Fish take more risks as ocean acidifies

If levels of CO2 continue to rise in our seas to predicted levels over the next 100 years, there could be a dramatic decline in fish populations.
New research reported in a recent issue of the Proceedings of the National Academy of Science (PNAS), shows that when larvae of the Clownfish, Amphiprion percula and the Damselfish, Pomacentrus wardi are subjected to increasing levels of CO2 their behaviour changes and they take more risks.

Furthermore, as those levels of CO2 increase, the larvae actively swim towards a predator! This could have dramatic effects on fish populations as there would be fewer fish surviving to sustain adult populations.

To conduct the experiment, the research team used tank-raised clownfish larvae and damselfish larvae captured from the reef. The larvae were split into groups, and reared in seawater with differing concentrations of CO2.

The larvae were then placed in a vessel known as a two channel flume – essentially two streams of water – with one channel of the flume receiving seawater with the chemical cues of a predator present, and the remaining channel with normal seawater with no predator cues.

Larvae that were reared in control water displayed a strong avoidance to the predator cues, likewise for larvae reared in water that had a CO2 concentration of 550 ppm.

However, larvae that were raised in water with 700 ppm CO2 concentration initially avoided the predator cue then, after a few days approximately 50% of the larvae began to migrate towards the stream with the predator cue.

Larvae that had been raised in water with an 850 ppm CO2 concentration exhibited the most remarkable response with nearly all the larvae spending over 90% of their time in the stream with the predator cue present.

The researchers continued their experiment by transplanting larvae to a reef environment, with similar results. The fish that had been exposed to CO2 levels of 700 ppm & 850 ppm were more active and ventured further away from shelter, thus leaving them open to predation, this led to a mortality rate in those larvae of 90% within the first 30 hours.

The authors note: “On the current emissions trajectory, atmospheric CO2 concentrations will exceed 500 ppm by 2050 and could reach 850 ppm by 2100. There seems to be little potential for adaptation of behavioural responses should 850 ppm CO2 be reached, with serious, irreversible consequences for marine biodiversity.”

For further information see: Philip L. Munday, Danielle L. Dixson, Mark I. McCormick, Mark Meekan, Maud C.O. Ferrari and Douglas P. Chivers. Replenishment of fish populations is threatened by ocean acidification. PNAS, July 6, 2010 DOI: 10.1073/pnas.1004519107

Gareth Bull, Practical Fishkeeping, 15 July 2010. Article.

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