An increasing number of species are migrating in response to global warming; some alpine organisms are climbing to higher altitudes, others animals are moving towards the poles.
A new study suggests that as sea temperatures rise, many fish may be electing to move into deeper, cooler waters, rather than moving to higher latitudes as many theorists had previously predicted.
Researchers led by Nicholas Dulvy of the Centre for Environment, Fisheries and Aquaculture in Lowestoft, UK, studied fish in the North Sea, where sea-bottom temperatures have climbed by 1.6 ºC in 25 years.
They analysed data gathered by large-scale research trawling operations to determine changes in the geographic distribution of 28 bottom-dwelling fish species during the past quarter-century. They found that fish have not universally shifted northwards.
The great escape
The researchers report in the Journal of Applied Ecology1, that rather than going north, many species are seeking deeper waters in a bid to continue living at their optimum temperature.
Over the past 25 years, North Sea fish have dived by an average of 9 metres, Dulvy and colleagues report. Some species, such as the megrim (Lepidorhombus whiffiagonus), have deepened their habitat by 35 metres.
Because the sea floor is not uniform in depth, the temperature changes are forcing bottom-dwelling fish to move to new locations so they can stay within the same underwater climate. This temperature-seeking behaviour is leading them to move in different directions.
This is similar to how alpine species are being forced to track their preferred temperature zones up mountains, explains Dulvy. When cool temperatures move up above the peak, these species effectively run out of habitat. “We don’t know if the fish will also reach the top of the 'mountain' or if they can just keep going deeper,” comments Dulvy, now based at Simon Fraser University in Burnaby, Canada.
The big question is what these results mean, says population biologist Marcel Visser at the Netherlands Institute of Ecology in Heteren. “How much should these species really be shifting for this increase in temperature? Are these results showing that species are adapting, which would be good, or are they showing that climate change harms species even in the sea?” he asks.
How low can you go?
Some experts do not think the outlook is good at all. “If these species keep going deeper they are going to run out of depth — the southern North Sea isn’t really all that deep,” comments fisheries oceanographer Brian MacKenzie at the Technical University of Denmark in Charlottenlund.
Even if the species go north, where the waters are deeper, “the fish are going to run into serious trouble with decreased light, increased pressure and changing habitats”, MacKenzie points out.
Warming temperatures are not the only pressure faced by North Sea ecosystems: overfishing is a serious problem. Teasing out the effects of these two factors is the next challenge, says marine ecologist Martin Edwards at the Sir Alister Hardy Foundation for Ocean Science in Plymouth, UK. “Determining which of these problems is causing what damage is going to prove extremely challenging,” he says.
Dulvy and his colleagues are already reasonably certain that the depth changes are not the result of over-fishing, because commonly fished species were changing depth just as much as species that were not fished at all, but all agree that further exploration of how these two threats are affecting populations is perhaps the greatest challenge ahead.
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