Marine heatwaves cause species to move 'thousands of miles'

Marine heatwaves are forcing sea turtles, whales and other wildlife to relocate thousands of miles away from their natural habitats, study shows

  • ‘Thermal displacement’  shifts ocean temperatures causing marine life to move 
  • Marine heatwaves such as ‘The Blob’ five years ago shifted sea lion prey north
  • Sea lion pups were stranded in record numbers on Southern California beaches

Marine heatwaves can shift suitable habitat for sea turtles, whales and other wildlife by thousands of miles in the world’s oceans, research suggests.

A US study found abnormally high temperatures in ocean waters cause many species to pack up and move – an effect known as thermal displacement. 

If a heatwave warms an area of sea, species may have to move great distances to find a more suitable habitat, depending on the rate of the temperature change.  

The changes may have implications if commercial fish species shift, as fishermen would have to travel hundreds of miles further to reach them. 

High-profile impacts of marine heatwaves include the closure of fisheries, large-scale population declines and unusual sightings of species thousands of miles from their natural range. 

Five years ago, for example, a major marine heatwave known as The Blob affected the northeast Pacific Ocean and shifted sharks north as much 1,700 miles. 

The range of smooth hammerhead sharks shifted north as much 2,800 kilometers, more than 1,700 miles, during a major marine heatwave that affected the northeast Pacific Ocean from 2013 into 2015. The heatwave was known as ‘The Blob’

‘When the environment changes, many species move,’ said research scientist Michael Jacox at NOAA Southwest Fisheries Science Center in the US.

‘This research helps us understand and measure the degree of change they may be responding to.’

Scientists have typically defined marine heatwaves based on how much they increase sea surface temperatures, and for how long. 

Such local warming particularly affects stationary organisms such as corals, which are becoming increasingly stressed due to rising water temperatures. 

In contrast, the new metric, thermal displacement, measures how far mobile species must move to find their preferred conditions. 

Researchers define thermal displacement during marine heatwaves as the minimum distance from a given position that is the same temperature as or cooler than the ‘normal’ historical temperature at that position. In (a), the normal temperature at the black circle in the Gulf of Alaska is 9.0 °C, but the marine heatwave temperature is 10.3 °C. The minimum distance to a region at 9.0 °C or less (darkest blue regions) is 521 kilometres. White regions indicate areas covered by sea ice. (b) shows examples of thermal displacements (red and blue circles) from two starting points (white circles) during different heatwaves

During marine heatwaves, thermal displacement is defined as the minimum distance from a given position that is the same temperature as or cooler than the ‘normal’ historical temperature at that position. 

Thermal displacement poses the same question that bothers an overheating fish, according to an accompanying article, published in Nature by Mark R Payne, a researcher a Denmark’s National Institute of Aquatic Resources.

That question is ‘How far do I have to go to find conditions that are at or cooler than normal temperatures?’ 

‘In other words, this metric quantifies the availability (or lack) of cooler temperatures in the waters surrounding a given point,’ Payne says. 

Some regions can therefore be expected to show reductions in thermal displacement due to marine heatwaves in the future, whereas others will see increases. 

‘It may give us an idea how the ecosystem may change in the future,’ said study co-author Michael Alexander, research meteorologist at NOAA.  

Researchers used a global sea surface temperature dataset to calculate thermal displacements for all marine heatwaves from 1982 to 2019.

Sea lion mothers struggled to feed their pups as a marine heatwave known as ‘The Blob’ shifted their most favoured prey north, far from rookeries in Southern California

They say thermal displacements during marine heatwaves vary from tens to thousands of miles across the world’s oceans and do not correlate spatially with marine heatwave intensity.

That means a higher increase in temperature does not necessarily equate to longer distances travelled.  

Short-term thermal displacements during marine heatwaves are of comparable magnitude to century-scale shifts inferred from global warming trends, they add. 

In other words, marine heatwaves can shift ocean temperatures at similar scales to what is anticipated over the long term for sea warming caused by climate change.    

This could spell major impacts for wildlife and natural systems, as well as for human activities such as fishing.

Across the world’s oceans, the average long-term temperature shift associated with ocean warming has been estimated at about 13 miles per decade. 

By comparison, marine heatwaves have displaced temperatures an average of approximately 120 miles in a matter of months. 

In effect, marine heatwaves are shifting ocean temperatures at similar scales to what is anticipated with climate change but in much shorter time frames. 

‘While these temperature shifts do not solely dictate species distributions, they do convey the scale of potential habitat disruption,’ the study authors say in their research paper, published in Nature.

Skinny California sea lion pups were stranded in record numbers on Southern California beaches 

One high profile impact of marine heatwaves was in 2012 in the north-west Atlantic, when a heatwave pushed commercial seafood species such as squid and flounder hundreds of miles north.   

At the same time it contributed to a lobster boom that led to record landings and a collapse in price. 

‘Given the complex political geography of the United States’ Eastern Seaboard, this event highlighted management questions introduced by marine heatwave-driven shifts across state and national lines,’ the scientists wrote. 

The Pacific marine heatwave known as ‘the Blob’ shifted surface temperatures more than 400 miles (700km) along the west coast of the US between 2014 and 2015.

This pushed the prey of California sea lions further from their breeding rookeries and left hundreds of starving pups to strand on beaches.

