Climate change and pollution are hand in hand, but we often forget the increasing warnings we have about how our emissions are acidifying the waters around us. In theory, acidity has no obvious effect on fish. The effects however seem to be on behaviour, which is hard to evaluate. Using some natural "seeps" of carbon dioxide near Papua New Guinea, young fish were seen with behavioural abnormalities that mirrored lab experiments. They could be attracted to predator odours, could not distinguish the odours of different habitats and were "bold" compared to normal fish on a control reef. The boldness was measured by the extent to which they could leave sheltered areas.

Metabolic activity and aerobic respiration appeared to be unaffected by high levels of the gas, with low pH values. Corals in the communities with these naturally acidic reefs were different from those in more normal reefs, but fish abundance and diversity was similar, though with different conditions. There were fewer predators in the seeps, which did seem to attract juvenile fish. The overall effect of extreme behavioural modification therefore didn't cause any loss of populations, but the lack of predators would compensate for any losses.

The increasingly acid ocean has concerned scientists for several years. Corals obviously suffer because they would lose their skeletal material, calcium carbonate to the carbonic acid in the water. Similar situations exist for both molluskan shellfish and the crustaceans. In the community, fish would seem to suffer less, but now it is claimed that the sensory systems of reef fish are affected by reduced olfactory ability. Increased mortality of these fish in a normal reef would be certain.

Damsel fish, Dascyllus aruanus and Pomacentrus moluccensis and cardinal fish, Apogon cyanosoma and Cheilodipterus quinquelineatus (both growing up to about 15cm) on two reefs with carbon dioxide seeps were investigated. All 4 species showed distinctively different behaviour at the seep reefs. The "smell" of a stream of water in which a rock cod (Lotella rhacina) had been living caused normal fish to avoid it completely. In contrast, the seep reef fish were strongly attracted to this fish's smell. This animal, up to 50cm long, is very predatory and would normally gobble up the small fish. Interestingly, normal reef fish also avoid the seep water, while seep juveniles were spending about 50% of their time in each of the two sea water samples.

All 4 species spent at least 80% of their time in hiding under normal reef conditions. The damsels from the seep spent <12% of their time under cover but were less active, while the cardinals spent their whole time in open water and were much more active than the normal fish.

The seep reefs were characterised by massive Porites coral formations with may caves and large holes formed by the acid erosion . Where there was no acidity, non-massive Acropora corals formed complex table and branching forms that attract predatory fish to hunt there. The damsels and cardinals are planktivorous, also preferring high positions near the massive corals. In future, as all reefs approach high acidity, the situation is likely to shift dramatically, as there would be no choice for reef organisms. At the moment larval fish are recruited from outside acidic areas, but populations would certainly drop when acid conditions prevailed.

As long as we emit so much carbon dioxide from fossil fuel burning, the fate of oceanic life is liable to be as dramatic as the products of global warming, According to the IPCC recently, we have to reduce carbon dioxide emissions before global warming exceeds 2 degrees Celsius. How many levels of pH will sea water go down to before we lose a vast amount of ocean life?