Oceans are becoming more acidic by absorbing atmospheric CO2, and research shows this hinders the sensory abilities of larval fishes. This means they will be unable to complete their life cycles.

Barramundi Cod, <i>Chromileptes altivelis</i>

A 15 mm long larval Barramundi Cod, Chromileptes altivelis, collected at Lizard Island, Queensland, February, 2001. Note the serrated dorsal fin spine.

Image: Stuart Humphreys
© Australian Museum

As human activities add more CO2 to the atmosphere, much is absorbed into our oceans, and this makes our oceans more acidic. Recent research on the effects of ocean acidification on the behaviour and sensory systems of fish larvae show that although this has little effect on the swimming ability of the larvae, it is deleterious to their sensory systems. Neurotransmitter function is disrupted, and the senses (smell, taste, hearing, and sight) will no longer function properly at CO2 levels predicted for the middle to end of the century under “business as usual” scenarios. Why is this important?

The marine bony fishes that live in association with bottom habitats like coral reefs, produce larvae that receive no parental care once the eggs hatch. Rather, the newly hatched larvae must survive and grow entirely on their own in open, pelagic waters for a few weeks to months, growing from about 2 mm to a cm or so, and then locate suitable habitat upon which to settle and become bottom-associated, like their parents.

Larval surgeonfish, Naso sp
A Larval surgeonfish, Naso sp, caught close to the surface during a fieldtrip to Lizard Island over January and February 2008. It is now registered in the Australian Museum Fish Collection (AMS I.44582-003). Image: Colin Wen
© Colin Wen

During this pelagic period, the larvae use their sensory systems to navigate through open water, and then to smell or hear their nursery habitat, and swim toward it. Once over that particular type of bottom, they must then run a gauntlet of predators and find that precise microhabitat – a particular species of coral, for example – upon which to take up residence.

If their senses are not functioning properly, they can do none of these things. Nearly every published study has shown that ocean acidification interferes with these essential senses of larval marine fishes. Therefore, the ability of larvae to navigate in the ocean and find suitable habitat will be impaired, and as a consequence, the number of larval fishes surviving to reach adulthood will decrease.

In a best-case scenario, marine fishes may be able to adapt to a more acidic ocean and retain the sensory abilities necessary for larvae to complete their life cycles. However, laboratory experiments with short-lived species such as plankton indicate it will take several hundred generations for any adaptation to occur to ocean acidification. Assuming that acclimation or adaptation will happen quickly enough to overcome the rapid rate at which atmospheric CO2 is rising is optimistic – to say the least - given our current level of understanding. A more certain result can be achieved by strongly reducing anthropogenic CO2 production so our oceans do not become as acidic as they will under a “business as usual” scenario.

Jeff Leis, AMRI Senior Fellow

More information:

  • Leis, J.M. (2018). Paradigm Lost: Ocean Acidification Will Overturn the Concept of Larval-Fish Biophysical Dispersal. Frontiers in Marine Science 5(47), 1-9. doi: 10.3389/fmars.2018.00047.