Page last updated at 19:13 GMT, Thursday, 7 May 2009 20:13 UK

Shrimp tuned to ocean temperature

By Victoria Gill
Science reporter, BBC News

Northern shrimp
Northern shrimp are sensitive to temperature changes at the seafloor

Stocks of northern shrimp, the essential ingredient in the ubiquitous prawn cocktail, could be badly affected if ocean temperatures rise.

Researchers report, in the journal Science, that shrimp eggs hatch within days of each spring phytoplankton bloom - the main food source for the larvae.

They conclude that shrimp are adapted to local temperature, which determines how long eggs take to develop.

If seas warm, as predicted, shrimp stocks could collapse, the team says.

The international team of scientists found that, throughout the north Atlantic - from Cape Cod in the US to to Svalbard in Norway - northern shrimp (Pandalus borealis) eggs hatched, on average, in time with the bloom.

This is the period when food is abundant, so the larvae have a far better chance of survival.

But to get the timing right, the shrimp must mate during exactly the right period during the previous year.

North Atlantic map
Further north, the algal bloom (green) happens later

"They don't do this on a year by year basis - deciding to mate a week later because the algal bloom will be a week later," said Peter Koeller, a researcher from the Bedford Institute of Oceanography, who led the study.

"This is on evolutionary time scales - they have adapted to local conditions."

This means it would be impossible for the shrimp to adapt to a rapid change in temperature at the seafloor, where they live.

Shrimp boats

Dr Koeller's team collected samples of shrimp daily and counted the proportion of females that were still carrying their eggs. With satellite imaging, they were able to compare the timing of the algal blooms to the release of the larvae.

Shrimp trawler
The tasty crustaceans make up 70% of a 500,000 tonne annual shrimp harvest

As Dr Koeller pointed out, an explosion in the northern shrimp population in the 1980s and 1990s was linked to a drop in sea temperatures at that time.

He said it was feasible that the opposite could happen "as the climate changes".

"As surface waters warm, this would eventually result in warmer water at the bottom, which would lead to faster development of eggs and earlier hatching," he explained.

"The larvae would be further removed from period of food abundance, which would mean poor survival rates and fewer shrimp."

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