Abstract
Spatiotemporal use of the SNE WEA by HMS was studied with passive acoustic telemetry via an array of acoustic receivers deployed from January 1, 2022 to December 31, 2022 (n = 75 receivers) and from January 1, 2023 to December 31, 2023 (n = 73 receivers). A total of 259 acoustic transmitters were deployed on 10 species, including blue shark (n = 86), bluefin tuna (n = 70), common thresher shark (n = 3), dusky shark (n = 12), little tunny (n = 1), sandbar shark (n = 3), shortfin mako (n = 50), smooth hammerhead (n = 1), spinner shark (n = 2), and yellowfin tuna (n = 31) in and around the WEA during summer 2020 (n = 29), 2021 (n = 69), 2022 (n = 70), and 2023 (n = 91). During 2022 and 2023, 164 of the 259 total tagged individuals were detected by the receiver array, including 144 of the 199 individuals tagged by developer-funded efforts and 20 of the 60 individuals (4 blue sharks, 10 bluefin tuna, 5 shortfin mako, and 1 smooth hammerhead) tagged in 2020 and 2021 during a pilot study funded by MACEC. A total of 33,098 detections were recorded over all individuals with multiple cases of fish detected in years subsequent to tagging. Tagged HMS inhabited the WEA during the warmer months of the year; blue sharks, bluefin tuna, and shortfin mako were the earliest species to arrive in June of both years, while bluefin tuna were the last species to depart in January 2023 and December 2023. Complete emigration occurred by mid-winter as no detections were logged from January through May 2022 and February through May 2023.
Tagged HMS ranged widely throughout the WEA and were detected by 70 receivers in 2022 and 71 receivers in 2023. A total of 3,077 residence events were documented, including 1,347 in 2022 (88 transmitters detected) and 1,730 in 2023 (103 transmitters detected). Residences lasted from 1 to 2,994 minutes (49.9 hours), with blue sharks and yellowfin tuna having the longest mean residence durations in both 2022 and 2023. Monthly residency indices, calculated as the cumulative number of days individuals of a species were detected at a given station divided by the cumulative number of receiver deployment days when each detected individual (of that species) was available for detection, varied across time and receiver stations for each species but generally indicate that HMS are more transient in the array during arrival/departure in late-spring, fall, and early winter and are more residential during the summer. For species for which interannual comparisons were available, bluefin tuna, blue shark, dusky shark, and shortfin mako exhibited residency throughout the WEA, while yellowfin tuna residency was largely limited to the southeasternmost extent of the WEA. Movement networks constructed by mapping paths between consecutive detections for bluefin tuna, blue shark, shortfin mako, and yellowfin tuna suggest that concentrations and spreads of activity may shift and vary throughout the WEA among species and years. Monitoring of both abiotic and biotic factors (i.e., bottom and surface temperature, thermal stratification, chlorophyll concentration) revealed species-specific associations in densities of occurrence that could potentially link presence, residency, and/or movements to certain ranges of environmental conditions and aid in prediction of habitat that may be of particular importance to HMS within leases, although further modeling efforts are needed. Lastly, a relatively high number of unidentified transmitters (n = 930) were recorded in the array that were deployed by other scientific groups; data sharing is currently underway, when permitted, to match these detections to their owners. Additional monitoring will continue, and forthcoming analyses will include data collected during the 2024 monitoring season.