The Southern Ocean envelops the Antarctic continent and is comprised of all global ocean waters south of 60°S latitude.
The Southern Ocean is home to the greatest ocean current, the Antarctic Circumpolar Current (ACC), which holds 150 times more water than all of the world’s rivers combined. Antarctic water is also a source of deep water, which plays a vital role in thermohaline circulation and climate regulation.
Temperatures in Antarctic waters can vary anywhere from -2 to 10°C and there is great seasonal variation in ice cover, water circulation, as well as phytoplankton biomass which determines the amount of food that is left for higher trophic level species.
These variations are a result of the polar day/night cycle, the phenomenon in which the polar regions go months with or without sunlight in their respective summer and winter seasons.
Despite its seemingly inhospitable conditions, the Southern Ocean is teeming with life. From gigantic shoals of Antarctic krill to dense kelps forests; Antarctica is far from desolate.
These waters are also home to several species of fish, including snailfish (Liparidae), icefish (Nototheniidae), and eelpouts (Zoarchidae), all of which account for almost 90% of the 300 plus Southern Ocean fish species.
Although these animals comprise approximately 3% of the Southern Ocean’s array of marine life, Antarctic fish play a crucial role in the marine food web. However, much like their fellow sub-zero neighbours, Antarctic fish are left vulnerable by the climate crisis.
Rapid climate change is perhaps most obvious in the Southern Ocean. Changes in water pH and temperatures, brought on by the increased reliance on fossil fuels, has the potential to cause drastic changes in climate and weather patterns across the world, as well as altering established climate cycles such as the ENSO cycle. A 2021 study suggests that by 2050, areas of the Antarctic continental shelf will have risen by a whole 1°C.
These changes present a number of challenges for life in the Southern Ocean; changes in the seasonal sea ice extent has the potential to disrupt the marine food web which is underpinned by ice-dwelling phytoplankton. Antarctic fish are one of many groups of animals that are at great risk.
But, there is still hope.
The team, led by VTC Health Sciences and Technology president, Michael Friedlander, observed a multitude of behavioural mechanisms employed by the species to combat increasing ambient water temperatures. Such behavioural adaptations include breathing at the surface, alternating periods of movement and rest, startle-like behaviour, and fanning and splaying their fins to aid in respiration.
Dr Friedlander had this to say,
“Remarkably, our team found that Antarctic fishes compensate for increasing metabolic demands by enhancing respiration through species-specific locomotor and respiratory responses, demonstrating resilience to environmental change and possibly to global warming“
Ambient warming presents a multi-faceted challenge to the fish, including increased temperature of the central nervous system and target tissues such as skeletal and cardiac muscles, but also reduced availability of dissolved oxygen in the water that passes through the gills during respiration. While these findings suggest that Antarctic fishes may be able to behaviourally adapt somewhat under extreme conditions, little is known about the effects of environmental warming on their predation habits, food availability, and fecundity.”
Blackfin icefish, Chaenocephalus aceratus, one of the two studied species, lack haemoglobin: the oxygen-carrying protein found in red blood cells. As a result of no haemoglobin, this species has translucent blood and cream-coloured gills.
Instead, blackfin icefish transport dissolved oxygen directly in the blood plasma. This results in an oxygen carrying capacity which is 90% less than that of haemoglobin.
White-blooded icefish are able to thrive in colder waters due to the fact that oxygen is more soluble in cold water. As such, rising temperatures decrease the solubility of oxygen as well as exerting increasing metabolic demands on Southern Ocean species. This in turn, may increase the vulnerability of icefish to climate change.
The team tested this hypothesis by examining the responses of blackfin icefish and red-blooded black rockcod, Notothenia coriiceps, to increasing water temperatures in controlled laboratory conditions.
The behavioural adaptations (detailed above) “provide a new perspective on the effects of rising temperature on these highly cold-adapted species,” said George Somero, a professor of marine biology at Stanford University.
This study suggests that species-specific behavioural responses in Antarctic fish fauna may just help these animals withstand the impacts of climate change and is a ray of hope in an otherwise despairing situation.
Written by Lucas King