Scyphozoan jellyfish are conspicuous components of marine ecosystems, which during a bloom, can impact food web structure and economically important fisheries. Jellyfish biomass in the southeastern Bering Sea (SEBS) is primarily comprised of Chrysaora melanaster and has varied widely over the past four decades, yet the underlying causes of these biomass fluctuations remain unclear. The present study investigated the spatial and seasonal dynamics of C. melanaster along the Alaska Peninsula in the SEBS using an Adaptive Resolution Imaging Sonar (ARIS) system and nets in June-July and September 2018. The abundance of C. melanaster was high in coastal waters near the Alaska Peninsula, with peak densities occurring north of Unimak Island in both sampling periods. The current pattern revealed by an acoustic Doppler current profiler (ADCP) showed that cold deep water from Bering Canyon flowed onto the shelf north of Unimak Island and influenced the distribution of C. melanaster by constraining the mixed coastal water to areas north of Unimak Island, flowing northeast along the Alaska Peninsula. The present study provides a new mechanistic understanding of how large-scale ocean variability in the SEBS affects C. melanaster distribution.

Temperature and salinity: Temperature and salinity profiles were measured at each station with a Seabird SBE911+ (Sea-Bird Electronics Inc., Bellevue, WA, USA). A conductivity-temperature-depth instrument (CTD) was deployed to either a maximum water depth of 500-m or within 5 m of the bottom. Data were recorded during the downward deployment, at a descent rate of 30 m min^–1. In total, 60 hydrographic casts were completed.

Ocean current from Acoustic Doppler Current Profilers (ADCPs): Ocean currents were measured continuously by vessel-mounted ADCPs. During the June-July cruise, R/V Sikuliaq carried 300 kHz Workhorse (WH300) and 150kHz Ocean Surveyor (OS150). In September cruise, R/V Oceanus was equipped with 300 kHz Workhorse (WH300) and 75 kHz Ocean Surveyor (OS75). The data acquisition, calibration and processing closely followed the protocols from the University of Hawaii Data Acquisition System (UHDAS) and Common Ocean Data Access System (CODAS). To examine the consistency of the ADCP data, we statistically compared the results of the WH300, OS150 and OS75. The ADCPs on both cruises displayed remarkable similarities in terms of magnitudes of zonal (east-west direction) and meridional (north-south direction) velocities and their vertical structures. The statistical square root of the differences between two ADCPs range from 1 to 3 cm s^-1, which is reasonable after taking their different vertical bin lengths into account. The 300 kHz ADCP had better vertical resolution (~2 m in bin length) and its total range (~100 m) can cover the whole water column in most of the survey area, therefore, we present data collected by the WH300 from both cruises.

The original ADCP data contains both tidal currents and sub-tidal flow. To determine ocean circulation, the ADCP data were de-tided by removing tidal currents (Data set S02). The tidal parameters were obtained from an ADCP deployed on M2 mooring during April - October, 1998 as part of NOAA’s Ecosystems and Fisheries-Oceanography Coordinated Investigations (Kachel et al. 2010). The M2 mooring is located 70 km from our sampling station 18. We assume that the tidal parameters (i.e., magnitude and phases) have not significantly changed from 1998 to 2018. These tidal parameters were used to estimate the tidal currents at each specific location and time of the ADCP profiles.

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