Bottlenose dolphin numbers declined by 12% following marine heatwave

An unprecedented spell of marine heat in Shark Bay, Australia, caused bottlenose dolphin numbers to decline for the following six years, a study finds.

Dolphin population numbers fell by up to 12% following the 2011 marine heatwave, says the research. The event also led to a decline in the number of dolphin calves being born.

Previous research has shown that marine heatwaves could become up to 41 times more likely by 2100 under a very high global-warming scenario.

The findings “suggest that the ecological consequences of extreme weather events may be too sudden or disruptive for even highly adaptable animals to respond,” the authors say.

Scorching seas

As with a heatwave on land, a marine heatwave is an extended period of unusually high temperatures.

Research suggests marine heatwaves have become 34% more likely over the past century, with rising global temperatures likely playing a key role in the increase.

In 2011, Shark Bay – a world heritage area in Western Australia famous for its seagrass meadows and unique wildlife – faced an unprecedented marine heatwave. For more than two months, coastal water temperatures soared to 2-4C above average, damaging around 36% of the region’s seagrass meadows. In some regions, almost 90% of seagrasses died.

The extreme heat also killed vast numbers of fish, including commercial populations of scallops and crabs.

The new research, published in Current Biology, explores how this ecological turmoil could have affected a species near the top of the food chain: the bottlenose dolphin.

From 2007-17, researchers monitored groups of dolphins living across a 1,500km area in the bay. The scientists took photographs of each dolphin, allowing them to be identified by their markings and the shape of their fins.

These continuous observations allowed the researchers to keep track of population numbers and the number of new calves being born – both before and after the 2011 event.

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