Jellyfish bloom peak shifts 43 days earlier
Phytoplankton growth delayed, out of sync with anchovy spawning
South Sea anchovy catches down about 25% over a decade
The ecological calendar of the South Sea is falling out of sync. Jellyfish are now appearing in large numbers roughly one and a half months earlier than before, while phytoplankton — the base of the anchovy food chain — are blooming later. The result is that young anchovies encounter jellyfish, their competitors and predators, far earlier in the season, just when they need to feed. Over the same period, anchovy catches in the South Sea have dropped by about 25 percent.
The Korea Institute of Ocean Science and Technology (KIOST) announced Thursday that a research team led by Kang Jeong-hun, director of the Southern Sea Research Center, had confirmed these changes after analyzing data on water temperatures, phytoplankton, jellyfish appearances and anchovy catches along the South Sea coast from 2010 to 2020.
The research team divided the study period into three intervals — 2010–2013, 2014–2016 and 2017–2020 — based on shifts in average water temperature. A multivariate statistical analysis found that rising water temperatures, the earlier appearance of Nomura's jellyfish and declining phytoplankton levels were all closely associated with fluctuations in anchovy catches.
The most striking change was in the timing of jellyfish blooms. The peak of Nomura's jellyfish mass occurrence shifted from late September to mid-August — about 43 days earlier. What had been the 268th day of the year moved to the 225th.
The research team noted a rise in Tsushima Current inflow during spring and summer, from April through July. Because jellyfish travel with ocean currents, the period when the Tsushima Current strengthened aligned with the period when jellyfish began arriving earlier along the South Sea coast.
The problem is that the earlier arrival of jellyfish has also shifted when they encounter anchovies. Since 2014, the overlap between peak jellyfish presence and the anchovy spawning season has grown longer.
Young anchovies and jellyfish both feed on zooplankton, so when they appear at the same time they compete for a limited food supply. Jellyfish can swallow prey smaller than 3 millimeters, making them capable of consuming anchovy eggs and newly hatched larvae.
The feeding environment has also turned against anchovies. Phytoplankton serve as food for zooplankton and form the foundation of the anchovy food chain. They typically proliferate in spring and summer, but their summer growth slowed sharply between 2017 and 2020.
The period of peak phytoplankton growth also shifted toward late summer, falling out of step with the June–July peak of anchovy spawning. Young anchovies are now exposed to competition and predation from jellyfish before sufficient food has had a chance to accumulate.
Anchovy catches in the South Sea have declined accordingly. The annual average output for the July–December period fell from 113,700 tons in 2010–2013 to 88,450 tons in 2014–2016, and further to 85,010 tons in 2017–2020 — a drop of about 25 percent between the first and last intervals.
The research team stopped short of concluding that the earlier jellyfish blooms and phytoplankton changes directly caused the decline in anchovy catches, noting that catch levels are influenced by many factors, including fishing activity. The study's finding is one of association — a link between seasonal shifts in marine organisms and anchovy catch trends.
The research is also notable for examining jellyfish damage not simply in terms of bloom size but through the lens of timing. According to existing research, annual fishery losses from jellyfish blooms in South Korea are estimated at up to $200 million.
Anchovies account for about a quarter of South Korea's total annual marine catch and serve as prey for other species such as mackerel, meaning changes in anchovy stocks can ripple through the broader marine ecosystem.
"This study shows that not only the scale of jellyfish blooms but also their timing may be linked to anchovy catches," Kang said. "When managing fishery resources, we need to consider together the seasonal changes in jellyfish appearance, phytoplankton growth and anchovy spawning."
Kang added that data on jellyfish and phytoplankton would become an important foundation for anchovy stock management going forward.
The findings were published in the international journal Marine Environmental Research.
adastra@heraldcorp.com
