: Spatial association between hotspots of baleen whales and demographic patterns of Antarctic krill Euphausia superba suggests size-dependent predation,
Description: We examined the spatial association between baleen whales and their principal prey, Antarctic krill Euphausia superba near the South Shetland Islands (Antarctic Peninsula) using data collected by the US Antarctic Marine Living Resources (AMLR) program during January surveys from 2003 through 2007. Whale distributions were determined using ship-based visual surveys, while data on krill distribution, abundance, and demographic characteristics were derived from net hauls. Approximately 25 000 km of transects and 500 net hauls were sampled over 5 yr. We defined hotspots based on statistical criteria to describe persistent areas of occurrence of both whales and krill. Hotspots were identified, and whales and krill length-maturity classes exhibited distinct spatial segregation in their distribution patterns. We found that baleen whales aggregated to krill hotspots that differed in size structure. Humpback whales Megaptera novaeangliae were associated with small (<35 mm) juvenile krill in Bransfield Strait, whereas fin whales Balaenoptera physalus were associated with large (>45 mm) mature krill located offshore. Overlapping these size-dependent krill distributions, Antarctic minke whales B. bonaerensis were associated with intermediate sized krill (35-44 mm). The correlation among different whale species and krill swarms of differing size composition presents an intriguing pattern that deserves further study. © Inter-Research 2010., Cited By (since 1996):16, Marine Mammals, Birds & Turtles, Antarctica, CODEN: MESED, ,
Author: Santora, Reiss, Loeb, Veit
Date: 2010-01-01T00:00:00Z

Title: Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996-2006,
Description: The time-series of acoustically surveyed Antarctic krill (Euphausia superba) biomass near the South Shetland Islands (SSI) between 1996 and 2006 is re-estimated using a validated physics-based model of target strength (TS), and a species-discrimination algorithm based on the length-range of krill in plankton samples to identify krill acoustically, derived from TS-model predictions. The SSI area is surveyed each austral summer by the US Antarctic Marine Living Resources Program, and the acoustic data are used to examine trends in krill biomass and to assess the potential impact of fishing to the reproductive success of land-based predators (seals and penguins). The time-series of recomputed biomass densities varies greatly from that computed using an empirical log-linear TS-model and fixed-ranges of differences in volume-backscattering strengths (ΔS v), conventionally used to identify krill acoustically. The new acoustic estimates of biomass are significantly correlated with both proportional recruitment and krill abundance estimated from zooplankton samples. Two distinct peaks in biomass (1996 and 2003) are in accord with recruitment events shown by net-based krill time-series. The foundation for the new TS-model and the associated krill-discrimination algorithm, coupled with the agreement between acoustic- and net-survey results, provides strong support for the use of the new analytical technique. Variable biases in the re-estimated krill biomass have been greatly reduced. However, survey variability increased as a result of the increased rejection of acoustic backscatter previously attributed to krill. Management of Southern Ocean krill stocks based on a precautionary approach may therefore result in decreased allocations of krill, given its dependence on the variability of survey estimates. © 2008 US Government and the Department of Commerce/National Oceanic and Atmospheric Administration/National Marine Fisheries Service/Southwest Fisheries Science Center., Cited By (since 1996):41, CODEN: ICESE, Antartica, ,
Author: Reiss, Cossio, Loeb, Demer
Date: 2008-01-01T00:00:00Z

Title: Long-term monitoring of krill recruitment and abundance indices in the Elephant Island area (Antarctic Peninsula),
Description: Krill distribution and density are reviewed for the Elephant Island area with regard to the representativeness of the study area (60°-62°30′S and 53°-57°30′W) for proportional recruit and density indices. Proportional recruitment indices were re-calculated applying the delta distribution approach introduced by de la Mare (1994a). The high interannual variability of krill recruitment is confirmed by the present analysis. Results are compared for one- and two-year-old krill (R1 and R2 respectively). Statistically significant fluctuations in krill density over the period 1977 to 1994 are also confirmed by this study using randomisation tests on an analysis of variance., Cited By (since 1996):33, Antarctica, Invertebrates, ,
Author: Siegel, De La Mare, Loeb
Date: 1997-01-01T00:00:00Z

