Young fish that rely on sound cues to navigate home across vast distances are getting lost at sea as our oceans acidify. According to new work published in Proceedings of the Royal Society B, ocean acidification boosts the development of young fish, but it also makes it harder for them to find a safe, suitable place where they can grow into adults.
Over evolutionary time, fish such as barramundi (Lates calcarifer) figured out a way to orient themselves and find their way to safety by listening for sounds, such as those produced by snapping shrimp that live among mangroves. "Locating appropriate homes is a crucial step in the life cycle of fish," University of Adelaide’s Tullio Rossi explained in a statement. "After hatching in the open ocean, baby fish travel to reefs or mangroves as safe havens to feed and grow into adults.”
Locating appropriate settlement habitat involves multiple senses: hearing, smelling, and seeing. Rising CO2 levels and ocean acidification are putting this age-old, but delicate, process at risk.
Rossi and colleagues conducted a series of experiments with barramundi hatchlings they reared from fertilized eggs obtained from a commercial hatchery. These popular tropical fish (also known as Asian seabass) are found between the eastern Indian Ocean and the western Central Pacific, and they migrate from fresh water to the ocean to spawn. The larvae then settle in estuaries and near-shore coastal areas.
The team examined the attraction of larval fish to acoustic cues in the environment daily over 15 days. They tested their response to recordings of estuary soundscapes that act as potential orientation cues leading towards settlement habitat. Larval barramundi are only attracted to the playback of settlement habitat sounds during a short window of just three days – which match the timing of their metamorphosis from larvae to juveniles.
Elevated CO2, the team found, causes larval barramundi to grow faster and initiate metamorphosis earlier – but it also reduces the window of opportunity for successful settlement. These hatchlings end up misinterpreting sound cues. "When ocean acidity increases due to increased CO2, the neurological pathways in their brain are affected and, instead of heading towards those sounds, they turn tail and swim away," Rossi said. They also moved slower and appeared to be more anxious – behaviors that would make it more difficult for them to find food or suitable habitat and also avoid predators.