WHAT ARE MARINE HEATWAVES AND WHAT DO WE KNOW ABOUT THEM?

On land, heatwaves can be deadly for humans and wildlife and can devastate crops and forests.

Unusually warm periods can also occur in the ocean. These can last for weeks or months, killing off kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries.

Yet until recently, the formation, distribution and frequency of marine heatwaves had received little research attention. 

Long-term change 

Climate change is warming ocean waters and causing shifts in the distribution and abundance of seaweeds, corals, fish and other marine species. For example, tropical fish species are now commonly found in Sydney Harbour.

But these changes in ocean temperatures are not steady or even, and scientists have lacked the tools to define, synthesize and understand the global patterns of marine heatwaves and their biological impacts.

At a meeting in early 2015, we convened a group of scientists with expertise in atmospheric climatology, oceanography and ecology to form a marine heatwaves working group to develop a definition for the phenomenon: A prolonged period of unusually warm water at a particular location for that time of the year. Importantly, marine heatwaves can occur at any time of the year, summer or winter.

Unusually warm periods can last for weeks or months, killing off kelp forests and corals, and producing other significant impacts on marine ecosystems, fishing and aquaculture industries worldwide (pictured)

With the definition in hand, we were finally able to analyse historical data to determine patterns in their occurrence.

Analysis of marine heatwave trends

Over the past century, marine heatwaves have become longer and more frequent around the world. The number of marine heatwave days increased by 54 per cent from 1925 to 2016, with an accelerating trend since 1982.

We collated more than 100 years of sea surface temperature data around the world from ship-based measurements, shore station records and satellite observations, and looked for changes in how often marine heatwaves occurred and how long they lasted.

This graph shows a yearly count of marine heatwave days from 1900 to 2016, as a global average.

We found that from 1925 to 1954 and 1987 to 2016, the frequency of heatwaves increased 34 per cent and their duration grew by 17 per cent.

These long-term trends can be explained by ongoing increases in ocean temperatures. Given the likelihood of continued ocean surface warming throughout the 21st century, we can expect to see more marine heatwaves globally in the future, with implications for marine biodiversity.

 ‘The Blob’ effect

Numbers and statistics are informative, but here’s what that means underwater.

A marine ecosystem that had 30 days of extreme heat in the early 20th century might now experience 45 days of extreme heat. That extra exposure can have detrimental effects on the health of the ecosystem and the economic benefits, such as fisheries and aquaculture, derived from it.

A number of recent marine heatwaves have done just that.

In 2011, a marine heatwave off western Australia killed off a kelp forest and replaced it with turf seaweed. The ecosystem shift remained even after water temperatures returned to normal, signalling a long-lasting or maybe even permanent change.

That same event led to widespread loss of seagrass meadows from the iconic Shark Bay area, with consequences for biodiversity including increased bacterial blooms, declines in blue crabs, scallops and the health of green turtles, and reductions in the long-term carbon storage of these important habitats.

Examples of marine heatwave impacts on ecosystems and species. Coral bleaching and seagrass die-back (top left and right). Mass mortality and changes in patterns of commercially important species s (bottom left and right)

Similarly, a marine heatwave in the Gulf of Maine disrupted the lucrative lobster fishery in 2012. The warm water in late spring allowed lobsters to move inshore earlier in the year than usual, which led to early landings, and an unexpected and significant price drop.

More recently, a persistent area of warm water in the North Pacific, nicknamed ‘The Blob’, stayed put for years (2014-2016), and caused fishery closures, mass strandings of marine mammals and harmful algal bloom outbreaks along the coast. It even changed large-scale weather patterns in the Pacific Northwest.

 As global ocean temperatures continue to rise and marine heatwaves become more widespread, the marine ecosystems many rely upon for food, livelihoods and recreation will become increasingly less stable and predictable.

The climate change link 

Anthropogenic, that is human-caused, climate change is linked to some of these recent marine heatwaves.

For example, human emissions of greenhouse gases made the 2016 marine heatwave in tropical Australia, which led to massive bleaching of the Great Barrier Reef, 53 times more likely to occur.

Even more dramatically, the 2015-16 marine heatwave in the Tasman Sea that persisted for more than eight months and disrupted Tasmanian fisheries and aquaculture industries was over 300 times more likely, thanks to anthropogenic climate change.

For scientists, the next step is to quantify future changes under different warming scenarios. How much more often will they occur? How much warmer will they be? And how much longer will they last?

Ultimately, scientists should develop forecasts for policy makers, managers and industry that could predict the future impacts of marine heatwaves for weeks or months ahead. Having that information would help fishery managers know when to open or close a fishery, aquaculture businesses to plan harvest dates and conservation managers to implement additional monitoring efforts.

Forecasts can help manage the risks, but in the end, we still need urgent action to curb greenhouse gas emissions and limit global warming. If not, marine ecosystems are set for an ever-increasing hammering from extreme ocean heat.

Source: Eric Oliver, Assistant Professor, Dalhousie University; Alistair Hobday, Senior Principal Research Scientist – Oceans and Atmosphere, CSIRO; Dan Smale, Research Fellow in Marine Ecology, Marine Biological Association; Neil Holbrook, Professor, University of Tasmania; Thomas Wernberg, ARC Future Fellow in Marine Ecology, University of Western Australia in a piece for The Conversation.


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