Title: Pteropods and climate off the antarctic Peninsula
Description: Shelled (thecosome) and naked (gymnosome) pteropods are regular, at times abundant, members of Southern Ocean zooplankton assemblages. Regionally, shelled species can play a major role in food webs and carbon cycling. Because of their aragonite shells thecosome pteropods may be vulnerable to the impacts of ocean acidification; without shells they cannot survive and their demise would have major implications for food webs and carbon cycling in the Southern Ocean. Additionally, pteropod species in the southwest Atlantic sector of the Southern Ocean inhabit a region of rapid warming and climate change, the impacts of which are predicted to be observed as poleward distribution shifts. Here we provide baseline information on intraseasonal, interannual and longer scale variability of pteropod populations off the Antarctic Peninsula between 1994 and 2009. Concentrations of the 4 dominant taxa, Limacina helicina antarctica f. antarctica, Clio pyramidata f. sulcata, Spongiobranchaea australis and Clione limacina antarctica, are similar to those monitored during the 1928-1935 Discovery Investigations and reflect generally low values but with episodic interannual abundance peaks that, except for C. pyr. sulcata, are related to basin-scale climate forcing associated with the El Niño-Southern Oscillation (ENSO) climate mode. Significant abundance increases of L. helicina and S. australis after 1998 were associated with a climate regime shift that initiated a period dominated by cool La Niña conditions and increased nearshore influence of the Antarctic Circumpolar Current (ACC). This background information is essential to assess potential future changes in pteropod species distribution and abundance associated with ocean warming and acidification. © 2013 Elsevier Ltd.
Author: Loeb, Santora
Date: 2013-01-01T00:00:00Z

Title: Early development of Diaphus garmani (Myctophidae) in the transition region of the western North Pacific,
Description: We present the descriptions of the larval and transforming stages of one of the most abundant Diaphus species in the transition region of the western North Pacific, Diaphus garmani. Species identification was achieved by tracing characters backward from identifiable juveniles through transforming and larval stages. Description of the larval development includes the morphometric characteristics, pigmentation, and photophore development, which help identify the early life stages of this species., Cited By (since 1996):6, Fish and Fisheries, ,
Author: Sassa, Kawaguchi, Loeb
Date: 2003-01-01T00:00:00Z

Title: Salp distribution and size composition in the Atlantic sector of the Southern Ocean,
Description: Salp abundance and length frequency were measured during the large-scale CCAMLR 2000 Survey conducted in the Atlantic Sector of the Southern Ocean in the 1999/2000 season. Results from regional surveys around Elephant Island in 1994/95 and 1996/97 seasons also were examined. During the CCAMLR 2000 Survey, salp abundance was higher in the Antarctic Peninsula and South Sandwich Island areas than in the central Scotia Sea. The probable reason for this pattern is a negative relationship with phytoplankton abundance; the central Scotia Sea having greater phytoplankton concentrations than required for optimal salp filter-feeding performance. Cluster analysis of salp size composition resulted in three cluster groups for each of the three surveys. Clusters comprising large salps occurred in warmer waters in all three surveys. The size composition of the salp populations suggests that the timing of intense asexual reproductive budding was earlier in warmer waters. As surface water temperatures generally decrease from north to south, and increase from spring to summer, the general spatio-temporal pattern of asexual reproduction by budding is likely to proceed from north to south as the summer season progresses. © 2004 Elsevier Ltd. All rights reserved., Cited By (since 1996):12, CODEN: DSROE, ,
Author: Kawaguchi, Siegel, Litvinov, Loeb, Watkins
Date: 2004-01-01T00:00:00Z

Title: Population dynamics of Salpa thompsoni near the Antarctic Peninsula: Growth rates and interannual variations in reproductive activity (1993-2009),
Description: The salp Salpa thompsoni has exhibited increased abundance in high latitude portions of the Southern Ocean in recent decades and is now frequently the numerically dominant zooplankton taxon in the Antarctic Peninsula region. The abundance increase of this species in high latitude waters is believed related to ocean warming. Due to its continuous filter feeding and production of dense rapidly sinking fecal pellets S. thompsoni is considered to be an important link in the export of particulate carbon from the surface waters. Hence basic information on the life history of this component of the Antarctic marine ecosystem is essential for assessing its impact given continued climate warming. Here we cover various aspects of the life history of S. thompsoni collected in the north Antarctic Peninsula during annual austral summer surveys of the US Antarctic Marine Living Resources (AMLR) Program between 1993 and 2009. We focus on seasonal and interannual variations in the size composition and abundance of the aggregate (sexual) and solitary (asexual) stages. This information is valuable for refining components of Southern Ocean food web models that explicitly deal with size-structured and life history information on zooplankton. Intraseasonal changes in length-frequency distribution of both stages are used to estimate their growth rates. These average 0.40mmday -1 for aggregates and 0.23mmday -1 for solitaries; together these represent ~7week and ~7.5month generation times, respectively, and a 9month life cycle (i.e., onset of aggregate production year 1 to aggregate production year 2). Based on the maximum lengths typically found during January-March, the life spans of the aggregate and solitary stages can reach at least ~5 and ~15months, respectively. Length-frequency distributions each year reflect interannual differences in timing of the initiation and peak reproductive output. Interannual differences in the abundance of total salps and proportions of the overwintering solitary stage are significantly correlated with El Niño Southern Oscillation indices (SOI and Nino3.4) prevailing over the previous 2years. Massive salp blooms result from two successive summers of elevated solitary production following a reversal from La Niña to El Niño conditions. These results indicate the role of basin-scale atmospheric-oceanic processes in establishing optimal conditions that support aggregate and solitary stage reproduction, development and growth. © 2011 Elsevier Ltd., Cited By (since 1996):4, CODEN: POCNA, ,
Author: Loeb, Santora
Date: 2012-01-01T00:00:00Z

