Habitat partitioning in Antarctic krill: Spawning hotspots and nursery areas

Abstract

Antarctic krill, Euphausia superba , have a circumpolar distribution but are concentrated within the south-west Atlantic sector, where they support a unique food web and a commercial fishery. Within this sector, our first goal was to produce quantitative distribution maps of all six ontogenetic life stages of krill (eggs, nauplii plus metanauplii, calyptopes, furcilia, juveniles, and adults), based on a compilation of all available post 1970s data. Using these maps, we then examined firstly whether “hotspots” of egg production and early stage nursery occurred, and secondly whether the available habitat was partitioned between the successive life stages during the austral summer and autumn, when krill densities can be high. To address these questions, we compiled larval krill density records and extracted data spanning 41 years (1976–2016) from the existing KRILLBASE-abundance and KRILLBASE-length-frequency databases. Although adult males and females of spawning age were widely distributed, the distribution of eggs, nauplii and metanauplii indicates that spawning is most intense over the shelf and shelf slope. This contrasts with the distributions of calyptope and furcilia larvae, which were concentrated further offshore, mainly in the Southern Scotia Sea. Juveniles, however, were strongly concentrated over shelves along the Scotia Arc. Simple environmental analyses based on water depth and mean water temperature suggest that krill associate with different habitats over the course of their life cycle. From the early to late part of the austral season, juvenile distribution moves from ocean to shelf, opposite in direction to that for adults. Such habitat partitioning may reduce intraspecific competition for food, which has been suggested to occur when densities are exceptionally high during years of strong recruitment. It also prevents any potential cannibalism by adults on younger stages. Understanding the location of krill spawning and juvenile development in relation to potentially overlapping fishing activities is needed to protect the health of the south-west Atlantic sector ecosystem.

Temperature–Induced Hatch Failure and Nauplii Malformation in Antarctic Krill

“…Because krill are a key species in the food web and exploited commercially, there is much interest in identifying areas of successful spawning and recruitment (Orsi et al, 1995;Hofmann and Hüsrevoglu, 2003;Murphy et al, 2004;Perry et al, 2019;Thorpe et al, 2019). Our results highlight the extremely variable and often low krill hatching success in the field, suggesting that we need to refine our models to account for maternal factors as well as temperature.…”

“…As one important example, krill are abundant at South Georgia, where the females are the largest anywhere in the Southern Ocean (Schmidt et al, 2014) and clearly benefit from the high food concentrations (Pond et al, 2005). Yet, larval abundances are typically low here (Ward et al, 2006;Tarling et al, 2007;Perry et al, 2019). A variety of explanations have been proposed, including high predation and mass advection (Tarling et al, 2007).…”

“…Overall, our results show that krill embryos are negatively impacted by incubation temperatures that exceed 3.0 • C. Below this temperature, other environmental factors and individual female variability dictate hatching success. Further understanding of these controls may help to explain the discrepancy in the spatial distributions of spawning females and larvae (Perry et al, 2019). To model the distribution of krill in a changing climate to the level required for effective fishing management to operate (Hill et al, 2006), we need to take account of the factors that dictate embryo hatching success.…”

“…Antarctic krill (Euphausia superba), hereafter "krill, " are an ecologically and commercially valuable stenothermal crustacean species in the Southern Ocean where they inhabit a temperature range of −2.0 • C to 4.0 • C (Tarling et al, 2006;Cuzin-Roudy et al, 2014). Within this 6.0 • C temperature envelope, krill are considered to preferentially inhabit regions with temperatures between −1.5 • C and ≤1.5 • C (Cuzin-Roudy et al, 2014;Perry et al, 2019). Experimental results by Atkinson et al (2006) show that adult krill growth rates peak at 0.5 • C and decrease rapidly to zero above 3.0 • C. Yet, the upper temperature tolerance of krill embryos has never been defined.…”

“…The south-west Atlantic sector of the Southern Ocean is key for krill, being the region with the highest krill densities, where the fishery operates and where the population extends to its warmest thermal limit around the island of South Georgia (Atkinson et al, 2008). Surface water temperatures around South Georgia can reach over 4.0 • C, yet its waters contain high densities of sexually mature adult krill that support the fishery (Marr, 1962;Perry et al, 2019). This adult population does not, however, translate to high larval recruitment as expected (Ward et al, 1990;Tarling et al, 2007;Siegel and Wakins, 2016).…”