Warmer seas to hit reef fish badly

Warmer seas triggered by the 1997-98 El Niño led to a dramatic drop in the number of reef fish around French Polynesia, scientists have found.

The fish may have starved, because higher water temperatures killed off tiny marine creatures called plankton, which the fish feed on when they're young.

During the El Niño – the second strongest on record – ocean temperatures shot up to 3.5°C above average. El Niño is the name given to the change in the temperature of the eastern Pacific Ocean which can bring huge changes in the world climate.

Scientists are predicting that warmer waters like those typified by an El Niño event will be the norm under climate change. If they're right, this could spell disaster for around 500 million people who rely on coral reef fish either for their food or for tourism.

'This is the same event that wiped out more than half of the Seychelles reefs. It was a substantial El Niño,' says Dr Steve Simpson from the University of Bristol, one of the authors of the study, published in Global Change Biology.

Although climate change is predicted to bring much warmer seas, it's difficult to be sure how this will affect ecosystems.

In this latest study, Simpson and an international team of experts managed to get hold of data on the numbers of coral reef fish in French Polynesia spanning four years – with the 1997-98 El Niño bang in the middle.

'This meant we could look at the effects of the warming and compare that to fish numbers before El Niño warmed the seas,' says Simpson.

Not just that, but the effects of an El Niño are far-reaching, with higher water temperatures happening over thousands of kilometres, and over relatively long periods. So the team realised that combined with the fish data, this was probably the best opportunity they were going to get to find out how reef fish cope with warmer-than-average water.

Simpson and his colleagues analysed satellite data on water temperatures, ocean currents and the amount of chlorophyll in the water from January 1996 to March 2000. Many plankton use chlorophyll to photosynthesize in the same way that plants do, so chlorophyll tells scientists how much plankton is in the water.

They found during the 1997-98 El Niño, water temperatures rose, concentrations of chlorophyll dropped and the direction in which the ocean currents flowed changed around French Polynesia.

But warmer waters didn't just hit young fish badly; lack of plankton may have meant adult fish couldn't produce as many offspring as usual. The team found that once plankton had recovered, it took three to four months before the numbers of young fish started to climb.

'Even after the current switched with El Niño, we still saw young fish on the reef, so the current didn't seem to be the problem. We're fairly sure this is all down to food availability,' says Simpson.

Young reef fish don't grow up on the reef. Instead, after the adults have laid eggs, they drift out to sea, where they feed on plankton until they're developed enough to come back to the reef. The numbers of predators around coral reefs are much higher than out at sea, so the young fish can grow up in relative safety.

Fisheries managers often monitor the numbers of young fish returning to the reef to get some idea of the likely numbers of fish the following season. It was this data that Simpson and his colleagues analysed.

While reef fish could be badly hit by climate change, Simpson is finding that other species, in the northern Atlantic for example, aren't doing quite as badly.

'There's bound to be winners and losers. Some species will find it easy to move north or south as temperatures rise; those that are more mobile. But reef fish are tied to the reef, so they won't be able to move so easily,' adds Simpson. 'It's not a simple story.'

by Tamera Jones

http://planetearth.nerc.ac.uk/news/story.aspx?id=896