Title: Mesozooplankton community structure in the Scotia Sea during the CCAMLR 2000 survey: January-February 2000,
Description: An analysis of mesozooplankton community structure in the Scotia Sea was carried out, based on 123 RMT1 double oblique hauls (0-200 m) taken during the CCAMLR 2000 Survey. Standardized sample data (log abundance per 1000 m 3) were grouped into taxonomic categories and subjected to cluster analysis and multi-dimensional scaling. Two ordinations were performed, the first based on a reduced taxonomic dataset (31 categories out of a full 120) obtained by pooling ontogenetic stages within species and by including only those taxa that contributed at least 4% to total abundance at any one station. This disclosed two major station groups, which separated north and south, forming 'warm' and 'cold' water communities respectively, whereas four minor groups were mainly associated with stations around the Antarctic Peninsula and within the Weddell Scotia Confluence. Mean zooplankton abundance (238 000 individuals per 1000 m 3) within the northerly group G1 was up to 12 times higher than in other groups. The second ordination using all taxonomic categories disclosed an additional intermediate group (G1a), which was geographically consistent with the southern part of the northern group 1 from the previous ordination. However, because of taxonomic similarities between all the major station groups it was concluded that they represented a single community, which differed only in its phenological development and the mass occurrence of patchily distributed organisms such as krill larvae. Testing the relationships of station groups with the position of water masses and frontal boundaries indicated that the Weddell Front was broadly coincident with the boundary of the northern and southern communities over much of its length. However, the presence of stations belonging to group G2, to the north of the Weddell Front, to the west of the Antarctic Peninsula, and around the South Sandwich Islands, was consistent with the distribution of ice-influenced surface water. Low zooplankton abundance and species developmental composition suggested that this 'community' was largely in an over-wintered state. Copepods and euphausiids dominated the mesozooplankton throughout the study area with small copepods (Oithona spp., Ctenocalanus spp. and Metridia spp.) particularly abundant. © 2004 Elsevier Ltd. All rights reserved., Cited By (since 1996):25, CODEN: DSROE, ,
Author: Ward, Grant, Brandon, Siegel, Sushin, Loeb, Griffiths
Date: 2004-01-01T00:00:00Z

Title: Using carapace measurements to determine the sex of Antarctic krill, Euphausia superba,
Description: Krill (Euphausia superba) carapace measurements (length and width; mm) collected from plankton tows in the South Shetland Islands (SSI), Antarctica are used to test the generality of a common discriminant function developed to reconstruct krill length frequencies in Antarctic fur seal diets for the area surrounding South Georgia (SG). Total length and sex ratio of krill in the SSI were overestimated by 5.6 and 154%, respectively, when the SG allometric equations were applied to 3 years (2003-2005) of data. These errors arise and increase as a result of krill population dynamics, specifically recruitment that contributes large proportions of immature krill, misclassified as males by the SG discriminant function. We develop sex-specific regression models based on separate discriminant functions that provide significantly better discriminatory power. However, our analysis indicates that reconstructions of krill sex ratio and length composition in the ocean environment are less reliable in years when the ratio of immature to mature krill is high. For the SSI area, five out of 14 years (35.7%) surveyed (1992-2005) had proportions of immature to mature adult krill ≥ 0.50. © 2006 Springer-Verlag., Cited By (since 1996):2, CODEN: POBID, ,
Author: Goebel, Lipsky, Reiss, Loeb
Date: 2007-01-01T00:00:00Z

Title: Seabird species assemblages reflect hydrographic and biogeographic zones within Drake Passage
Description: Drake Passage, extending from the southern tip of South America to the northern Antarctic Peninsula, is a dynamic oceanographic region with well-defined habitats delineated by the three strong frontal jets of the Antarctic Circumpolar Current (ACC). Here, we describe seabird species distribution patterns across Drake Passage and test the hypothesis that species assemblages broadly reflect physical characteristics of the hydrographic fronts. Strip-transect seabird surveys were conducted between Tierra del Fuego and the South Shetland Islands (700 km track line) during January–March (austral summer) over 14 years (48 crossings). Locations of the latitudinally variable fronts were assessed using in situ shipboard data on sea surface temperature and salinity; areas of high variance were used to indicate frontal features. We quantified five distinct species assemblages that correspond to biogeographic regions and relate to the positions of the Sub-Antarctic Front, Polar Front and ACC Southern Front. Dense seabird concentrations coincided with regions characterized by highly variable sea surface temperature and salinity, suggesting that associated species assemblages reflect the mesoscale hydrographic surface as indicated by sea surface conditions.
Author: Force, Santora, Reiss, Loeb

