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Climate Change globalization

A Dead Sea: How Climate Change is Forcing Unnatural Migration of Marine Life

Increased oxygen intake of warmer-watered fish, coupled with the decreased oxygen levels in warming water, create the perfect recipe for mass marine dead zones.

When extreme numbers of loggerhead turtles began to show up in Southern California fishery bycatch, scientists and policymakers alike were shocked. 

These turtles are rarely seen in Southern California, and virtually never in such large quantities. In the United States, loggerhead turtles are traditionally found on the southern Atlantic coast and  the Gulf of Mexico. Pacific loggerheads only nest in Japan and Oceania, not California. Further, Pacific loggerheads are endangered according to the U.S. Endangered Species Act, making their appearance in bycatch all the more troubling.

What caused the sudden migration of loggerheads to Southern California waters? The answer lies in our changing climate.

El Niño, the recurring climatic event that carries warm ocean water to the east Pacific coast about every four years, has been exacerbated by climate change. A study conducted through the University of Hawaii’s International Pacific Research Center expects anthropogenic warming (also known as human-induced climate change) to increase the intensity of  El Niño in the future. Even the current oceanic impacts of El Niño present a dangerous situation for marine animals, like the loggerhead turtles.

This marine heatwave impacted the distribution of turtles traditionally located in the West Pacific, attracting them to unprecedented (and therefore non-protected) fishing areas. Heather Welch of UCSC’s Institute of Marine Sciences and her colleagues suggest that climatic events like El Niño are early indicators of species distribution changes, and fishery management should take steps to avoid and avert incoming bycatch surges.

While the formation of The Loggerhead Conservation Area in California–an offshore no-catch area enacted during El Niño years–has helped reduce the significant bycatch of loggerheads, changing marine temperatures present broad-scale challenges to marine conservation and protection efforts.

Many of these challenges stem from the difficulties of protecting species in new regions. When animals with specific conservation needs enter unexpected marine areas, it is difficult for conservationists to plan for and enact policy in time to prevent disastrous human interactions. However, with rising temperatures, displaced sea animals are becoming all the more common, and vulnerable.

The term thermotolerance is defined as the habitable temperature range of an organism. With ocean temperatures rising due to anthropogenic warming, species are forced to migrate away from the equator to remain within their thermotolerance range. Further, short-term spikes in local aquatic temperatures, defined as marine heatwaves by Welch and colleagues, are increasingly common alongside changing ocean current patterns. As the ecosystem makeup changes in a given marine area, fisheries both lose economically vital populations to migration and gain increasing amounts of unwanted bycatch, just like the loggerhead turtles. 

And loggerheads are just one of the many species impacted by the changes in pelagic temperatures.

Changing Species Distribution

While the rising extremity of El Niño attracted loggerheads to the Southern California coast, changing ocean temperatures consistently impact and destroy habitats across the planet. Chaya Chaudhary of the University of Auckland identifies anthropogenic warming as the cause of lost biodiversity and declines in the number of different species in a given ecosystem at the equator. Michael Burrows from the Scottish Marine Institute and his colleagues identify phenological changes and shifting biological ranges as the key ways animals can cope with the changing water temperatures. However, at the equator, the temperature increases are sufficient to surpass adaptations – completely extraditing some previously endemic species.

Even across latitudes, water temperature heavily affects species richness. The global impacts of climate change means even species outside the intense heating of the equator are forced to migrate to stay within their thermotolerance range. 

This is not the first time warming waters have led to wide-scale biodiversity changes. The mass extinction marking the end of Earth’s Permian period (about 252 million years ago) followed skyrocketing global temperatures. The mass marine extinction was directly linked to the warming ocean waters, which subsequently saw a decrease in dissolved oxygen concentrations. This extinction event, named the “Great Dying,” saw the extinction of as much as 96% of marine species, coupled with 70% of terrestrial species. 

