Josh M. London^1^,✉, Heather L. Ziel^1^, Lorrie D. Rea^2^, Stacie M. Koslovsky^1^, Michael F. Cameron^1^, and Peter L. Boveng^1^

1. Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, Seattle, Washington, USA
2. Water and Environmental Research Center, Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, Alaska, USA

✉ Correspondence: Josh M. London [email protected]

This repository serves as the research compendium in support of the above titled working paper. As the manuscript works its way through development, peer review, and the publications process, additional reproducibility features (e.g. devcontainer) and documentation of key revisions and releases will be available.

Please note this analysis and manuscript are still in draft form and under active development. Changes to results, code, and the manuscript are likely and this should not be cited or used for any reason. We are sharing the work and development of this manuscript in the spirit of open science, improved transparency, and scientific reproducibility.

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In deep-diving seals (Phocidae) niche partitioning has been observed as delineation in time, multi-dimensional use of the ocean, or diet composition. Here, we focus on two species of seals in the Bering Sea -- ribbon seals (Histriophoca fasciata) and spotted seals (Phoca largha) -- and evidence for niche partitioning from two decades of bio-logger deployments (n=110 ribbon; n=82 spotted) and stable isotope sampling (n=29 ribbon; n=43 spotted). Whiskers of dependent pups in the spring reflect the isotopic space of adult female diet in the winter (when the pup was developing in-utero) and sampling from the whisker base of adults in the spring corresponds with the isotopic space of their recent diet. In both seasons, spotted seals had higher mean δ13C (winter: +6.9%; spring: +3.8%) and δ15N (winter: +10.5%; spring = +12.1%) values, which are reflective of on-shelf and coastal foraging at a higher trophic level. Two-dimensional utilization distributions (UD) were estimated from bio-logger geolocations for each species during similar seasonal periods ('spring' and 'fall-winter'). Optimally weighted auto-correlated kernel density estimates were combined into a population UD to test spatial overlap. Greater overlap was observed in the spring when both species rely on the marginal sea-ice zone for pupping, breeding, and molting. More separation was observed during the fall-winter season when spotted seals remained largely on the continental shelf and ribbon seals shifted to the shelf break and Bering Sea basin. Dive behavior records showed ribbon seals consistently diving to deeper depths (max dive depth = ~600m) compared to spotted seals (max dive depth = ~300m) indicating additional partitioning of resources within the water column. Changes in the extent and timing of sea ice in the Bering Sea along with anomalous warming events could disrupt the niche partitioning between these seal species and, thus, challenge their resilience to climate change.

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