Valerie J. Loeb

Krill (Euphausia superba) density, proportional and absolute recruitment and biomass in the Elephant Island region (Antarctic Peninsula) during the period 1977 to 1997

Abstract 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.

ENSO and variability of the Antarctic Peninsula pelagic marine ecosystem

Abstract The West Antarctic Peninsula region is an important source of Antarctic krill (Euphausia superba) in the Southern Ocean. From 1980–2004 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 Niño–Southern 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 1990–2004, 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 1980–1996 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 Peninsula–Weddell Sea region.

A LARGE SWARM OF EUPHAUSIA SUPERBA: OVERVIEW OF PATCH STRUCTURE AND COMPOSITION

ABSTRACT Twenty-five MOCNESS (Multiple Open/Closing Net and Environmental Sensing System) tows were taken in and around a large swarm of Euphausia superba near Elephant Island. Krill from all 25 tows (201 samples) and the copepods from 9 tows (63 samples) have been analyzed. Significant agreement in rank order of abundance is found for both krill and nonkrill fractions. Most copepod species show significant covariation; there is less significant covariation among euphausiid categories. Larvae of E. superba and Thysanoessa spp. cooccur and show strong positive covariation. There are large day-night differences in catches of both adult and, to a lesser degree, larval euphausiids. Vertical boundaries between larval and adult E. superba are often sharp, and larvae may occur either above or below significant adult concentrations.

Vertical distributions and relations of euphausiid populations off Elephant Island, March 1984

SummaryDistributional 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.

Distribution and abundance of larval fishes collected in the western Bransfield Strait region, 1986–87

Abstract 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 ).

Variability of Euphausia superba populations near Elephant Island and the South Shetlands: 1981 vs. 1984

SummaryMarch 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).

Early development of Diaphus garmani (Myctophidae) in the transition region of the western North Pacific

Abstract 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.