Title: Hydrographic control of the marine ecosystem in the South Shetland-Elephant Island and Bransfield Strait region
Description: The South Shetland-Elephant Island and Bransfield Strait region of the West Antarctic Peninsula is an important spawning and nursery ground of Antarctic krill (Euphausia superba) and is an important source of krill to the Southern Ocean. Krill reproductive and recruitment success, hence supply of krill to predator populations locally and in downstream areas, are extremely variable on interannual and longer time scales. Interannual ecosystem variability in this region has long been recognized and thought related to El Niño Southern Oscillation (ENSO) events, but understanding of how has been limited by the hydrographic complexity of the region and lack of appropriate ocean-atmosphere interaction models. This study utilizes multidisciplinary data sets collected in the region from 1990 to 2004 by the U.S. Antarctic Living Marine Resources (AMLR) Program. We focus on hydrographic conditions associated with changes in the distribution, abundance and composition of salp- and copepod-dominated zooplankton assemblages during 1998 and 1999, years characterized respectively by a strong El Niño event and La Niña conditions. We provide detailed analyses of hydrographic, biological and ecological conditions during these dichotomous years in order to identify previously elusive oceanographic processes underlying ecosystem variability. We found that fluctuations between salp-dominated coastal zooplankton assemblages and copepod-dominated oceanic zooplankton assemblages result from the relative influence of Weddell Sea and oceanic waters and that these fluctuations are associated with latitudinal movement of the Southern Antarctic Circumpolar Current Front (sACCf). Latitudinal movements of the sACCf can be explained by meridional atmosphere teleconnections instigated in the western tropical Pacific Ocean by ENSO variability and are consistent with out-of-phase forcing in the South Pacific and South Atlantic Oceans by the Antarctic Dipole high-latitude climate mode. During El Niño decreased northwest winds, equatorward movement of the sACCf and an intensified Weddell Gyre allow Weddell Sea water to flow into eastern Bransfield Strait. During these periods mixing between oceanic and coastal waters is reduced, chlorophyll a concentrations are low, salps numerically dominate the zooplankton, and krill recruitment success is poor. During La Niña increased and more frequent northwest winds and poleward movement of the sACCf allows increased influence of oceanic waters and mixing of these with cold coastal waters. These periods are characterized by numerical dominance of copepods, elevated concentrations of oceanic zooplankton taxa and phytoplankton blooms that promote krill reproduction and recruitment success. Hydrographic and ecological changes after the 1998 El Niño are associated with a shift from frequent El Niños to the prevalence of La Niña and neutral conditions and conform to a decadal-scale climate regime shift in the Antarctic Peninsula region., Cited By (since 1996):8, CODEN: DSROE
Author: Loeb, Hofmann, Klinck, Holm-Hansen
Date: 2010-01-01T00:00:00Z

Title: Variability of Euphausia superba populations near Elephant Island and the South Shetlands: 1981 vs. 1984,
Description: March 1981 and 1984 Euphausia superba populations were compared using acoustics and net catches near Elephant Island, the South Shetlands, and in the Bransfield Strait. In 1981, krill tended to form large, thick swarms and in 1984, smaller, more dispersed, shallower patches. March body lengths of juvenile-adult krill were 22-59 mm in 1981 and 13-59 mm in 1984. Near Elephant I. in 1981, krill >45 mm were most numerous; in 1984 sizes <45 mm were dominant. In March 1984, the larger (>45 mm) body-size group was prereproductive and occurred from just west of Elephant I. westward into waters north of the South Shetlands; in 1981 the larger krill were postreproductive and more widely distributed in the Elephant I. area. Overall, netted postlarval krill, 1981 vs. 1984, averaged 73 vs. 48 individuals/m2, or 54 vs. 16 g/m2; acoustic biomass estimates were 229 vs. 134-201 g/m2. Larvae near Elephant I. averaged >2000/m2 in 1981 vs. <1/m2 in 1984-compatible with respective March reproductive states. Net-type comparisons revealed short-term (15 min to 6h) variability of a similar scale in both MOCNESS and bongo net catches, but bongo abundances averaged greater. Variation in maturity composition across 1981 swarms, patches, and random transects was like variation among the random 1984 tows; spatial distributions were more heterogeneous in 1984. The March 1984 krill of 20-44 mm (Year-2, mode 34 mm) relate to November 1983 krill of 9-30 mm (mode 21 mm), indicating growth averaging 12 mm during the season. Body-lengths and size-frequency modes of Year-2 and combined Years-3,3+ krill from comparable Feb-Mar data collected since 1968 suggest trends between times when (1) Year-2 krill average small and peak reproduction seems to be late in the season and/or weak (1979, 1982-1984), and (2) Year-2 krill are larger, and reproduction is possibly earlier and more successful (1976, 1980, 1981). © 1987 Springer-Verlag., Cited By (since 1996):9, CODEN: POBID, ,
Author: Brinton, Loeb, Macaulay, Shulenberger
Date: 1987-01-01T00:00:00Z

