Despite looking like harmless translucent blobs, jellyfish have an appetite that researchers worry could wreak havoc in the Salish Sea.
In 2019 and 2020, University of Washington student Haila Schultz spent some time observing moon jellies as they feasted on zooplankton. Her findings, which were published in April, could help researchers understand how the jellies’ eating habits could affect fisheries and the survival of endangered Pacific Northwest icons like Chinook salmon and Southern resident orcas.
Whereas human-made conditions such as nutrient pollution, oxygen depletion and warming temperatures are not ideal for the survival of many marine species, they create the ideal concoction for some jellyfish, moon jellies included, to thrive.
Research shows a surge that has been taking place over the course of the past few decades, particularly in inland seas. The marine heatwave that hit Puget Sound in 2015 and 2016 was especially impactful, leading to “larger, more abundant and more persistent” aggregations of moon jellyfish compared to previous years, as researchers concluded by analyzing aerial images.
This prompted experts like Correigh Greene, who at the time worked as a research biologist at the National Oceanic and Atmospheric Administration, to wonder how this population growth would affect the availability of zooplankton — a staple in the diets of jellyfish as well as juvenile and forage fish — and, consequently, the surrounding ecosystem.
According to NOAA, zooplankton are a group of microscopic animals that includes krill, sea snails and young fish, as well as “weak swimmers” like jellyfish.
Greene, who retired this year but still serves as a volunteer, described moon jellies as “extremely good predators” of zooplankton, adding that NOAA has been concerned about their potential impacts on fisheries nationwide.
Greene worked on the study’s original design and helped get the project started in 2019. Schultz, who is pursuing a PhD at UW’s School of Oceanography, conducted the work with financial support from UW, NOAA, Western Washington University and Highline College.
At the Highline College MaST Center Aquarium, located in Des Moines, Schultz set up some regular-sized fish tanks that contained moon jellies borrowed from the aquarium, then poured in a batch of zooplankton that was scooped up from the sea. After two hours, she would measure how much plankton was left.
Schultz also visited the jellyfish smacks — or aggregations — in their natural environment, surveying Budd Inlet, Eld Inlet, Quartermaster Harbor and Sinclair Inlet in Central and South Puget Sound. First, she deployed a drone to locate them as they can be difficult to find by just sailing around.
“I would be flying the drone, looking down at the jellyfish, and it just kept on going, and I couldn’t find the edge of the aggregation,” she said, estimating the smacks extended for hundreds of feet. “Which was really cool.”
She would then reach the thousands of jellies by boat, collecting data on zooplankton densities and measuring water quality parameters both inside and outside of the aggregations to compare results.
The chain transferring energy from the bottom
to the top of the food chain is disrupted, causing
predators to compete over less food.
By the end of the two-hour lab experiment, the jellies had swallowed about 75% of the copepods. Similarly, the field experiment showed that the copepod population was 73% lower inside the wild jelly aggregation compared to the outside sampling site, Schultz said, estimating that it could only take two days for the smacks to clear copepod populations if the jellies stayed in the same place.
It was already known that jellyfish were voracious, she said, but there wasn’t much data on local jellyfish.
“I was surprised that they were able to eat so many zooplankton so quickly,” she said.
This is a win for phytoplanktons, the zooplankton’s smaller plant relatives that photosynthesize and are at the bottom of the food web.
As the zooplankton populations declined within the smacks, Schultz observed, populations of phytoplankton populations bloomed, suggesting that a trophic cascade — which happens when predators affect the density or behavior of their prey’s prey — was taking place.
An example of this phenomenon can be found at Yellowstone National Park, where the extirpation of wolves by 1930 led to the elk population growing and over-eating the young aspen trees, which in turn affected species like beavers and cavity-nesting birds, according to a study conducted by researchers at Oregon State University. Thirty years after the reintroduction of the wolves to the park in 1995, the trees have finally been showing signs of recovery.
While phytoplankton might be thriving as a result of jellyfish preying on their predators, their proliferation could also be fueled by the assimilation of nitrogenous waste that is produced by jellyfish, according to Schultz.
When dinoflagellates and diatoms — two main classes of phytoplankton — have too many nutrients available, they can grow out of control and result in harmful algal blooms, which release toxins that can harm wildlife and people.
The depletion of zooplankton means that the chain transferring energy from the bottom to the top of the food chain is disrupted, causing predators to compete over less food.
Juvenile salmon, for example, can’t eat phytoplankton due to their microscopic size, Schultz said. In the Salish Sea, nonprofit Long Live The Kings reports that the marine survival of Chinook, coho salmon and steelhead salmon populations has already declined by up to 90% due to a multitude of factors, threatening the fishing industry, Indigenous traditions and the endangered Southern Resident orcas.
To further complicate the matter, jellyfish may also eat small fish that salmon and other fish eat, Greene said, or even unintentionally swallow tiny salmon when they’re preying on zooplankton, as said by Hem Nalini Morzaria-Luna, a marine ecologist at Long Live The Kings.
The jellyfish problem seems more prevalent in central and southern Puget Sound, where the water is more protected from the open ocean’s influence, thus allowing jelly aggregations to build up. The northern portion that is closer to the Pacific Ocean experiences more “flushing,” Greene said, though he recalled the San Juan Islands have experienced lots of smacks.
Another contributor could be the presence of hard artificial surfaces underwater, Greene said, like structures made of metal, concrete or plastic, where jellyfish spores attach and transform into a polyp until finally developing into a chain of jellyfish clones that will repeat the cycle.
While researchers continue to investigate what other artificial and natural factors are contributing to the jellyfish population booms, Long Live The Kings is using Schultz’s data to update the Puget Sound Integrated Modeling Network, an environmental model that solves equations to three-dimensionally simulate processes and their effects on the entire watershed.
This tool can be helpful in assessing future threats, identifying the best solutions and shaping policy, according to Morzaria-Luna, who is collaborating with other researchers in developing the ecosystem model.
While the Puget Sound region alone can’t stop climate change and warming water temperatures, as Morzaria-Luna pointed out, a solution worth looking into is commercially harvesting and exporting jellyfish, she, Greene and Schultz said.
South Carolina, Georgia and Florida, for example, have already been harvesting their own jellyfish pests, known as cannonball or jellyballs, which were notorious for getting tangled in shrimping nets. Because there isn’t much demand in the U.S., the jellyballs are exported to countries like Japan and China where they are eaten or used to treat arthritis pain, bronchitis and blood pressure.
— By Luisa Loi
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