Winter distribution and size structure of Antarctic krill Euphausia superba populations in-shore along the West Antarctic Peninsula

Abstract

Antarctic krill Euphausia superba are a key component of food webs in the maritime West Antarctic Peninsula, and their life history is tied to the seasonal cycles of sea ice and primary production in the region. Previous work has shown a general in-shore migration of krill in winter in this region; however, the very near-shore has not often been sampled as part of these surveys. We investigated distribution, abundance, and size structure of krill in 3 fjordic bays along the peninsula, and in the adjacent Gerlache Strait area using vertically stratified MOCNESS net tows and ADCP acoustic biomass estimates. Krill abundance was high within bays, with net estimated densities exceeding 60 krill m −3 , while acoustic estimates were an order of magnitude higher. Krill within bays were larger than krill in the Gerlache Strait. Within bays, krill aggregations were observed near the seafloor during the day with aggregations extending to the sediment interface, and exhibited diel vertical migration higher into the water column at night. We suggest these high winter krill abundances within fjords are indicative of an active seasonal migration by krill in the peninsula region. Potential drivers for such a migration include reduced advective losses and costs, and availability of sediment food resources within fjords. Seasonally near-shore krill may also affect stock and recruitment assessments and may have implications for managing the krill fishery in this area.

Krill motion in the Southern Ocean: quantifying in situ krill movement behaviors and distributions during the late austral autumn and spring

“…Other visible organisms, such as amphipods and ctenophores, amounted to << 1% of organisms observed. Based on MOCNESS and ring net tows conducted during the same cruise, the krill observed in the images were most likely Euphausia superba (Cleary et al ).…”

“…Turning rates were measured as changes in swimming directionality over time (e.g., Harvey and Menden‐Deuer ). Krill lengths were measured from krill collected by the MOCNESS tows of the late austral autumn and ranged between 9 and 51 mm, with an average of 29 mm (Cleary et al ). These lengths can be used to convert swimming speeds and vertical velocities from apparent BL per second (BL s −1 ) to centimeters per second (cm s −1 ); for example, a swimming speed of 2 BL s −1 would result in a swimming speed range of 1.8 to 10.2 cm s −1 , with an average swimming speed of 5.8 cm s −1 .…”

KRILLBASE: a circumpolar database of Antarctic krill and salp numerical densities, 1926–2016

“…3c). Time of year of sampling has a potentially strong influence on the abundance of zooplankton, due to life cycle-and behavioural traits such as seasonal vertical migration (Foxton, 1966;Atkinson et al, 2012a;Cleary et al 2016). While samples were obtained during most months of the year, 89% of the hauls were conducted in the period December to March (Fig 4), with no longitudinal bias in timing (Fig 3a).…”

“…horizontal distribution below 200m. These deeper and near-seabed zones are being increasingly recognised as important habitats for krill (Gutt and Siegel, 1994;Clarke and Tyler, 2008;Schmidt et al, 2011;Cleary et al, 2016).…”

Overwinter habitat selection by Antarctic krill under varying sea-ice conditions: implications for top predators and fishery management

“…There is considerable discussion in the literature (Siegel 1988, Lascara et al 1999, Cleary et al 2016 regarding the importance of the seasonal shoreward migration of krill to coastal waters. Yet, there is less information on the magnitude of this difference and the resulting changes in ecosystem structure (Atkinson et al 2008).…”