Land-based predator impact on vertically migrating zooplankton and micronekton advected to a Southern Ocean archipelago

Abstract

Combined acoustic (12 and 120 kHz) and trawling (RMT-2 and Bongo net) surveys in offshore waters of the Prince Edward Archipelago revealed diurnal vertical migration of major prey species (the euphausiid Euphausia vallentini and the myctophid fish Krefftichthys andersson~) of landbased predators. Migration was from 200-400 m in the day to near-surface levels at night and was linked to feeding periodicity. Consistently high winds result in drift of surface waters (down to 10-20 m) and advection of prey over the shelf Within the seasonal limits of the investigation (autumn), an estimated 3200 t of zooplankton and 310 t of myctophid fish were carried over the shelf every 24 h. Predators in the archipelago consume ca 900 t of crustaceans and 1700 t of fish d-' Thus, advection could supply sufficient crustaceans to the shelf area for the needs of the land-based predators, but is inadequate to support myctophid-feeding piscivores. Once over the shelf, prey attempting to migrate downwards ~vould be trapped by bottom topography and vulnerable to predation. The fit between estimates of advect~on and consumption of different prey types is supported by observed prey densities and predator behaviour Prey density in shelf and oceanic waters was similar at night, but decreased dramatically over, and downstream of, the shelf in the day. Rockhopper and Macaroni penguins are the most important crustacean predators and feed mainly over the shelf, with foraging ranges of about 30 and 100 km respectively. In contrast, King penguins account for >70 O/o of the fish consumed in the area and forage in deep waters, having an average foraging range of 300 km. This spatial segregation of the main predators is consistent with the availability of their prey.

Movement and diving of killer whales ( Orcinus orca ) at a Southern Ocean archipelago

“…Seals and seabirds forage in diverse areas, often far from the PEIs, where they target frontal and bathymetric features at which their crustacean, fish and cephalopod prey are presumably concentrated (de Bruyn et al, 2009;Nel et al, 2001;Tosh et al, 2012). The shelf between the islands is somewhat enriched by topographic trapping of vertically migrating zooplankton and dense phytoplankton blooms (Perissinotto and Mcquaid, 1992) but the broad distribution pattern among seals and seabirds, however, reflects that macrozooplankton biomass over the inter-island shelf may be 3-8 times lower than in the offshore region. Also, adult mesopelagic fishes are virtually absent from the inter-island region, whereas they predominate the open waters of the Polar Frontal Zone (Mcquaid and Froneman, 2008;Pakhomov and Froneman, 1999;Pakhomov et al, 2000).…”

School characteristics of mesopelagic fish at South Georgia

“…anderssoni form distinct schools in the Southern Ocean (Perissinotto and McQuaid, 1992;Zasel'sliy et al, 1985) and there is evidence from fish larval surveys that K. anderssoni also exhibit this behaviour around South Georgia (Belchier and Lawson, 2013). E. carlsbergi and K. anderssoni were found regularly in net samples at South Georgia during the timeframe of this study, but there was clear variation in their presence at the island between years (Collins et al 2008;Collins et al 2012; Figure 8).…”

“…Myctophid schools, regardless of species or life cycle stage, were deeper and acoustically stronger in onshore regions where predation pressure from shore-based predators can be expected to be highest (Staniland et al, 2004). An acoustic study at Marion Island, Southern Ocean, reported an onshelf/offshelf gradient in the presence of a deep scattering layer (DSL) attributable to K. anderssoni whereby the DSL was strong in offshelf waters but faded gradually towards the island's shelf (Perissinotto and McQuaid, 1992). These authors suggested that increased predation pressure from land-based predators in the inshore regions was responsible for this trend.…”

“…A further explanation could be that schools in the inshore regions reduce their horizontal dimensions by extending vertically so that they become narrower in shape and are therefore less visible to predators from above. Such behaviour has been suggested for K. anderssoni in shallow waters off Marion Island as an anti-predatory mechanism to avoid predation from above when DVM becomes constrained by bathymetry (Perissinotto and McQuaid, 1992). It is also possible that the onshore and offshore trends were simply a function of variations in school shape and size due to schooling of different species or different life stages within the same species.…”

Advection in polar and sub-polar environments: Impacts on high latitude marine ecosystems

“…When there are few krill, the condition of mackerel icefish is negatively affected and growth rates are reduced (Everson et al, 1997(Everson et al, , 1999. In sub-Antarctic regions, Antarctic krill are less important, and the advection of zooplankton and myctophid fish into shelf areas helps maintain local food webs (Perissinotto and McQuaid, 1992;Pakhomov and Froneman, 1999b).…”