Abstract
The duration of maternal foraging trips has been regarded as an indicator of foraging conditions in many marine mammals, including northern fur seals Callorhinus ursinus (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 Gadus chalcogrammus, 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.
Highlights
Like other otariids, maternal northern fur seals Callorhinus ursinus (NFS) employ a central-place foraging strategy, in which they alternate foraging trips to sea in support of milk production with brief suckling bouts ashore (Orian & Pearson 1979, Gentry & Holt 1986, Gentry 1998, Sterling 2009)
Benoit-Bird et al (2013) examined the patchiness of forage fish distribution in relation to NFS movements and concluded that time spent within forage patches by fur seals was relatively constant within each trip, and that variation in maternal foraging trip duration (MFTD) was largely a function of transit times to and from the source rookery, and between forage patches
Average female pup mass was negatively correlated with rookery-averaged MFTD through to the date pups were weighed (F1,6 = 6.81, p = 0.038), with MFTD explaining 53.95% of the variation in female pup mass (Fig. 5)
Summary
Maternal northern fur seals Callorhinus ursinus (NFS) employ a central-place foraging strategy, in which they alternate foraging trips to sea in support of milk production with brief suckling bouts ashore (Orian & Pearson 1979, Gentry & Holt 1986, Gentry 1998, Sterling 2009). The largest population of NFS occupies the Pribilof Islands (St. George [STG] and St. Paul [SNP] Islands), Alaska (USA), during the summer, breeding on 1 of 19 rookeries. Many studies have linked maternal foraging trip duration (MFTD) of otariids to direct and indirect measures of prey availability and/or abundance, as well as to the environmental conditions known to influence prey abundance
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