Title: Variation in the biomass density and demography of Antarctic krill in the vicinity of the South Shetland Islands during the 1999/2000 austral summer,
Description: Vessels from Japan, Peru, and the USA conducted four sequential surveys designed to estimate the biomass density and demography of Antarctic krill in the vicinity of the South Shetland Islands between late December 1999 and early March 2000. The surveys were conducted during the same austral summer as the CCAMLR 2000 Survey in the Scotia Sea (Watkins et al., Deep-Sea Research, II, this issue [doi: 10.1016/j.dsr2.2004.06.010]), and the data were analyzed in a similar manner. Biomass densities were not significantly different between the surveys and averaged 49 g m -2. Maps of krill biomass indicate three areas of consistently high density: one near the eastern end of Elephant Island, one mid-way between Elephant Island and King George Island, and one near Cape Shirreff on the north side of Livingston Island. The areas of highest krill density appeared to move closer to the shelf break as the season progressed. This apparent movement was accompanied by a change in the demographic structure of the population, with smaller krill absent and a larger proportion of sexually mature animals present in late summer., Cited By (since 1996):8, CODEN: DSROE, Antarctica, ,
Author: Hewitt, Kim, Naganobu, Gutierrez, Kang, Takao, Quinones, Lee, Shin, Kawaguchi, Emery, Demer, Loeb
Date: 2004-01-01T00:00:00Z

Title: Antarctic krill stock distribution and composition in the Elephant Island and King George Island areas, January-February, 1988,
Description: Information is provided on the distribution, size and maturity composition of Antarctic krill (Euphausia superba) stocks in the Elephant Island and King George Island areas, and at repeatedly sampled sites to the north of each island, during January-February, 1988. The overall distributional patterns of different sizes and maturity stages demonstrated a seasonal progression of those observed in the Antarctic Peninsula region during November-December, 1987 by Siegel (1989). The krill sampled at each island site represented different size-maturity groups and demonstrated different horizontal and vertical distributional characteristics. These distributional differences may be related to the demographic differences and/or hydrographie differences between the two sites. © 1993 Springer-Verlag., Cited By (since 1996):2, CODEN: POBID, Invertebrates, ,
Author: Loeb, Amos, Macaulay, Wormuth
Date: 1993-01-01T00:00:00Z

Title: Vertical distributions and relations of euphausiid populations off Elephant Island, March 1984,
Description: Distributional relationships are described for post-larval and larval Euphausia superba and Thysanoessa sp. (probably macrura) and post-larval Euphausia frigida collected in 0-70/80 m and 0-175/200 m depth ranges with a MOCNESS sampler north of Elephant Island (61°S, 55°W) during 17-23 March 1984. Larval E. superba (predominantly calyptopes stage 2 and 3) were rare shallower than 80 m at night. Day catches of post-larval E. suberba were small and night catches were primarily near the top of the thermocline above 50 m depth. Thysanoessa sp. occurred throughout the 0-200 m depth range and was abundant in the upper 80 m both night and day. E. frigida migrated to the upper 80 m at night from deeper day depths. Larval stages of E. superba and bost-larval stages of all three species demonstrated independent and variable vertical distribution patterns both night and day. Changes in E. superba abundance and distributional patterns could to a certain extent be associated with observed environmental changes. An increase in larval and decrease in post-larval E. superba abundances between 0-80 m was associated with an intrusion of cold water at depth. At night, vertically restricted concentrations of post-larval E. superba were associated with shallow mixed layer depths, and a significant vertical separation of developmental stages and size categories was observed only during periods of stratification in the upper 80 m. Fluctuations in the distribution and abundance of Thysanoessa sp. and distribution of E. frigida did not appear to be influenced by physical parameters within the upper 80 m. Within the 0-80 m depth range, the distributions of these two species differed from each other and from E. superba and showed large tow to tow variability that could not be related to physical parameters in the upper water column. © 1987 Springer-Verlag., Cited By (since 1996):8, CODEN: POBID, ,
Author: Loeb, Shulenberger
Date: 1987-01-01T00:00:00Z

