The jellyfish buffet: jellyfish enhance seabird foraging opportunities by concentrating prey

Abstract

High levels of jellyfish biomass have been reported in marine ecosystems around the world, but understanding of their ecological role remains in its infancy. Jellyfish are generally thought to have indirect negative impacts on higher trophic-level predators, through changes in lower trophic pathways. However, high densities of jellyfish in the water column may affect the foraging behaviour of marine predators more directly, and the effects may not always be negative. Here, we present novel observations of a diving seabird, the thick-billed murre, feeding on fish aggregating among the long tentacles of large jellyfish, by using small video loggers attached to the birds. We show that the birds encountered large jellyfish, Chrysaora melanaster, during most of their dives, commonly fed on fish associated with jellyfish, and appeared to specifically target jellyfish with a high number of fish aggregating in their tentacles, suggesting the use of jellyfish may provide significant energetic benefits to foraging murres. We conclude that jellyfish provide feeding opportunities for diving seabirds by concentrating forage fish, and that the impacts of jellyfish on marine ecosystems are more complex than previously anticipated and may be beneficial to seabirds.

Keywords: animal-borne video logger; jellyfish; seabird.

Figure 1.

(a) Vertical changes in the water temperature (i). Vertical distribution of encounter events with solitary juvenile fish (ii), jellyfish without fish (iii) and jellyfish with fish (iv) for 36 dives from two birds where both video and diving behaviour data were collected. The estimated depth of the thermocline was from 9.6 to 24.8 m. (b) Effect of number of fish among tentacles of jellyfish (x-axis) on the probability of attack behaviour of birds towards fish (y-axis). Occurrence/non-occurrence of attack behaviour was determined by the bird's head movements towards prey. The probability of attack increased as the number of fish among the tentacles of a jellyfish increased. Please see table 1 for the statistical results. (Online version in colour.)

Figure 2.

Changes in depth, heaving acceleration, vertical speed and wingbeat frequency during typical dives of thick-billed murres. The bird stroked its wings continuously during the descent and bottom phases (a,b). However, during the ascent phase, birds scarcely stroked and ascended with a constant vertical speed when there were no feeding events (c). In contrast, when birds fed on prey during ascent (d, arrowhead), the number of wing strokes increased up to 2 Hz, and vertical speed decreased instantaneously (d). (Online version in colour.)