April 18, 2011

Warming Oceans Pose Risk To Inshore Fish Species

The growth of the banded morwong, a long-lived southeast Australian and New Zealand inshore species, is negatively affected by the warming of our oceans, a study published in Nature Climate Change found.

Scientists have been monitoring temperatures at Maria Island off the east coast of Tasmania since 1944. They found that the surface water temperature in the Tasman Sea has been rising by nearly 2°C over the past sixty years.

The study found that the rapid jump in sea surface temperature has caused the morwong's growth in some areas to slow.

According to the study, rapid warming in the southern hemisphere oceans is due to "globally increasing sea-surface temperatures and local effects caused by southward extension of the east Australian Current."

As the seas heat up and become more acidic, coral reefs and multi-billion dollar commercial fisheries could be affected.

The study says that a lot of commercial fish do not move very much. "They tend to return to the same spawning grounds or they live on the same reefs. And those are the ones that are going to be most affected," says CSIRO marine ecologist Dr. Ron Thresher, a co-author of the study with colleagues from the University of Tasmania's Institute for Marine and Antarctic Studies.

This was particularly true for the morwong and other fish species that live near the shore and at shallow depths, reports Reuters.

Dr. Jeremy Lyle, researcher at the University of Tasmania and co-author of the study says that morwong were used in the study because they can live for almost 100 years, and adult morwongs essentially stay in the same area even if the water temperature shifts.

Lyle says that the study showed that the growth performance in banded morwong started to suffer above average annual water temperatures at about 17°C.

"Generally, cold-blooded animals respond to warming conditions by increasing growth rates as temperatures rise," says Thresher.

However there is a limit.

He says, "As temperatures get too high, we begin to see increased signs of stress, possibly eventually leading to death. We are looking at whether climate change is beginning to push fish past their physiological limits."

"By examining growth across a range that species inhabit, we found evidence of both slowing growth and increased physiological stress as higher temperatures impose a higher metabolic cost on fish at the warm edge of the range."

"In this case, off northern New Zealand, ocean warming has pushed the banded morwong "“ which inhabits temperate reefs in waters 10-50m deep "“ past the point where increasing temperatures are beneficial to growth."

Thresher adds, "Preliminary field and laboratory studies suggested that this decline in growth may be related to temperature induced physiological stress, resulting in increased oxygen consumption and reduced ability to sustain swimming activity."


Image 1: Banded morwong. Image credit "“ Rick Stuart-Smith, University of Tasmania

Image 2: Sectioned banded morwong otolith showing growth rings. Image credit "“ IMAS, University of Tasmania


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