Title: Climate variability and spatiotemporal dynamics of five Southern Ocean krill species
Description: Understanding the ecological response of marine organisms to future climate change will benefit from quantifying spatiotemporal aspects of their distribution and abundance as well as the influence of ocean-atmospheric climate modes on their population cycles. Our study provides a synthesis of 18. years of data (1992-2009) for 5 krill (euphausiid) species monitored near the North Antarctic Peninsula (NAP) during austral summer. Distribution and abundance data are presented for postlarval stages of Euphausia crystallorophias, E. frigida, E. superba, E. triacantha and Thysanoessa macrura and larval E. superba and T. macrura. Intraseasonal, interannual and longer-term distribution and abundance patterns are quantified relative to climate modes driving ecosystem variability off the Antarctic Peninsula: El Niño-Southern Oscillation (ENSO), Southern Annual Mode (SAM) and associated zonal and meridional winds. Interannual abundance variations of all 5 species are significantly correlated with seasonally averaged ENSO indices and, with the exception of E. triacantha, elevated population sizes are associated with the higher productivity La Niña phase. Time-lagged responses of each species to ENSO indices approximate their generation times and suggest evolution of their life histories and reproductive efforts in accordance with the ENSO cycle. Postlarval E. crystallorophias and E. frigida and larval T. macrura demonstrate significant abundance increases after 1998 associated with a shift from an El Niño dominated period to predominantly La Niña and "Nino-neutral" conditions. Seasonal changes in species distributions and co-occurrence indicate portions of the southernmost E. frigida, E. triacantha and T. macrura populations move poleward with E. superba during late-summer, suggesting that environmental conditions associated with sea ice development (e.g., food, retention) may be more favorable than within the Antarctic Circumpolar Current during low productivity seasons. Spatial distributions of larval and postlarval T. macrura suggest 2 separate spawning populations within oceanic and coastal waters. Lastly, mean euphausiid species concentrations and abundance relationships encountered during 1992-2009 are remarkably similar to those reported for the Antarctic Peninsula during the 1928-1935 Discovery Investigations. Circumpolar observations of E. superba "superswarms" associated with a major climatically related ecosystem change in the late 1970s may have represented an episodic period of anomalous peak abundance. Subsequent abundance estimates compared to these anomalies would indicate a significant decrease, but should not be taken to reflect the impact of climate warming.
Author: Loeb, Santora

Title: Nearshore assemblages of larval rockfishes and their physical environment off central California during an extended El Niño event, 1991-1993,
Description: Rockfish are among the most prevalent members of nearshore ichthyoplankton assemblages off central California, yet their abundance varies greatly from year to year. Warm events, like El Niño, can have pronounced effects on the success of a year class. We evaluate distribution, abundance, and species composition of rockfish larvae on small spatial and temporal scales in the upwelling center north of Monterey Bay during an extended El Niño (December 1991 through June 1993) relative to regional hydrography. Anomalously warm, low-salinity water to depths greater than 50 m during much of our study was indicative of an onshore displacement of the California Current. Upwelling was reduced and delayed relative to other years. The two years differed, however, in the intensity, duration, frequency, and direction of wind events. Larval rockfish abundance was similar in both years of the El Niño, peaking in early February, and was among the highest when compared with estimates from CalCOFI surveys off central California (1951-84). Using larval ages, we determined that median birthdates of shortbelly rockfish were in early- to mid-February both years. Growth rates of larval shortbelly rockfish did not differ among months or between years. Relative to 1993, rockfish larvae were more abundant, and sizes of larval shortbelly were significantly greater at onshore stations in 1992. This coincided with onshore advection of water during the onset of the El Niño, suggesting retention of larvae nearshore. Initiation of upwelling in March and April 1993 and fewer larvae at onshore stations are indicative of greater offshore transport during the second year. Juvenile rockfishes were extremely rare in summer of 1992; in 1993 they were twenty times more abundant and larger. Surviving juvenile shortbelly rockfish were born late during both years; upwelling occurred coincidentally during this period in 1993 but not in 1992. We suggest that substantially higher survival and recruitment of juvenile rockfishes in 1993 was due to increased offshore transport and perhaps lower predation during the larval stages., Cited By (since 1996):34, CODEN: FSYBA, ,
Author: Yoklavich, Loeb, Nishimoto, Daly
Date: 1996-01-01T00:00:00Z

Title: Krill population dynamics in the Scotia Sea: Variability in growth and mortality within a single population,
Description: Understanding the demographics of Antarctic krill over large scales may be complicated by regional differences in the processes that govern population structure. The influence of regional differences in growth and mortality on population size structure was examined using data on the length-frequency distribution of krill in the Scotia Sea using samples from the South Shetland Islands and South Georgia collected annually from 1991 to 2000. A correction function, which took account of the higher growth rate at South Georgia, produced a consistent similarity in the position of the modal size classes that was not present in the raw data. Optimising the mortality rate, to minimise the differences in the growth corrected length-frequency distribution, suggested a higher mortality rate at South Georgia that the South Shetlands. The intra-specific variations in growth and mortality rates are consistent with published values and with other Euphausiids species. Having accounted for the demographic plasticity, it is apparent that strong recruitment of the smallest size class of krill is represented in both populations simultaneously. It appears that first-year krill are advected into different regions of the Scotia Sea where the resultant population size structure is determined by regional differences in growth and mortality. The majority of the commercial harvest of krill in the Antarctic occurs in a relatively small number of regional fisheries within the Scotia Sea and is managed using population models based on a single set of demographic parameters. Where substantial differences in these parameters exist between fishing areas, the calculation of catch limits should take these differences into account., Cited By (since 1996):28, CODEN: JMASE, ,
Author: Reid, Murphy, Loeb, Hewitt
Date: 2002-01-01T00:00:00Z

