Predation on jellyfish and their associates by seabirds

We investigated the foraging distribution of 2 species of murres (Uria lomvia and U. aalge) in relation to Acoustically Determined Biomass (ADB) and hydrographic structure surrounding the Pribilof Islands, Bering Sea, Alaska, USA. The distribution and abundance of both murre species were similar; therefore, we combined data for thick-billed and common murres in the analysis. We found that murre densities were higher in frontal regions than in non-frontal regions. Maximum ADB was also found to be elevated in the frontal zone. Within frontal regions, high densities of rnurres were associated with regions containing relatively high levels of ADB. In contrast, we did not find significant correlations between murre density and ADB in the study area as a whole. We postulate that rnurres restrict their foraging to the frontal region because prey can be located more predictably within this zone than in other areas in the vicinity of the Pribilof Islands.

Section: Discussionmentioning

confidence: 99%

Foraging by murres (Uria spp.) at tidal fronts surrounding the Pribilof Islands, Alaska, USA

Decker¹,

Hunt²

1996

“…Jellyfishes can reproduce both sexually and asexually, which can further lead to a rapid increase in population size. Other than seabirds (Harrison 1984) and other jellyfishes (Hamner 1983, and present study) consuming smaller individuals, we know of no other abundant predators on these medusae. Some fish predators, including adult pollock, are known to prey on jellyfishes in both the Bering Sea (Geoff Lang, Alaska Fisheries Science Center, pers.…”

Section: Potential Causes Of Jellyfish Increasesmentioning

confidence: 99%

Increases in jellyfish biomass in the Bering Sea: implications for the ecosystem

Brodeur¹,

Sugisaki²,

Hunt³

2002

306

195

There has been a dramatic increase in jellyfish biomass over the eastern Bering Sea shelf since the early 1990s, which was previously hypothesized to have been triggered by changing climate and ocean conditions. We examine the hypothesis that the presence of these large carnivores has affected fisheries resources, either through direct predation on larval stages, or through competition for zooplankton prey. In this paper, we explore the impact of this jellyfish increase on zooplankton and fish communities based on field data on the composition of the jellyfish community, and the abundance, size, stable isotopic signatures, and feeding habits of the principal scyphomedusae in the region. These data, together with those on zooplankton biomass, are used to estimate the ecosystem impacts of this increase. The center of jellyfish biomass has shifted from the SE Middle Shelf Domain in the early 1980s to the NW in the late 1990s. In recent years, the species composition of large medusae caught in trawls was dominated (> 80% by number and > 95% by weight) by the scyphozoan Chrysaora melanaster. Dense aggregations of this species occupied the water column in daytime between 10 and 40 m. Their food habits consisted mainly of pelagic crustaceans (euphausiids, copepods, amphipods), although other jellyfish and juvenile pollock were also consumed. Based on stable isotope ratios, the trophic level of this scyphozoan is equivalent to, or higher than, that of Age 0 pollock. Preliminary estimates showed that medusae have a moderate grazing impact on zooplankton in the area around the Pribilof Islands; C. melanaster was estimated on average to consume seasonally about one-third of the standing stock and 4.7% of the annual production of zooplankton in this region. Daily consumption of Age 0 pollock was estimated to be 2.8% of the standing stock around the Pribilof Islands during 1999. A hypothesis for the increase in jellyfishes observed in the eastern Bering Sea, based on release from competition from planktivorous forage fishes, is proposed.

“…In general, least auklets specialize on copepods, particularly Neocalanus plumchrus, when available, and crested auklets specialize on euphausiids, particularly Thysanoessa raschii. In contrast, parakeet auklets are more generalized in their diet and take a variety of larval fishes and zooplankton, including gelatinous zooplankton and their commensals (Harrison 1984(Harrison , 1987 …”

Section: Study Area and Focal Speciesmentioning

confidence: 99%

Comparative foraging ecology of planktivorous auklets in relation to ocean physics and prey availability

Hunt¹,

Russell²,

Coyle³

et al. 1998

141

ABSTRACT. We tested the hypothesis that the spatial distributions of foraging least, crested and parakeet auklets (Aethia pusilla, A. cristatella and A. psittacula, respectively) in the shallow passes of the Aleutian Islands would be determined by physical mechanisms that control near-surface prey concentrations. We recorded currents using an Acoustic Doppler Current Profiler, volume scattering using 200 and 420 kHz scientific echosounders, and the numbers of foraging birds. Zooplankton were sampled using a multiple opening/closing net and environmental sampling system (MOCNESS). Prey choice of birds was ascertained by collecting foraging birds and examining their stomach contents. Most sampling occurred between 8 July and 6 August 1993, when we conducted 50 passages along a transect that crossed a sill between Unalga and Kavalga Islands, western Aleutian Islands, thereby samplinq the loraglng activity ot auklets at a variety of times of day and tidal phases. We found that the abundance of foraging individuals of each of the 3 auklet species was a function of tidal speed. Auklet species were selective about the species of prey taken. Regardless of tidal direct~on, crested auklets foraged on euphausiids upwclled on the upstream side of the pass, whereas least auklets consumed copepods concentrated in near-surface convergences on the downstream side. Parakeet auklets foraged over the top of the pass and took fish and invertebrates. Tidal speed and direction influenced the distance between the peak numbers of some, but not all, species of auklets. Auklet prey preferences dictated where they foraged in the pass and the physical mechanisms exploited for successful foraging. Thus, in this instance, resource partitioning by these closely related planktivores was enhanced by a spatial segregation forced by the physical processes that enhanced the availability of prey. Our findings emphasize the important role of physical processes in the structuring of marine communities.