Warmer oceans make some fish smaller

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Warmer oceans are not good news for some fish, according to research to be published in a forthcoming issue of the journal Nature Climate Change.

Anna Neuheimer and coauthors examined the effects of long-term changes in sea surface temperatures (SSTs) on the growth of the Banded morwong (Cheilodactylus spectabilis), a long-lived species found in coastal waters of New Zealand and southeastern Australia.  

The authors focused on the Tasman Sea, one of the fastest warming areas in the South Pacific Ocean, and assessed the impacts of this temperature increase on long-term growth rates of the Banded morwong.

The authors estimated the growth rates of the fish over a 90-year period (1910–2000) by studying their otoliths (bones in the head that the fish use for orientation and the detection of movement), which grow incrementally (analogous to growth rings in trees). They then compared these changes to temperature trends across the species’ distribution, using SSTs obtained from a dataset and validated with those recorded over a period of 60 years from Maria Island, off the eastern coast of Tasmania.

The authors found that mean temperatures were the warmest off New Zealand and coldest off southeastern Tasmania. A strong relationship was observed when comparing temperature data to the growth rates of the Banded morwong, with an increase in growth rate with temperature for the cooler water Australian populations but a decrease with temperature for the warmer water New Zealand populations.

This demonstrates that increasing water temperatures have pushed the Banded morwong past the point where warming is beneficial to growth.

In addition to decreased growth, the authors also found evidence that increasing temperatures impose more physiological stress on the Banded morwong.  

The fishes consumed 44% more oxygen when approaching spawning swimming speeds (about 0.9 metres per second) at temperatures above 16°C compared to those at temperatures below 14°C. Furthermore, the fish were unable to sustain spawning swimming speed above 16°C and showed signs of stress due to lack of oxygen.

Based on their results, the authors predict that increasing ocean temperatures would cause the range of the Banded morwong to contract, the average size of the fish to decrease, and fecundity and egg quality (which is correlated to the size of the fish) to correspondingly suffer as well.

For more information, see the paper: Neuheimer, AB, RE Thresher, JM Lyle and JM Semmens (2011) Tolerance limit for fish growth exceeded by warming waters. Nature Climate Change doi:10.1038/nclimate1084