Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996–2006

Abstract

Reiss, C. S., Cossio, A. M., Loeb, V., and Demer, D. A. 2008. Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996–2006. – ICES Journal of Marine Science, 65: 497–508. 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 (ΔSv), 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.

Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica

“…Sea-ice extent and duration have been correlated with the reproductive success of krill, and in years following winters with expansive ice area and temporal duration, krill reproductive success increases (34,36). Mean annual sea-ice extent in the WAP and Scotia Sea is inversely related to mean annual air temperature (Fig.…”

“…Results from annual net surveys of krill populations around the South Shetland Islands suggest that the size of recruiting cohorts has declined, but the average time between recruitment events (4-5 y) has remained fairly consistent (Fig. 3B) (36). The decline in recruitment strength is an important factor determining the amount and mean size of krill available for predators and may be especially important for fledging penguins.…”

“…If the warming trend in the WAP and Scotia Sea continues (25,26), winter sea ice will be absent from much of this region in the near future, krill abundance may be reduced further, krill recruitment events will remain episodic (34)(35)(36), and Adélie and chinstrap penguin populations probably will continue to decline. These conditions are particularly critical for chinstrap penguins, because this species breeds almost exclusively in the WAP and Scotia Sea, where they have sustained declines in excess of 50% throughout their breeding range.…”

Spatial association between hotspots of baleen whales and demographic patterns of Antarctic krill Euphausia superba suggests size-dependent predation

“…Despite their ecological importance, past exploitation, and dominant role in the ecosystem, rather little is known about the foraging habitats of each species of baleen whale near the South Shetland Islands, Antarctica ( Fig. 1), a region known for its elevated krill biomass (Laws 1977, Siegel 1988, Loeb et al 1997, Reilly et al 2004, Reiss et al 2008, active krill fishery (Marin & Delgado 2001, Jones & Ramm 2004, and historical whaling (Mackintosh 1965, Laws 1977. Whales are often associated with krill (Reid et al 2000, Murase et al 2002, Friedlaender et al 2006, and there is evidence that whales exhibit preferences for ABSTRACT: 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.…”

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

“…S v at 120 kHz (S v120kHz ) were apportioned to krill using a variable dB window differencing identification technique using the 120 and 38 kHz (S v120-38 ) frequencies (Method 2, Reiss et al 2008). Window ranges for different size ranges of krill (identified from the net samples in each survey region in each year) were taken from the current agreed Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) protocol for krill B 0 assessment (CCAMLR 2009).…”