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Smaller fish are likely to fare best as the world's oceans become increasingly acidic, suggest Australian researchers.

Seawater is predicted to drop in pH from the current 8.1 to 7.8 by the end of this century as a consequence of ongoing CO2 absorption and fears are held for the ability of much marine life to adapt.

But a new study of Australia's freshwater ornate rainbowfish (Rhadinocentrus ornatus) by a group at the University of Queensland in Brisbane, has found that changes in pH affect the metabolic rate of larger fish significantly more than their smaller counterparts.

This suggests that increasing acidification may impact most on populations of bigger fish.

The ornate rainbowfish study, by Dr Craig White and honours student Fabian Vaca of the university's School of Biological Sciences, has been published in Royal Society journal Biology Letters

"It's a big picture examination of a small fish," says White.

"These fish can exist over a much wider range of pH than [that predicted for] ocean acidification because they've had a chance to evolve ways of dealing with such a variable environment."

"The question is what will happen over long time spans to ... species that haven't evolved to deal with these kinds of pH."

When the acidity of water increases, the mucus on fish gills generally thickens, restricting the level of oxygen that can enter. This is why changes in pH frequently lead to fish kills.

But there are fish that have evolved to be able to live in a wide pH range by reducing their metabolic rate and thus need for oxygen in acidic conditions.

In some parts of Australia, species such as the ornate rainbowfish can exist in water as acidic as orange juice.

White and Vaca found that when pH is changed within a tolerable range, the metabolic rate of the fish alters, but it doesn't alter consistently across the fish population.

Instead, the metabolic rates of larger fish drop a lot further than that of their smaller cousins.

This implies that the relationship between the rainbowfish mass and metabolic rate is also changing.

This challenges the generally accepted wisdom that metabolic rate changes according to body mass in a constant way across the animal kingdom.

White says that pH is likely to be only one of a number of environmental factors, such as temperature, that may influence the scaling of the relationship between metabolic rate and body mass.
Species survival

In this study, the fish were monitored over a month-long period. It remains to be seen what the consequences would be for a large fish with a chronically depressed metabolism.

"It's a coping mechanism, but not necessarily an ideal," says White.

Unfortunately, he says that in the wild it is the larger fish that produce the most eggs and the most offspring, contributing most to ongoing generations.

"If it's those big fish who have the biggest problems, then that's possibly a bad thing for the continuation of those species because they're the ones who contribute most."

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