Evidence that summer jellyfish blooms impact Pacific Northwest salmon production

Abstract

Interannual variability in salmon (Oncorhynchus spp.) production in the northeast Pacific is understood to be driven by oceanographic variability and bottom‐up processes affecting prey availability to juvenile salmon. Scyphozoan jellyfish have an important role in shaping the pathways of energy flow through pelagic food webs. While jellyfish obtain high production rates and biomasses as major consumers of zooplankton production, they have few predators and may divert plankton production away from higher trophic levels. Although jellyfish are planktivorous and juvenile coho (O. kisutch) and Chinook (O. tshawytscha) salmon are mainly piscivorous, they may be indirect competitors for plankton production. Ecosystem model simulations suggested that among all trophic interactions within the Pacific Northwest coastal food web, juvenile salmon are particularly sensitive to jellyfish blooms, and that salmon production will be suppressed in years of high summer jellyfish biomass. Pelagic surveys off Oregon and Washington (1999–2012) were used to examine the interannual relationship between salmon production and the dominant jellyfish species, the sea nettle Chrysaora fuscescens, off the Pacific Northwest coast. There was a significant, negative correlation between sea nettle biomass and the strength of adult coho and Chinook salmon returns to the Columbia River. Examination of spatial distributions across years showed a positive association between sea nettles and salmon. Within individual years, significant differences between the distribution of sea nettles and yearling coho and Chinook salmon generally occurred during cooler ocean summers, perhaps due to the greater expanse of optimal salmon habitat resulting from more upwelling. Whether the association is behavioral or a product of oceanographic processes, association enhances the opportunity for indirect competition. Examination of feeding incidence in September showed that salmon stomachs were less full at locations with higher sea nettle biomass.

Distinct gelatinous zooplankton communities across a dynamic shelf sea

“…They can exert substantial top‐down pressure on prey species, and on occasion, intense blooms of particular species can cause a trophic cascade, transforming an entire ecosystem into a less desirable state, at least from a human perspective (Greve ; Kideys and Romanova ; Kideys ). Just as important is the role of gelatinous zooplankton as competitors, whereby their rapid growth potentially allows them to outcompete planktivorous fish species (Brodeur et al ; Ruzicka et al ). It is also increasingly apparent that gelatinous zooplankton are consumed by a substantial number of marine predators (Hays et al ), including a large number of important commercial fish species (Arai ; Purcell and Arai ; Lamb et al ).…”

Jellyfish and forage fish spatial overlap on the eastern Bering Sea shelf during periods of high and low jellyfish biomass

“…This may be attributed to predation upon larval herring or to competition with larval herring for zooplankton (Lynam et al 2005). In the NCC, high local abundances of the jellyfish Chrysaora fuscesens are associated with poor feeding conditions for juvenile salmon Oncorhynchus spp., as evidenced by less food in salmon stomachs in the presence of jellyfish blooms (Ruzicka et al 2016). Even smaller juvenile salmon do not themselves prey heavily upon zooplankton within the NCC.…”

Interannual variability in the feeding and condition of subyearling Chinook salmon off Oregon and Washington in relation to fluctuating ocean conditions

“…Accurately forecasting the number of returning adult fish is essential for establishing a sustainable fishery. Adult Chinook salmon return values are from Ruzicka et al (2016), who back-calculated annual returns of adult Chinook salmon to Bonneville Dam (http://www.cbr.washington.edu/dart/) to include both 2-and 3-yr returns. The adult return value for 2011 was 3.4 standard deviations above the long-term mean and was not included in the analysis.…”

“…Four-month (May through August) averages of oceanographic variables and adult returns fromRuzicka et al (2016) used in the regression analyses by year. See Methods for explanation of variables and their sources, see text.…”