| Summary: | Understanding prey quality and prey selection by predators is critical for management efforts aimed at identification and protection of essential habitats and prey. Marine predators must make daily foraging choices in a heterogenous and dynamic environment in order to meet the high energetic demands of migration, reproduction and foraging. With shifting ecosystems and food-web dynamics, it is important to know what whales prioritize in their foraging decision-making. Here, two studies were conducted to assess the drivers of gray whale foraging ecology relative to (1) comparative prey caloric content between foraging grounds, and (2) individual prey patch selection based on available prey quality and quantity within a foraging ground. The first study was addressed through assessment of the energetic value of six zooplankton species collected within the Oregon, USA range of the Pacific Coast Feeding Group (PCFG) gray whale (Eschrichtius robustus) feeding grounds. These results were compared to the energetic value of the predominant amphipod species fed on by Eastern North Pacific (ENP) gray whales in the Arctic, and extrapolated to daily energetic requirements of gray whales. Prey in the PCFG range were found to be of comparable, and in some cases higher, energetic value than Arctic prey, suggesting that other factors, such as prey density and accessibility, or natal philopatry, play a role in the large difference in the population sizes using these foraging grounds. The second study paired theodolite tracking of PCFG gray whales in Port Orford, Oregon, USA, with concurrent sampling of their zooplankton prey from a research kayak to examine the micro scale (40 m) foraging decisions of whales relative to available prey quantity (abundance) and quality (caloric content). Our analysis relates whale tracks and behavior states to prey, demonstrating that (1) foraging whales were significantly more associated with higher relative prey abundance and calories than searching and transiting whales, (2) whales significantly ...
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