Abstract
The Antarctic krill, Euphausia superba, is a major component of the Southern Ocean’s ecosystem. Limited high-resolution data on the relative importance of oceanographic processes on the behavioral responses of krill at traditional predator foraging grounds constitutes a major obstacle in the understanding of krill-environment coupling and ecosystem-based management of this resource. Aggregation structures of krill and predator interactions were investigated using active acoustic data collected by WBAT echosounders deployed on moorings in two hydrographically different sites in Bransfield Strait. Near Nelson Island, water flows from the northwest to southeast while Deception Island is influenced by stronger net current velocities from the southwest to northeast. Krill aggregations were identified and then classified in three clusters using a swarm-identification algorithm and hierarchical clustering using aggregation morphological characteristics: acoustic density, mean depth, center of mass, inertia, equivalent area, aggregation index, and proportion occupied. A total of 693 and 736 aggregations were detected at the mooring sites close to Nelson and Deception Islands. The three aggregation categories ranged from high to low densities, evenness, and dispersion and were distributed throughout the water column. Krill aggregation density distribution and mean thickness are influenced by krill mean depth, current velocities and direction. The majority of observed predator dive profiles occurred over the aggregation type with highest krill densities at both Nelson and Deception Islands, and within the first 25 m of the water column. The heterogeneity of krill aggregations potentially impacts predator foraging strategies and predator–krill interactions in the hydrodynamically active Bransfield Strait.
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The authors acknowledge the captain, crew, and scientific staff of the RV Kronprins Haakon for deployment and recovery of the moorings. The first author is the beneficiary of a postdoctoral research fellowship granted by the Cooperative Institute for Climate, Ocean, and Ecosystem Studies (CICOES, University of Washington, Seattle, USA). This publication is funded by CICOES under NOAA Cooperative Agreement NA20OAR4320271, Contribution No. 2023-1251.
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CSR and GJM conceived and designed this study, and provided data with input from GRC. PA processed and analyzed the data and prepared the figures, advised by JKH. PA wrote the manuscript, JKH, CSR, GRC, and GJM revised the manuscript.
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Annasawmy, P., Horne, J.K., Reiss, C.S. et al. Antarctic krill (Euphausia superba) distributions, aggregation structures, and predator interactions in Bransfield Strait. Polar Biol 46, 151–168 (2023). https://doi.org/10.1007/s00300-023-03113-z
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DOI: https://doi.org/10.1007/s00300-023-03113-z