Oceanographic variability and changes in Antarctic krill (Euphausia superba) abundance at South Georgia

Abstract

Oceanographic data collected to the north of South Georgia were examined for three consecutive summers (1996/97, 1997/98, 1998/99). The results show the existence of a shelf break front during each period. The most reliable means of defining the front was the potential density anomaly at the near‐surface potential temperature minimum. In each year, off‐shelf waters were separated from on‐shelf waters by water with a potential density anomaly between 27.22 and 27.29 kg m−3. During 1997/98, the near‐surface potential temperature minimum was much colder and much shallower than in other years and was consistent with waters originating from much further south than South Georgia; these differences were further evident at a single deep off‐shelf station. The oceanographic changes during 1997/98 were consistent with a mesoscale or large‐scale movement of the southern Antarctic Circumpolar Current front. Acoustically determined densities of Antarctic krill, Euphausia superba, at South Georgia showed consistent patterns between years. Densities were substantially higher over the shelf compared with off‐shelf, with the highest densities at the shelf edge; densities were also higher to the east of the island. During 1997/98, acoustic densities of krill were substantially higher than in other years. The coincidence of the elevated acoustic density and the cooler oceanographic conditions was explored. When data from all years were combined and analysed by Generalized Additive Model, an inverse relationship between acoustic density and temperature was apparent. Historical data were also examined and it was noted that the only other occurrence of such a high estimate of krill density at South Georgia, was when oceanographic conditions were also colder.

Foraging behaviour in two Antarctic fur seal colonies with differing population recoveries

“…The shelf break region has a major influence on water mass distribution to the north of South Georgia, with a frontal zone near the 500 m isobath between more productive, warmer, less stratified shelf water and colder more saline water off shelf ). This front is an area characterised by higher krill densities (Trathan et al 2003). The effect of travelling dives can be seen at both locations, with a higher frequency of shallower dives in shallow water, and at Cooper Bay close to the breeding beach.…”

Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

“…Statistical analysis of environmental influences on krill distribution also revealed a consistent negative association with the distance to nearest land. Lascara et al (1999) At the finer scales of 0.5 km acoustic integration intervals, Trathan et al (2003) found no consistent effect of temperature, and instead bathymetry was the only consistent predictor of abundance, with abundance greatest near the shelf-break (similar to the qualitative summertime observations of Ichii et al, 1998 andPauly et al, 2000). Krill in summer may be associated with the shelf break in order to be positioned above CDW, due to the latter being favorable to successful spawning (Hofmann et al, 1992).…”

“…Furthermore, GAMs proceed by fitting smoothing functions to the relationship between the response and each predictor variable, and thereby allow for nonlinear relationships (Hastie and Tibshirani 1990). The use of GAMs to examine associations between the results of acoustic surveys and environmental features has been employed previously for krill (Trathan et al, 2003), allowing interesting ecological insight.…”

“…Although the Antarctic krill is a well-studied species in many respects, few studies have been able to identify clear and consistent relationships between krill distribution and environmental properties (Trathan et al, 2003;Siegel 2005). Furthermore, in comparison to the well-studied time periods of austral spring and summer, when many antarctic regions are ice-free and more easily accessible to survey vessels, relatively few studies have examined krill distribution during winter (Siegel, 1989;Nordhausen, 1994;Ross et al, 1996;Lascara et al, 1999).…”

Role of krill versus bottom-up factors in controlling phytoplankton biomass in the northern Antarctic waters of South Georgia

“…Second, there is good background knowledge of this area, with extensive datasets on physics, nutrients, phytoplankton and krill (e.g. Whitehouse et al 1996, Brierley et al 1997, Trathan et al 2003, Meredith et al 2005.…”