Antarctic krill (Euphausia superba) distributions, aggregation structures and predator interactions in Bransfield Strait

Abstract

Abstract Aggregation structures of Antarctic krill (Euphausia superba) and predator interactions were investigated using active acoustic data collected for approximately one month by WBAT and Signature100 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 the Bransfield Front with 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 densities and mean thickness are influenced by current velocities, direction, mean depth, and predator foraging. 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.