Cyclic variability of eastern Bering Sea jellyfish relates to regional physical conditions

Abstract

Globally, there have been few long-term (>30 years), shelf-wide surveys of jellyfish that have allowed for the examination of how populations might respond to changing climatic and oceanographic conditions. One region where investigation of jellyfish responses to climate variability is possible is the eastern Bering Sea, where jellyfish biomass, primarily that of Chrysaora melanaster, has fluctuated dramatically since 1982, when systematic collections of these medusae began. Our previous investigations of a 27-year time series indicated that the timing of the jellyfish biomass increases and declines coincided with transitions between climatic regimes. In the current study, we used updated jellyfish catch and environmental data from 1982 to 2017 and reran our generalized additive model (GAM) analyses to determine if models using solely physical variables and lag of jellyfish biomass could describe accurately the increases and subsequent decreases observed in this jellyfish biomass index. GAMs hindcasting jellyfish biomass for the period 1982–2017 explained a large fraction of the variance, 92.3 % and 86.4 %, for the southeast (SE) and northwest (NW) portions of the survey area, respectively, using jellyfish biomass in the preceding year and physical variables (SE: ice retreat, sea-surface temperature, wind mixing, wind stress and current displacement; NW: sea-surface temperature, ice cover, wind stress and current displacement). We developed more parsimonious models by calculating the variance inflation factor for each term and dropping highly correlated terms from the models. The resulting GAMs continued to explain a significant portion of the variance in jellyfish biomass, i.e., 78.2 % and 73.5 %, in the southeast and northwest survey areas, respectively. Jellyfish biomass in the SE region was correlated with the jellyfish biomass in the preceding year and with wind mixing, wind stress and current displacement. In the NW region, jellyfish biomass was correlated with biomass from the preceding year, and with summer sea-surface temperature and current displacement. Jellyfish biomass in the eastern Bering Sea did not increase during warm periods, as has been speculated to occur elsewhere. Jellyfish, which are both predators and competitors of fish, appear to be responding to changes in physical conditions and are important indicators of ecosystem change in the eastern Bering Sea. The development of models that use physical parameters, as opposed to biological variables that are often not readily available, is key to predicting jellyfish abundance and their impacts on commercially important species.