Fish pay price as parasites thrive in warmer waters, warn scientists
December 05 2011 Lewis Smith
Fish populations are likely to slump under global warming because parasitic worms thrive in warmer temperatures, a study shows.
Salmon, eels, cod and haddock are among the fish likely to suffer as a rise in water temperatures encourages a boom in the numbers and sizes of parasites.
Parasitic worms such as the bird tapeworm, Schistocephalus solidus, have a larval stage in fish and a study of sticklebacks shows that they are likely to benefit from warmer waters.
Not only are they able to grow to maturity much faster but they are also thought to be able to change a fish’s behaviour to make it swim to warmer waters. Moreover, the fish grew more slowly in the warmer conditions.
Researchers issued their warning after carrying out a study into how the bird tapeworm affects sticklebacks. Fish were kept in waters of 15C and of 20C and the researchers found that the 5C difference enabled the parasites to grow four times as fast.
Sticklebacks are not eaten by people in the UK but Dr Iain Barber, of the University of Leicester, said the advantage global warning gives to the parasites will be the same in commercial stocks.
“Fish like cod, salmon and haddock all have worm parasites that would be expected to benefit from increases in temperatures,” he said. “The principles will still hold. What we are showing is parasites grow better with increases of temperature. Fish are much more sensitive to higher temperatures and don’t always do better.”
Cod are among the species that have already reacted to warmer water temperatures caused by global warming. They are increasingly moving northwards. Similarly, mackerel are shifting to more northerly waters as temperatures rise.
Fish found in rivers and lakes are likely to be the worst affected because when temperatures rise they are less likely to be able to swim to somewhere cooler.
Dr Barber, who carried out the study with Vicki Macnab at Leicester University, added: “Sticklebacks are fairly unimportant on a commercial level but are ecologically important.”
The consequences of the tapeworm larvae growing faster means that the parasites reach maturity quicker, survive in greater numbers and can lay many more eggs. This could have the effect of causing sticklebacks and other species to suffer population crashes, with a knock on effect for other species, such as heron and kingfishers which eat them.
Ms Macnab said: “The research shows a dramatic effect of increased environmental temperatures on the growth rates of parasites in fish hosts. The size these parasites attain in their fish hosts determines how severely fish reproduction is affected, so our results suggest that parasite will have a more serious effect on fish reproduction if temperatures rise.
“In addition, our paper documents behavioural changes in infected fish that suggests the parasites are manipulating host behaviour to make them seek out warmer temperatures, creating a positive feedback mechanism to exacerbate the effects of global warming.
“This research shows that global warming could shift the balance between parasites and their hosts with potentially serious implications for fish populations.”
The study was supported with funding from the Biotechnology and Biological Sciences Research Council (BBSRC) and the Centre for Environment, Fisheries and Aquaculture Science (Cefas), and is published today in the journal Global Change Biology.
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