Title: Interannual variation of ichthyoplankton composition and abundance relations off northern Chile, 1964-83,
Description: The 19-yr timespan included a wide variety of hydrographic conditions in the Humboldt Current area (cold years, El Nino events, and intervening transition years); it also included the decline and collapse of the anchoveta Engraulis fisheries and increases of sardine Sardinops, mackerel Scomber and jack mackerel Trachurus stocks off northern Chile and Peru. A marked shift in relative abundances of nonfished mesopelagic species in 1969-70 is associated with changes within long-term physical data bases from Chile and Peru suggesting a large-scale environmental change. Sardine stock growth began with successful larval survival of 1968-69 and later year classes. Anchoveta stock decline began in 1972 probably due to poor larval survival. Affiliation of anchoveta and coastal species larval abundance implies that they are similarly influenced by coastal processes. An atmospherically driven oceanic circulation change beginning in the late 1960s and possibly involving onshore presence of subtropical and/or oceanic waters and altered coastal processes may have been responsible for the changes in the northern Chilean fish assemblages., Cited By (since 1996):25, ,
Author: Loeb, Rojas
Date: 1988-01-01T00:00:00Z

Title: Krill (Euphausia superba) density, proportional and absolute recruitment and biomass in the Elephant Island region (Antarctic Peninsula) during the period 1977 to 1997,
Description: The Elephant Island region (Antarctic Peninsula) was selected as a long-term monitoring site to describe the interannual variability of important krill stock parameters. The analysis reviewed and updated krill density and proportional recruitment indices. Krill absolute recruitment and biomass from net sampling surveys are introduced as additional indices from this time series. New survey results from the past two seasons indicate a very successful 1994/1995 year-class and slightly below average proportional recruitment of the 1995/1996 krill year-class. Absolute recruitment of the 1995/1996 year-class was high compared to preceding years, because total stock size was relatively high in 1996/1997. After a period of low krill density and biomass in the area for almost a decade, krill density and biomass have increased. Caution is expressed as to whether this observed increase represents a long-term recovery of the stock., Cited By (since 1996):55, CODEN: POBID, ,
Author: Siegel, Loeb, Gröger
Date: 1998-01-01T00:00:00Z

Title: Effects of sea-ice extent and krill or salp dominance on the Antarctic food web,
Description: Krill (Euphausia superba) provide a direct link between primary producers and higher trophic levels in the Antarctic marine food web. The pelagic tunicate Salpa thompsoni can also be important during spring and summer through the formation of extensive and dense blooms. Although salps are not a major dietary item for Antarctic vertebrate predators, their blooms can affect adult krill reproduction and survival of krill larvae. Here we provide data from 1995 and 1996 that support hypothesized relationships between krill, salps and region-wide sea-ice conditions. We have assessed salp consumption as a proportion of net primary production, and found correlations between herbivore densities and integrated chlorophyll-a that indicate that there is a degree of competition between krill and salps. Our analysis of the relationship between annual sea-ice cover and a longer time series of air temperature measurements indicates a decreased frequency of winters with extensive sea-ice development over the last five decades. Our data suggest that decreased krill availability may affect the levels of their vertebrate predators. Regional warming and reduced krill abundance therefore affect the marine food web and krill resource management., Cited By (since 1996):361, Antarctica, Invertebrates, CODEN: NATUA, ,
Author: Loeb, Siegel, Holm-Hansen, Hewitt, Fraser, Trivelpiece, Trivelpiece
Date: 1997-01-01T00:00:00Z

