In the cold fjords of the sub-Antarctic island of South Georgia, the survival of entire generations of fish is decided. New research shows that changes in ocean currents caused by climate change can determine whether young mackerel icefish are able to grow and survive, or are swept out into the open ocean.
Mackerel icefish are a key component of South Georgia’s marine ecosystem. They are an important food source for seals, penguins, and other marine animals. If their offspring are threatened, the effects ripple through the entire food chain.
Glacier melt alters currents
The study was led by Dr. Joanna Zanker of the British Antarctic Survey. As part of her doctoral research, she investigated for the first time in detail how ocean currents in South Georgia’s largest fjord respond to climatic changes.
In November 2021, the research team collected extensive oceanographic data during an expedition aboard the fisheries patrol vessel Pharos SG. Combined with computer simulations, the results revealed a clear pattern: increasing glacier melt is releasing more freshwater into the fjords. This change strengthens currents that pull young fish out of the sheltered fjord areas and into the open ocean, where their chances of survival drop dramatically.
Wind as an unexpected ally
However, the study also offers a glimmer of hope. Strong, warm winds can alter currents in ways that keep fish larvae trapped inside the fjord. This natural “retention mechanism” significantly increases their chances of reaching adulthood.
“Our results show just how sensitive fjord systems are to changes in the ocean,” explains Dr. Zanker. “As the climate continues to warm, these processes could have major impacts on local ecosystems and on commercial fisheries as well.”
Implications for marine conservation
South Georgia is home to one of the largest marine protected areas in the Southern Ocean and is internationally regarded as a model for strictly regulated, sustainable fisheries. The new insights into the role of ocean currents during the early life stages of fish provide an important foundation for future conservation measures — both in South Georgia and in comparable regions worldwide.
The study was funded by the Natural Environment Research Council, Polar Seafish Ltd, and UK Research and Innovation. The findings were published under the title “Sub-Antarctic fjord circulation and associated icefish larval retention in a changing climate” in the journal Progress in Oceanography.
Heiner Kubny, PolarJournal