Rebels without a cause?

Carbon dioxide, when dissolved in water, creates an acid. The continued rise in greenhouse-gas emissions means that since recordings began in the 1980s, the acidity of the oceans has risen from pH 8.11 to pH 8.06 (on the pH scale, lower numbers mean more acid). This equates to a 12% increase.

How this will affect fish is of huge concern: billions depend on them as a source of protein and for income.

A study published online today for Nature Climate Change, led by Philip Munday from James Cook University in Australia, suggests that elevated levels of carbon dioxide in seawater affect the behaviour of young damselfish and cardinalfish in a manner that may ultimately, in natural ecosystems, harm larger marine communities. Juveniles living in more acidic environments appeared oblivious to the dangers therein, acting more rashly than fish from less acidic areas as a result.

Previous studies, conducted in laboratories, have found that elevated rates of carbon dioxide in water affect the sense of smell and decision-making capabilities of fish. But without the presence of larger communities, these findings have told scientists little about the larger implications of carbon dioxide on them.

Dr Munday and his team therefore studied three sites off Papua New Guinea to test the effects of continuous carbon dioxide exposure on reef fish behaviour.

At the sites, reefs housed fish near cool volcanic seeps that acidified surrounding water to levels akin to those projected for 2100: pH 7.72 to pH 7.95. Nearby were areas unaffected by the carbon dioxide seeps—perfect control communities for experiments with their lower acidity: pH 7.98 to 8.02.

Two damselfish species and two cardinalfishes were scrutinised. “Highly sedentary” and with small home ranges, these fish are recruited to reefs after the larval phase of their development.

First a tank was prepared containing a two-channel choice flume filled with water streams either conditioned with or without odour from predatory fish. Then the team captured ten individuals from each of the species under consideration and monitored which stream they would prefer to swim in over four minutes. The participants were taken from all three sites.

Results across the species were similar. But they differed when fish from more acidic waters were compared with those from control communities. Fish used to higher levels of carbon dioxide spent 90% of their time in water perfumed with predators; control fish spent all their time in the other stream.

Large amounts of carbon dioxide interfere with the neurotransmitter receptor function in fish, explaining why those from more acidic water apparently smelled little danger. The acid-base relevant ion concentration in their blood and tissue had been altered.

The rash behaviour of fish accustomed to higher acidity did not stop there, however. After being placed individually within another tank that contained a small coral colony for shelter, the scaly participants were left for two hours (so they could get used to their new surroundings). Their activity, and the distance they travelled from the coral, were then observed for five minutes after which the fish were “chased” back home with the help of a pencil. Scary stuff.

Damselfish busy themselves in the day, and cardinalfish at night, so the experiments were timed accordingly. All four species of fish from control areas spent 80% of their time in shelter. Damselfish used to greater acidity did the same 12% of the time but their cardinalfish peers never did. This latter group also boldly came out from behind the coral six times faster than the more nervous control fish.

Overall the communities to which all the fish belonged, and their structures seemed little affected by carbon dioxide variations. But that is no reason for a sigh of relief. Experimental studies have previously shown that carbon dioxide-induced behaviour increases mortality in fish newly-settled at a reef by fivefold. As the three sites studied were small, Dr Munday and his team believe that fish who were casualties of their own rash behaviour could have been easily replaced. Predatory fish were also fewer at more acidic sites.

But as ocean acidification increases, reefs will not be able to recruit new inhabitants from unaffected areas so easily. Predators will also grow accustomed to the changes in pH.

Bold behaviour in juvenile fish does not improve given prolonged exposure to elevated levels of carbon dioxide dissolved in sea water. The implications of the study are more troubling than even pencils acting aggressively.

The Economist, 13 April 2014. Article.

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