Geographe bay as an opportunistic foraging habitat for baleen whales

Genty, G., C.D. Jolliffe, J.C. Perkins, and C.R. McPherson

Marine Biology 173(6): 91 (2026)

DOI: 10.1007/s00227-026-04830-9

Geographe Bay, located along the southwest coast of Western Australia, is increasingly recognised as a region of ecological significance for migrating baleen whales. This study investigates the bay’s potential role as an opportunistic feeding ground, particularly for species such as pygmy blue whales, which are typically considered krill specialists but may exhibit dietary flexibility under varying prey conditions. Over three years, drone-based observations recorded frequent whale defecation events, with faecal colouration consistently ranging from green to yellow, hues commonly associated with copepod digestion due to their carotenoid content. To examine the underlying prey availability, this study provides the first investigation of applying environmental DNA metabarcoding on plankton tows within bay to assess food availability for baleen whales. A total of 3.2 million quality-filtered reads were assigned to 600 amplicon sequence variants (ASVs), with Arthropoda dominating the assemblage. Notably, the copepod order Calanoida accounted for over 20% of total reads and up to 95% of the relative abundance at individual transects. Key calanoid taxa included Clausocalanus furcatus, Temora turbinata, and Paracalanus indicus, all consistently detected across the transects. The strong dominance of copepods, paired with behavioural evidence and faecal pigmentation, supports the hypothesis that whales may be actively feeding in Geographe Bay. These findings highlight Geographe Bay as not only a sheltered resting habitat, particularly for mother/calf pairs, but also an energetically valuable area that may help buffer whales against variable offshore prey availability.