Abundance, composition and distribution of carnivorous gelatinous zooplankton in the Northern Gulf of Alaska

Abundance, biomass, size and distribution of macro-jellyfish were measured in the Northern Gulf of Alaska (NGA). Nearly 1000 kg dispersed among similar to 13 800 jellyfish were collected using a 5-m(2) Methot net. We present length-weight regressions for seven most-common taxa. Catches were dominated by the hydrozoan Aequorea victoria and the scyphozoan Chrysaora melanaster. During 2018, epipelagic macro-jellyfish 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 seasons within a year, July abundances were fivefold greater than abundances in September, with July catches dominated by smaller-sized jellyfish over the inner shelf, while during September larger jellyfish were more prominent and most predominant at offshore stations. Comparison to 20 years of data from standard towed nets allowed determination of the relative magnitude of the dominant carnivorous zooplankton components: scyphozoans, hydrozoans and chaetognaths in the NGA. The biomass of these smaller epipelagic predators (5.4 mg WW m(-3) for hydrozoans and 10.5 mg WW m(-3) for chaetognaths) is a low percentage of the macro-jellyfish, despite their much higher numerical abundance.

Automated summary

The study measured the abundance, biomass, size, and distribution of macro-jellyfish in the Northern Gulf of Alaska. A total of approximately 13,800 jellyfish weighing about 1000 kg were collected using a 5-m(2) Methot net. The most common taxa were Aequorea victoria and Chrysaora melanaster. The biomass of macro-jellyfish in the epipelagic zone showed seasonal variations, with July abundances five times greater than September abundances. Despite higher numerical abundance, the biomass of smaller epipelagic predators, such as hydrozoans and chaetognaths, was relatively low compared to the macro-jellyfish.