Over the past four decades, oceans have already gotten warmer, which has had a significant effect on infectious diseases in marine life, shows new research, published Wednesday in Proceedings of the Royal Society B. In short, the warming oceans seem to have led to an increase in diseases for plants and animals that live in the sea. But some existing protections have proven to help defend against the worst effects of these diseases.
The study looked at the rates of disease in corals, urchins, mammals, decapods (think lobsters), fish, mollusks, elasmobranchs (sharks and rays), turtles, and seagrass. The findings establish a baseline of diseases over the time period, which stretched from 1970 to 2013, and add to the list of effects global warming has on marine life.
Tracking long-term can be challenging in “globally distributed, often inaccessible underwater ecosystems,” the researchers write. “However, understanding these trends is critical for evaluating threats to ocean ecosystems.”
During the 44-year span, diseases in corals and urchins increased significantly.
“The correlation between coral disease reports and temperature anomalies in the Caribbean is evidence of a long-term link between warming and coral disease,” Allison Tracy, Ph.D., a postdoctoral researcher at the Smithsonian Environmental Research Center and the study’s first author, tells Inverse in an email.
“It’s clear from previous studies that warming can drive coral disease over shorter time scales, but this multi-decadal relationship is a new piece of the puzzle,” Tracy writes. “It suggests that warming oceans can drive increases in disease in marine life over long time periods.”
By contrast, sharks, rays, and fish saw a drop in disease reports; but that may not be a good thing, the researchers say. It’s unclear exactly why diseases dropped for those species, but it may have to do with humans fishing too much.
“The fact that this happens in two groups of marine animals that have experienced drastic human-induced population declines is suspicious,” Tracy says.
“Disease outbreaks often rely on big enough groups and animals coming in contact with each other. Overfishing might have depleted fish populations so severely that there are not enough fish to support disease.”
Major shifts, then, due to warming and overfishing, seem to be having a big effect on disease in marine life, the authors say.
“Our findings indicate that rapidly changing oceans have diverse but potent effects on disease,” they write. “Unfortunately, these effects will probably intensify in the coming years.”
The study offers new perspective on how humans are disrupting ocean ecosystems on a global scale.
So, how do we stop us?
Preventing overfishing can help marine ecosystems bounce back, some evidence shows. As Inverse reported last week, new research shows that marine protected areas are thriving compared to unmanaged neighboring places.
Not allowing fishing in those safeguarded areas proved to be key, that study found. With more fish around to eat algae off the reefs, the ecosystems seemed to be healthier, with stronger immune responses to infections; reefs without fishing protections were found bleached by pathogens and covered in algae.
The corals in the marine protected areas were significantly less susceptible to coral bleaching, a death spiral often caused by warming waters, which is now happening at a massive scale.
Reducing overfishing in a way that will have a real impact on global marine life will require global cooperation. The World Trade Organization has spent most of this year trying to make that happen.
This week, the WTO restarted negotiations to create an international agreement to curb overfishing by reducing major government subsidies that encourage it. Those harmful subsidies rose 6 percent this year, now totaling $22 billion.
If they’re successful, those negotiations could pave the way for a huge, universal shift in fishing practices. As the studies on disease and coral bleaching both suggest, that kind of increased management is a crucial step toward promoting healthier marine ecosystems worldwide.
Abstract: Outbreaks of marine infectious diseases have caused widespread mass mortalities, but the lack of baseline data has precluded evaluating whether disease is increasing or decreasing in the ocean. We use an established literature proxy method from Ward and Lafferty (Ward and Lafferty 2004 PLoS Biology2, e120 (doi:10.1371/journal.pbio.0020120)) to analyse a 44-year global record of normalized disease reports from 1970 to 2013. Major marine hosts are combined into nine taxonomic groups, from seagrasses to marine mammals, to assess disease swings, defined as positive or negative multi-decadal shifts in disease reports across related hosts. Normalized disease reports increased significantly between 1970 and 2013 in corals and urchins, indicating positive disease swings in these environmentally sensitive ectotherms. Coral disease reports in the Caribbean correlated with increasing temperature anomalies, supporting the hypothesis that warming oceans drive infectious coral diseases. Meanwhile, disease risk may also decrease in a changing ocean. Disease reports decreased significantly in fishes and elasmobranchs, which have experienced steep human-induced population declines and diminishing population density that, while concerning, may reduce disease. The increases and decreases in disease reports across the 44-year record transcend short-term fluctuations and regional variation. Our results show that long-term changes in disease reports coincide with recent decades of widespread environmental change in the ocean.