Environmental correlates of Antarctic krill distribution in the Scotia Sea and southern Drake Passage

Abstract

Antarctic krill is a key prey species for many vertebrate and invertebrate predators in the Southern Ocean; it is also an abundant fishery resource in the Scotia Sea and southern Drake Passage. Here, we identify environmental correlates of krill distribution utilizing acoustic data collected during an extensive international survey in January 2000. Separate models (at scales of 10-80 nautical miles) were derived for the full study area and for each of four subregions: northern and southern shelf waters, the seasonally ice-covered open ocean, and the generally ice-free open ocean. Krill distribution was strongly correlated with bathymetry; densities were higher over island shelves and shelf breaks and decreased with increasing distance offshore. Low krill densities occurred in areas of low chlorophyll concentration and high geostrophic velocity. Krill distribution was also related to sea level anomaly but relationships were not consistent between subregions. The models explained a maximum of 44% of the observed deviance in krill density, but did not reliably identify areas of high krill density in the open ocean, and explained a small proportion of the deviance (16%) in offshore areas covered seasonally by sea ice, probably because of the strong, residual influence of retreated ice. The commercial krill fishery is currently concentrated in shelf areas, where high densities of krill are most predictable. As krill are not predictable in the open ocean, the fishery is likely to remain principally a near-shore operation, and should be managed accordingly.

Predicting krill swarm characteristics important for marine predators foraging off East Antarctica

“…Higher swarm densities during night time may be due to a reduced visual contact distance between individuals leading to more tightly packed swarms (Tarling et al ). Relationships between krill and bathymetry have been widely reported, notably high krill biomass over shelves and near the shelf break (Trathan et al , Jarvis et al , Silk et al ), and larger, denser swarms closest to shore (Klevjer et al ). These associations likely reflect interacting processes including food availability, sea ice properties, and predator avoidance; see Silk et al () and Nicol () for a detailed discussion of these factors.…”

“…Relationships between krill and bathymetry have been widely reported, notably high krill biomass over shelves and near the shelf break (Trathan et al , Jarvis et al , Silk et al ), and larger, denser swarms closest to shore (Klevjer et al ). These associations likely reflect interacting processes including food availability, sea ice properties, and predator avoidance; see Silk et al () and Nicol () for a detailed discussion of these factors. In high slope areas, our models predicted that swarms were also shallower and therefore potentially more accessible to predators, similar to findings elsewhere in Antarctica (Klevjer et al , Cox et al ).…”

“…Our results suggest that the model set developed here may be informative for predicting spatial fields for at least the summer season in the East Antarctic, suitable for use in further predator movement and distribution studies, as well as the parameterisation of krill individual based models (Santora et al ). Scaling up beyond this would require caution (Torres et al ), since the Southern Ocean is characterised by strongly regionalised oceanography (Silk et al ), and regional differences in climatic conditions, physical dynamics and drivers (Murphy et al , Constable et al ). Krill distributions are also known to show strong seasonal changes (Siegel , Lascara et al ).…”

“…There is strong interest, therefore, in identifying biological and physical factors that influence prey field characteristics (Tarling et al 2009, O'Toole et al 2015, Silk et al 2016, such as spatial distribution and density (Kloser et al 2009). These factors may be of broader predictive use for both conservation planning purposes and in evaluating effects of environmental change (Trathan et al 2012, Hill et al 2013, Constable et al 2014a, Gutt et al 2015.…”

The importance of Southern Ocean frontal systems for the improvement of body condition in southern elephant seals

“…Therefore, individuals most likely target another prey type-possibly myctophids (Gymnoscopelus nicholsi Gilbert 1911)-by frequently diving to depths at which mCDW can be found. In our area of study, the SB follows the shelf break along the Antarctic Peninsula and the southern Scotia Arc, which is often associated with higher Antarctic krill densities (Silk et al, 2016). Antarctic krill is a major food source for other mesopelagic fish, which in turn is another prey for southern elephant seals (Cherel, Ducatez, Fontaine, Richard, & Guinet, 2008;Saunders, Collins, Stowasser, & Tarling, 2017).…”