Jellyfish are thriving in our warming seas, and their rise is thought to have negative impacts on marine ecosystems. According to new findings published in Biology Letters, at least one species of diving seabird appears to benefit from jellyfish increases. By targeting high-density jellyfish clusters, bird predators get to feast on the little fish aggregating within the tangles of tentacles.  

Until now, observations of interactions between jellyfish and bird predators have been rare. A team led by Nobuhiko Sato from the Sokendai Graduate University for Advanced Studies in Tokyo attached data and video loggers on to eight thick-billed murres (Uria lomvia) on Alaska’s St. George Island in the eastern Bering Sea back in August of 2014. The bird-borne video loggers recorded 97 daytime dives from four of the murres. On average, they dove 78 meters in 150 seconds.

After analyzing footage of 197 feeding events, the team found that murres frequently fed on fish swimming among tentacles of the northern sea nettle (Chrysaora melanaster, pictured to the right). During their ascent in 82 of the dives (that’s 85% of all the dives recorded), the birds encountered the large jellyfish a total of 179 times. Of those murre-jellyfish encounters, 49 of the jellyfish hid juvenile fish among their tentacles. Most of these fish were young walleye pollock that were less than 50 millimeters long, and they might have been feeding on plankton trapped by the jellies' tendrils.

The majority of the time, the birds fed on solitary fish not associated with jellyfish. But in about a fifth of all the fish-feeding events recorded for the four birds, the murres approached jellies and fed on fish swimming around their tentacles. As Pacific Standard points out, this is a relatively high number for a previously unobserved behavior.

Jellyfish enhance feeding opportunities of seabirds by concentrating their prey, the team concludes, and some marine predators may benefit from the rise in jelly numbers – a jellyfish buffet, as they describe. 

Image in text: Miloslav Petrtyl CC BY-NC via BioLib.cz