Warming Oceans are Increasing the Risk of Disease Outbreak Among Marine Life

A warming climate means warmer oceans, which has resulted in increased disease outbreaks among marine life.These outbreaks start at species low on the food chain, such as eelgrass meadows, that provide essential elements like spawning habitat to ocean life.

The Salish Sea stretches from Olympia, Washington all the way into northern British Columbia in Canada. According to The SeaDoc Society, 113 marine species that call the Salish Sea home are threatened, endangered, or at risk of being so. One of the current contributing factors to the decline of marine species is outbreaks of ocean disease.

Eelgrass wasting disease (EWD), scientifically known as labyrinthula zosterae, is a parasitic disease that kills patches of eelgrass blades and spreads through eelgrass meadows.

A team of scientists conducted field studies involving the disease from between 2013 and 2017 in the San Juan Islands off of Washington’s coast. To study this disease, a variety of both healthy and diseased eelgrass blades are observed or taken from beds. They are examined through microscopes to find the distinct black patches, called lesions, that this disease creates. From this point, scientists confirm whether or not it is diseased.

Drew Harvell, a marine ecologist at Cornell University, found from this study that over 75 percent of the locations in the San Juan Islands all had eelgrass wasting disease present. Harvell and her team found not only that the presence and severity of the disease is steadily increasing, but also there was more disease in eelgrass beds that were less dense. In both the field and the lab, Harvell found that infections increased with warming temperatures..

“This is a very important habitat with important ecosystem services,” said Harvell at the 2020 American Association for the Advancement of Science (AAAS) meeting in Seattle, Washington. “We can’t afford to lose it, and it needs better monitoring.”

Salmon use eelgrass beds as spawning and juvenile habitat. Salmon are one of the Pacific Northwest’s iconic and keystone animals with shrinking population numbers. Kristina Miller, a scientist at for Fisheries and Oceans Canada, attributes some recent salmon population declines to diseases caused by increasing ocean temperatures.

Miller and a team of scientists found that changes in temperature and salinity are the most primary stressors to salmon and are most likely to initiate disease. They developed a technology called salmon Fit-Chips, which are plates on which molecular biomarkers are placed that can track 47 pathogens, viruses, bacteria, and parasites at the same time. This technology allows scientists to quickly identify when, where, and how viruses are happening within a salmon’s body.

“If you look at all 47 agents, we found that on average, there was an increase of the probability of infection overall with warmer water,” said Miller. She added that the probability of infection is 25% higher with only a 2.7 degree celsius change in water temperature.

“Some pathogens are transmitted in freshwater, suggesting that fish are already compromised when they’re entering the marine environment,” said Miller. “Maybe [they’re already] at a greater risk of developing disease and mortality.”

To gauge stress, Miller monitored the fishes’ heart, kidney, liver, and brain. Miller and her team studied 12 different viruses across 10,000 fish. These fish included a variety of wild, farmed, and hatchery fish. It was in these contained conditions that Miller and her team were able to identify that viruses can be prevalent in salmon at a very young age.

“The real goal of this is to identify what kinds of stressors and diseases we can actually mitigate,” said Miller.

There are things already known to improve survival rates of salmon, which Miller said include restoring degraded habitats, increasing flow from dams during warm periods, and minimizing contact between farmed and wild fish.

“With overall survival rates plummeting to less than one percent for some stocks in some years, if we could affect even a 2 percent rise in early marine survival, it could make the difference between sustained populations and extinction,” said Miller in a presentation at the 2020 AAAS meeting in Seattle, Washington.