Title: Krill demography and large-scale distribution in the southwest Atlantic during January/February 2000,
Description: This paper summarizes the results of krill demographic studies from the Commission for the Conservation of Antarctic Marine Living Resources 2000 Survey - a large-scale krill survey across the Scotia Sea conducted in January/February 2000. Standard Rectangular Midwater Trawl net tows were carried out at midnight and midday stations between 20° and 70°W south of the Polar Front. The overall mean numerical density and biomass of krill estimated from nets (247 krill per 1000 m 3 and 18.7 g m -2, respectively) were similar to lower values reported previously for the Southwest Atlantic, and comparable with high values for other regions of the Southern Ocean. Krill varied in mean size across the survey area, with cluster analyses showing three distinct groups of length-frequency distribution. Small juvenile and immature krill occurred east of the South Orkney Islands. Adult krill <50 mm dominated the shelf areas of the Antarctic Peninsula and to the north of the juvenile stock across the Scotia Sea. Adult krill >50 mm were mostly restricted to the west of the South Orkney Islands. Maturity stage composition indicated that peak spawning occurred before early February. Distribution of the spawning stock showed two hotspots, the first between the South Shetland and South Orkney Islands, and the second around the South Sandwich Islands. Reproductive krill were largely absent in the central Scotia Sea and around South Georgia. Krill larvae concentrations occurred slightly to the east of the spawning stock. Mean density of larvae in the western Scotia Sea was 2044 m -2, but only scattered aggregations of larvae were recorded east of 36°W (2 m -2). Recruitment indices for 1- and 2-year-old krill were low in the western part of the survey area, the outflow from the Bellingshausen Sea/Antarctic Peninsula region (R1=0.0, R2=0.11) indicating spawning failure and/or poor recruitment for several years. In the eastern part of the survey area, mostly the outflow of the Weddell Sea, recruitment indices were high and above the long-term average (R1=0.60, R2=0.72), suggesting a population with constant and successful reproduction, recruitment and mortality. The distribution and structure of the krill population did not show any clear relationship to the position of the major oceanic fronts in the Scotia Sea., Cited By (since 1996):31, CODEN: DSROE, ,
Author: Siegel, Kawaguchi, Ward, Litvinov, Sushin, Loeb, Watkins
Date: 2004-01-01T00:00:00Z

Title: ENSO and variability of the Antarctic peninsula pelagic marine ecosystem,
Description: The West Antarctic Peninsula region is an important source of Antarctic krill (Euphausia superba) in the Southern Ocean. From 19802004 abundance and concentration of phytoplankton and zooplankton, krill reproductive and recruitment success and seasonal sea ice extent here were significantly correlated with the atmospheric Southern Oscillation Index and exhibited three- to five-year frequencies characteristic of El NioSouthern Oscillation (ENSO) variability. This linkage was associated with movements of the Southern Antarctic Circumpolar Current Front and Boundary, a changing influence of Antarctic Circumpolar Current and Weddell Sea waters, and eastward versus westward flow and mixing processes that are consistent with forcing by the Antarctic Dipole high-latitude climate mode. Identification of hydrographic processes underlying ecosystem variability presented here were derived primarily from multi-disciplinary data collected during 19902004, a period with relatively stable year-to-year sea ice conditions. These results differ from the overwhelming importance of seasonal sea ice development previously established using 19801996 data, a period marked by a major decrease in sea ice from the Antarctic Peninsula region in the late 1980s. These newer results reveal the more subtle consequences of ENSO variability on biological responses. They highlight the necessity of internally consistent long-term multidisciplinary datasets for understanding ecosystem variability and ultimately for establishing well-founded ecosystem management. Furthermore, natural environmental variability associated with interannual- and decadal-scale changes in ENSO forcing must be considered when assessing impacts of climate warming in the Antarctic PeninsulaWeddell Sea region. © Antarctic Science Ltd 2009., Cited By (since 1996):20, Antarctica, Invertebrates, Oceanography, CODEN: ANTSE, ,
Author: Loeb, Hofmann, Klinck, Holm-Hansen, White
Date: 2009-01-01T00:00:00Z

Title: A re-appraisal of the total biomass and annual production of Antarctic krill,
Description: Despite much research on Euphausia superba, estimates of their total biomass and production are still very uncertain. Recently, circumpolar krill databases, combined with growth models and revisions in acoustics have made it possible to refine previous estimates. Net-based databases of density and length frequency (KRILLBASE) yield a summer distributional range of ∼19×10 6km 2 and a mean total abundance of 8×10 14 post-larvae with biomass of 379 million tonnes (Mt). These values are based on a standardised net sampling method but they average over the period 1926-2004, during which krill abundance has fluctuated. To estimate krill biomass at the end of last century we combined the KRILLBASE map of relative krill density around Antarctica with an acoustics-derived biomass estimate of 37.3Mt derived for the Scotia Sea area in 2000 by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Thus the CCAMLR 2000 survey area contains 28% of the total stock, with total biomass of ∼133Mt in January-February 2000. Gross postlarval production is estimated conservatively at 342-536Mtyr -1, based on three independent methods. These are high values, within the upper range of recent estimates, but consistent with the concept of high energy throughput for a species of this size. The similarity between the three production estimates reflects a broad agreement between the three growth models used, plus the fact that, for a given population size, production is relatively insensitive to the size distribution of krill at the start of the growth season. These production values lie within the envelope of what can be supported from the Southern Ocean primary production system and what is required to support an estimated predator consumption of 128-470Mtyr -1. Given the range of recent acoustics estimates, plus the need for precautionary management of the developing krill fishery, our net-based data provide an alternative estimate of total krill biomass. © 2008 Elsevier Ltd., Cited By (since 1996):34, CODEN: DRORE, , , Antarctica, Invertebrates
Author: Atkinson, Siegel, Pakhomov, Jessopp, Loeb
Date: 2009-01-01T00:00:00Z

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