Fish numbers under threat from climate change

Jellyfish and small fish
Juvenile fish rely on the jellyfish for protection against predators. Photo: James Brook

Fish populations could be threatened by climate change and its effect on a special relationship between jellyfish and juvenile fish.

New research from Griffith University, published today in the journal Proceedings of the Royal Society B, has found that predicted ocean acidification may alter symbiotic relationships between juvenile fish and their jellyfish host.

Similar to the symbiotic relationship between anemones and clownfish, as seen in Finding Nemo, the juvenile fish rely on the jellyfish for protection against predators in the open ocean where there is not much shelter for them.

“By protecting the juveniles the jellyfish are probably enhancing the survival rates of the fish and that’s likely to lead to more fish in the future,” says Associate Professor Kylie Pitt of the Griffith School of Environment.

“If this symbiosis breaks down we actually may find this could reduce the number of adult fish in the future.”

The study, led by the University of Adelaide, is the first to demonstrate how ocean acidification will disturb the symbiotic relationship between two animals that interact closely for survival.

Ocean acidification and animal behaviour

Ocean acidification can reverse or alter a wide range of animal behaviours by interfering with the brain neurotransmitter function.

Jack mackerel treated with elevated seawater CO2 concentrations, as forecast for the end of the century, spent only one third of the time they normally spent close to their jellyfish host compared with ambient CO2 conditions.

The jack mackerel, commonly found along the east coast of Australia, and about 80 different juvenile fish species including important commercial varieties such as pollock and trevallies, form important symbiotic relationships with jellyfish.

“They’re pretty nimble and agile swimmers so they’re probably just good at avoiding the stingy bits,” says Associate Professor Pitt on how the baby fish somehow manage to avoid the poisonous tentacles of their jellyfish protectors.

Marine life such as the clownfish is protected from the anemone because of mucus coating.

Study leader Associate Professor Ivan Nagelkerken, from the University of Adelaide’s Environment Institute, says these intricate, interdependent relationships between different species are common in both the marine and terrestrial environments.

“But, apart from the well-known relationship between coral and microalgae and what happens during a bleaching event, little is known about how climate change and predicted ocean acidification will affect such relationships,” he says.

The research shows just how important jellyfish could be in sustaining fish populations in the next 100 years.