Transport of Antarctic krill (Euphausia superba) across the Scotia Sea. Part II: Krill growth and survival

Abstract

A time-dependent, size-structured, physiologically based krill growth model was used in conjunction with a circulation model to test the hypothesis that Antarctic krill (Euphausia superba) populations at South Georgia are sustained by import of individuals from upstream regions. Surface phytoplankton concentrations along the simulated drifter trajectories were extracted from historical Coastal Zone Color Scanner (CZCS) measurements and sea ice biota concentrations were calculated from sea ice concentration and extent extracted along drifter trajectories from Special Sensor Microwave/ Imager measurements. As additional food sources, a time series of heterotrophic food was constructed from historical data, and time series of detritus concentrations were calculated using phytoplankton concentrations extracted from CZCS measurements together with measured particulate organic carbon to chlorophyll a ratios. These food resources along specified drifter trajectories were then input to the krill growth model to determine the size and viability of krill during transport from the source region to South Georgia. The krill growth model simulations showed that no single food source can support continuous growth of krill during the 58-306 days needed for transport to South Georgia. However, under the current assumptions results indicate that combinations of food sources during the transport time enhanced krill survival, with heterotrophic food and detritus being particularly important during periods of low phytoplankton concentrations. The growth model simulations also showed that larval and juvenile krill originating along the western Antarctic Peninsula can grow to 1þ (14-36 mm) and 2þ (26-45 mm) age and size classes observed at South Georgia during the time needed for transport to this region. Krill originating in the Weddell Sea need 20 months for transport, which allows retention in a potentially high food environment, provided by sea ice, for almost 1 year. Krill then complete transport to South Georgia in the following year and larval and juvenile krill grow to 2þ (26-45 mm) and 3þ (35-60 mm) age and size classes during transport. The results of this study show that the successful transport of krill to South Georgia depends on a multitude of factors, such as the location of the spawning area and timing of spawning, food concentrations during transport, predation, and variations in the location of the Southern Antarctic Circumpolar Current Front (SACCF) and in sea ice extent. r

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“…Though this study finds a high dispersal potential of anchovy larvae up to 500 km across the Black Sea in just 36 days, as well as a strong climateinduced interannual variability, these results are on similar scales to other connectivity studies that have been carried out in different seas, such as on anchovy larvae in the Benguela Parada et al, 2003) and Peru regions (Xu et al, 2013), benthic larvae in the Caribbean Sea (Cowan et al, 2006), Antarctic krill larvae in the Scotia Sea (Fach and Klinck, 2006;, as well as Lobster larvae in the Gulf of Maine (Xue et al, 2008;Incze et al, 2010). For example, it has been shown that in the southern Benguela important spawning grounds of anchovy may be hundreds of kilometers away from nursery areas (Parada et al, 2003), while for different reef fishes in the Caribbean, typical dispersal distances were on the order of tens to hundreds of kilometers (Cowen et al, 2006).…”

“…For example, it has been shown that in the southern Benguela important spawning grounds of anchovy may be hundreds of kilometers away from nursery areas (Parada et al, 2003), while for different reef fishes in the Caribbean, typical dispersal distances were on the order of tens to hundreds of kilometers (Cowen et al, 2006). In addition, it was shown that the Antarctic krill population at South Georgia in the Southern Ocean is supplied by larvae originating more than 1500km upstream along the Antarctic Peninsula (Fach and Klinck, 2006;.…”

“…An alternative approach is to test connectivity using particle simulations (Cowen et al, 2006, Fach andKlinck, 2006;Ashford et al, 2010). In this study, such tracking of the large-scale physical transport pathways in the Black Sea together with a simple, temperature dependent anchovy larvae growth model was applied to assess the dispersal of pelagic anchovy larvae.…”

Transport of Antarctic krill (Euphausia superba) across the Scotia Sea. Part I: Circulation and particle tracking simulations

“…A shorter transport time would benefit krill and result in higher numbers of krill surviving transport, as discussed in the following paper (Fach et al, 2006).…”

“…Drifters are released into the simulated surface flow field at 50 m at 332 initial positions and starting at three different times because krill are found at average depths of 50-70 m (Godlewska, 1996). The trajectories provide transport pathways across the Scotia Sea that can be used for simulated circulation field validation, as well as for krill growth simulations discussed in Part II (Fach et al, 2006). Vertical migration is not included here assuming that the effect on drift trajectories is minimal because the velocity shear in the near surface ð$100 mÞ is very small.…”