Jellyfish numbers growing in many coastal ecosystems

Study finds increased abundance in areas affected by human development

Map of population trends of native and invasive species of jellyfish by Large Marine Ecosystems. Red: increase (high certainty); orange: increase (low certainty); green stable/variable; blue decrease, grey: no data. Circles represent jellyfish populations with relative sizes reflecting confidence in the data. (Brotz et al, Hydrobiologia).
Giant jellyfish (Nemopilema nomurai) interfere with fishing in Japan. PHOTO COURTESY SHIN-ICHI UYE.

By Bob Berwyn

SUMMIT COUNTY — Jellyfish populations are growing  in  many of the world’s coastal ecosystems, especially in areas affected by pollution, overfishing, and warming waters, according to a new study done by researchers with the University of British Columbia.

“We found numerous types and species of jellyfish that appear to be increasing, so the reported increases are certainly not due to one type jellyfish in particular,” said Lucas Brotz, a PhD student with the Sea Around Us Project at UBC and lead author of the study.

“That being said, there are several species of jellyfish that appear to be highly invasive are invading new regions around the globe all the time (probably due to transport from cargo ships),” Brotz said via email. ” The most notorious of these is a comb jellyfish called Mnemiopsis leidyi which is showing up in new places every year and often rapidly increases in abundance, potentially altering the ecosystems it invades,” he said.

Diving with jellyfish. PHOTO COURTESY NOAA.

The study, published in this month’s edition of the journal Hydrobiologia, examined data for numerous species of jellyfish for 45 of the world’s 66 Large Marine Ecosystems. They found increasing jellyfish populations in 62 per cent of the regions analyzed, including East Asia, the Black Sea, the Mediterranean, the Northeast U.S. Shelf, Hawaii, and Antarctica.

“There has been anecdotal evidence that jellyfish were on the rise in recent decades, but there hasn’t been a global study that gathered together all the existing data until now,”  Brotz said. “Our study confirms these observations scientifically after analysis of available information from 1950 to the present for more than 138 different jellyfish populations around the world.”

A bloom of moon jellyfish (Aurelia sp.) near Denmark. PHOTO COURTESY CASPER TYBJERG. (

Jellyfish directly interfere with many human activities – by stinging swimmers, clogging intakes of power plants, and interfering with fishing. Some species of jellyfish are now a food source in some parts of the world.

Other research has suggested that increases are due to nutrient loading or other human factors, but Brotz and other researchers are still studying that part of the equation.

“This study was designed to identify whether jellyfish populations are changing or not. Our next step is to look at “why,” Brotz said. “We are investigating that now, but we still need to do some additional analyses before we have any results. However … many areas with increasing jellyfish populations appear to regions impacted by human development. There are suggestions that the increases may be due to overfishing, pollution, coastal development, aquaculture, and global warming.”

“By combining published scientific data with other unpublished data and observations, we could make this study truly global – and offer the best available scientific estimate of a phenomenon that has been widely discussed,” said Daniel Pauly, principal investigator of the Sea Around Us Project and co-author of the study. “We can also see that the places where we see rising numbers of jellyfish are often areas heavily impacted by humans, through pollution, overfishing, and warming waters.”

Pauly  the study provides a concrete baseline for future studies.

The study also notes decreases in jellyfish abundance in seven per cent of coastal regions, while the remainder of the marine ecosystems showed no obvious trend.

“We only found a few places where the overall biomass of jellyfish appears to be declining. However, there are some species of jellyfish that appear to be quite sensitive to changes.  While these species may decline in areas highly impacted by human development, other species may thrive, and therefore the overall biomass of jellyfish in the region will still increase,” Brotz said.

“Our study focused on coastal areas, which is where we have at least minimal information.  We know virtually nothing about changes in abundance for jellies in the open ocean and the deep sea, so we couldn’t include those areas in our study,” he added.

The research made allowances for possible geographic shifts in populations wrought by changing coastal currents, Brotz said, acknowledging that jellyfish abundance can be locally affected by such changes.

“However, these interactions are difficult to identify, and many jellyfish populations in coastal areas are also contained within seas and embayments. If we found any evidence that a change in a particular jellyfish population was likely due to a spatial relocation, we did not classify it as an increase,” he explained.