Abundance, composition and distribution of predatory gelatinous zooplankton in the northern Gulf of Alaska

Abstract

Jellyfish are conspicuous yet under-studied components of marine zooplankton communities. Abundance, biomass, size, and distribution of large-jellyfish were measured during July and September of 2018 and 2019 as part of the Northern Gulf of Alaska Long-Term Ecological Research (NGA-LTER) cruises. Nearly 1000 kg dispersed among ~13,800 jellies were collected using a 5 m² Methot net. Catches were dominated by two macro-jellies, the hydrozoan Aequorea sp. and the scyphozoan Chrysaora sp. During 2018, epipelagic macro-jellies biomass averaged 1.46 ± 0.36 g WW m⁻³ for July and 1.14 ± 0.23 g WW m⁻³ for September, while during 2019 they averaged 0.86 ± 0.19 g WW m⁻³ for July and 0.72 ± 0.21 g WW m⁻³ by September. Despite similar biomass among sampling seasons within the same year, July abundances were fivefold greater than abundances in September, with July catches dominated by juvenile jellyfish over the inner shelf, while during September jellyfish adults were more prominent and most predominant at offshore stations. Comparison to over 20 years of data from standard towed nets allowed determination of the relative magnitude of the three dominant predatory zooplankton components: Scyphozoans, Hydrozoans, and Chaetognaths in the NGA. The biomass of these smaller epipelagic predators (10 mg WW m⁻³ for hydrozoans and 8 mg WW m⁻³ for chaetognaths) is a low percentage of the macro-jellies, despite their much higher numerical abundance. Acknowledging that changes in gelatinous biomass could have profound effects on fisheries, we argue that jellyfish should be quantitatively monitored in ecosystems with high fisheries productivity.