Ontogenetic changes in the buoyancy and salinity tolerance of eggs and larvae of polar cod (Boreogadus saida) and other gadids

West Greenland ichthyoplankton and how melting glaciers could allow Arctic cod larvae to survive extreme summer temperatures

“…The high metabolic rates of larval Arctic cod in warm waters (5 ºC) make them highly sensitive (in terms of condition as measured by morphology or lipid storage) to low food availability (Koenker et al 2018a), which could result in highly challenging conditions if early ice breakup and warm summer SST, in addition to impact directly larval survival, decrease the abundance of prey available to the larvae. Finally, as suggested for other seas (Christiansen 2017;Spencer et al 2020), a very early ice breakup could expose the buoyant and transparent Arctic cod eggs to damaging UV radiation.…”

“…Arctic cod larvae were found at stations with surface (0 -5 m) temperature ranging from 3.1 to 10.3 °C and surface salinity ranging from 22.1 to 33.1. At most stations (17/21), the density of surface water exceeded the density of larval Arctic cod as determined in the laboratory for larvae aged 0-14 days (1.024 g cm -3 , Spencer et al 2020). Hence at these stations, larval Arctic cod were most likely in the surface waters with SST ranging from 3.5 to 9.4 °C and surface salinity ranging from 30.6 to 33.1 (Fig.…”

“…For each station, the depth of Arctic cod larvae in the water column were estimated from CTD profiles, assuming a specific density of 1.024 g cm -3 , as measured from larvae aged 0-14 days in the laboratory (Spencer et al 2020), and assuming no directional swimming behavior modified significantly the vertical position of the larvae in the water column. Stations with surface (0-5 m) density (σ t ) > 1.024 g cm -3 were described as having low influence of freshwater (LF) while stations with surface σ t < 1.024 g cm -3 were identified as highly influenced by freshwater (HF).…”

“…Arctic cod is highly adapted to the extreme conditions of Arctic seas. Documented adaptations include the production of antifreeze proteins (Osuga and Feeney 1978;Enevoldsen et al 2003), spiny bony plates protecting the skin from sharp ice edges (Evseenko and Bolshakova 2020) and euryhaline larvae able to tolerate important salinity fluctuations associated with ice melt and freshwater discharge (Doroshev and Aronovich 1974;Spencer et al 2020). The species' long hatching season (up to 7 months) is seen as an adaptation to maximize annual recruitment in an environment characterized by high interannual variations in the timing of sea-ice breakup, primary and secondary production (Fortier et al 2006;Bouchard and Fortier 2008;.…”

“…In addition to the direct effect of temperature, many factors could contribute to declining feeding success, growth, and survival of Arctic cod early life stages in the future. Arctic cod spawn in the winter, hence the positively buoyant eggs generally develop under the sea ice (Aronovich et al 1975;Spencer et al 2020). With current and future sea-ice cover declines, exposure to UV radiation could damage the highly sunlight-sensitive, unpigmented Arctic cod eggs floating at the sea surface (Altukhov 1981;Christiansen 2017;Spencer et al 2020).…”