The Great Dying, like modern warming events, was caused by rapid greenhouse gas emissions. While the end-Permian period was marked by emissions from mass volcanic activity across the planet, anthropogenic carbon emissions are the new sponsor of extinction. Interestingly, Permian fossil records show mass die-outs of species furthest from the equator; these organisms tend to have inherently higher oxygen intakes and could not survive the 80% decrease in oxygen levels caused by the warming. But as waters warm, Curtis Deutsch – a researcher at the University of Washington – and his colleagues confirm, marine animals at all latitudes require more oxygen intake.

This poses a problem.

The Fight for Oxygen

With marine animals already forced to “choose” between migration and adaptation to warming waters, it becomes increasingly clear that adaptation is not an option. Increased oxygen intake of warmer-watered fish, coupled with the decreased oxygen levels in warming water, create the perfect recipe for mass marine dead zones. Further, phytoplankton–the aquatic autotroph that forms the base of nearly all marine food webs–is not immune to the dangers of warming waters. These organisms must also remain within their thermotolerance to survive. 

However, phytoplankton and other aquatic producers do not have the luxury of migration. By definition, plankton are floating organisms that do not have enough autonomy of motion to overpower ocean water movements. Other aquatic plants, which may float in the pelagic layer or be rooted to the ocean floor, are also incapable of moving beyond the fluctuations of ocean currents. Therefore, unable to escape, marine plants will die out as waters warm past their thermotolerance.

The damages of local producer extinction are manifold. Without converting carbon dioxide into dissolved oxygen, marine animals will not be able to breathe, and are forced to leave or die. This reduction in photosynthesis also perpetuates warming, as marine photosynthesis makes up 50-60% of oxygen in the biosphere. Without this drawdown of carbon dioxide, the greenhouse effect will be exacerbated, forming a positive feedback loop that continues the damaging effects of climate change.

The loss of a producer species also creates a form of habitat destruction for food webs dependent on that energy source. When a given primary producer is wiped out, species dependent on that food source are threatened with extinction as well, and competition over other food sources increases. Increased competition leads to a decline in other producer populations, perpetuating the cycle of endangerment and extinction. The lack of food resources forces many marine animals to leave their homes. 

While many migrate, some endemic marine animals cannot replace their original habitats and niches, and go extinct as a result.

The death of marine plants also leads to a spike in dissolved oxygen consumed by decomposers and detritivores. These organisms not only decompose the remaining organic matter, but also deplete oxygen levels in the surrounding water. When coupled with the loss of photosynthetic activity, marine dead zones form. Here, hypoxic (oxygen-deficient) conditions cannot support any form of marine life. To prevent these dead zones and extinctions from expanding into another “Great Dying,” human intervention must occur.

Preventing Extinction

First, it is important to recognize how the changing distribution of marine animals is already impacting these populations. As noted with the loggerhead turtles, threatened and endangered species are now found in new regions. This gives rise to difficulties within conservation efforts as fishing regions are unprepared for new populations and potential bycatch. For instance, tuna fisheries in the Indian Ocean must work to reduce dolphin bycatch to be within international guidelines. However, they are unprepared to handle other threatened species, such as whales potentially entering new territories due to warming-forced migration. Welch identifies the lack of environmental precautions in migration anomalies as a major challenge of marine conservation going forward. Implementing more marine protected areas and stricter fishing regulations can lessen unintended human interactions.

Marine animals are still vulnerable to extinction even without the threat of overfishing. On top of marine temperature increases, human-induced climate change has produced changes to sea level, ocean acidity, sea ice cover, and salinity. The only way to truly avert the severe impacts that climate change will have on marine life is to significantly reduce anthropogenic greenhouse gas emissions and work to sequester the carbon dioxide already in the atmosphere.

For this to be possible, extreme investments into clean energy and technology will need to happen, and soon. Current mitigation efforts are a start, but they do not do enough to prevent mass global species extermination, starting with the death of marine life. The international community must invest in non-carbon energy sources and carbon sequestration to reduce the spiraling greenhouse effect. Without strict emission restrictions across the globe, the impacts of climate change will be devastating. Humans must take immediate and strict action to prevent the Anthropocene from ending with unprecedented extinction.





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By Natalie Brown

Natalie is a second year undergraduate student at the University of California, Los Angeles. She is studying History with a minor in Environmental Systems and Societies and plans to go into the field of law.

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