Buoyancy of post‐fertilised Dissostichus mawsoni eggs and implications for early life history

Abstract

Gametes from gravid Antarctic toothfish (Dissostichus mawsoni) were combined in vitro and buoyancy measurements were made on fertilised eggs during early development. Eggs were strongly positively buoyant, indicating that they would ascend quickly in the water column and reside near or in association with the underside of sea ice, which covers most of the spawning habitat during winter. An association with sea ice may provide: protection from the turbulence of ice‐free surface waters, a mechanism that modifies the velocity of advection by surface currents, and a habitat with a concentrated planktonic food source during spring months. An association with sea ice would also create spatial differences in egg advection patterns for eggs produced spanning a large geographic area. The effects of climate change on sea ice formation and melting patterns, or fishery‐induced changes in the spatial density of adults could then influence juvenile advection patterns and influence survival.

The Impact of Sea‐Ice Drift and Ocean Circulation on Dispersal of Toothfish Eggs and Juveniles in the Ross Gyre and Amundsen Sea

“…In the present study, a series of Lagrangian particle tracking simulations based on ocean and sea-ice velocities obtained from a high-resolution ocean model hindcast is used to determine the pathways of particles from five spawning regions within the Ross Gyre (Figure 1b) under different advection schemes, which mimic different hypothetical juvenile behaviors. The present study builds on and expands the previous work by investigating how certain juvenile behaviors and spawning regions influence particle trajectories and recruitment success using empirical data on buoyancy and timing of spawning (Parker et al, 2019(Parker et al, , 2021. The various advection schemes were designed to explore the impact of ocean circulation and sea-ice drift as well as various biological strategies of toothfish (i.e., active swimming and diel vertical migration [DVM]) on the trajectories and variability in recruitment success of particles in reaching the "target region."…”

“…It is likely that swimming abilities of larval and juvenile toothfish would increase during this period (Hanchet et al., 2008). Although no studies have been conducted linking toothfish juveniles and sea‐ice, an association has been hypothesized based on the strongly buoyant eggs, timing of spawning, and likely locations of spawning (Parker et al., 2021).…”

“…

Antarctic toothfish (Dissostichus mawsoni) is one of the few commercially exploited notothenioid species endemic to the seas around Antarctica. In the northern Ross Sea region (Figures 1a and 1b), it spawns on bathymetric features, such as seamounts and ridges during winter between early July and late August (Hanchet et al, 2008(Hanchet et al, , 2015Parker et al, 2019Parker et al, , 2021. Spawning and spent fish have been observed spanning from 150°W to 150°E longitude at latitudes near 65°S (Parker et al, 2019), while juveniles (50-100 cm size class) are abundant on the shelf and slope between the Ross and Amundsen Sea.

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Genome-wide analysis reveals striking lack of genetic differentiation over long distances for the Antarctic toothfish Dissostichus mawsoni: High genetic connectivity or shared spawning grounds?

“…Hanchet et al, 2015), but its reproductive strategies and the early life-history stages of development are much less known. Some evidence indicates the life cycle of the Antarctic toothfish to be closely linked to the seasonal Antarctic environment, thus the viability of this species may additionally be threatened by global climate change (Parker et al, 2021;Hanchet et al, 2015;Ainley et al, 2016;Griffiths et al, 2017). Antarctic environmental conditions such as air and water temperatures, and sea ice dynamics are in fact experiencing unprecedented rates of change (Clem et al, 2020;Stammerjohn et al, 2012;Vaughan et al, 2003).…”

“…It has also been suggested that deep water currents may shape or influence the distribution of adult toothfish around the Antarctic continent (Hanchet et al, 2015) as they attain neutral buoyancy in adulthood (Near et al, 2003), although this hypothesis remains a matter of debate (Ainley et al, 2016). The recent discovery that adults release pelagic eggs in open waters, which freely drift downstream over long distances (Parker et al, 2021;La Mesa et al, 2019), suggests gene flow across the distribution range of Antarctic toothfish could occur.…”