| Summary: | <jats:p>The duration of maternal foraging trips has been regarded as an indicator of foraging conditions in many marine mammals, including northern fur seals <jats:italic>Callorhinus ursinus</jats:italic> (NFS). However, previous work has focused on individual variation, was conducted during limited portions of the lactation period, and/or reached conclusions based on relatively small sample sizes. Here, we build upon the substantial foundations of this previous work to establish maternal foraging trip duration (MFTD) as an index of foraging success at the rookery level. We found that a 1 d increase in rookery-averaged MFTD corresponded to a 6.52% reduction in the average mass of female pups. Furthermore, rookery-averaged MFTD increased by 0.34 d per 1°C increase in average ocean bottom temperature. The magnitude of variation observed in both MFTD and pup mass is likely too small to help explain the general decline in population size seen over recent decades. However, the correlation between rookery-averaged MFTD and pup mass highlights the potential power of the MFTD index to detect bottom-up effects on pup growth, a likely indicator of survival. Furthermore, when compared with concurrently conducted studies on prey distribution, availability, and quality, the relationship between MFTD and bottom temperature suggests a northward shift in distribution of NFS preferred prey, walleye pollock <jats:italic>Gadus chalcogrammus</jats:italic>, that is associated with an increase in trip duration across the study period. Thus, rookery-averaged MFTD is a promising metric for tracking broad environmental changes, such as northerly shifts in the Eastern Bering Sea cold pool.</jats:p>
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