The mesopelagic zone of the northeast Pacific Ocean is an important foraging habitat for many predators, yet few studies have addressed the factors driving basin-scale predator distributions or inter-annual variability in foraging and breeding success. Understanding these processes is critical to reveal how conditions at sea cascade to population-level effects. To begin addressing these challenging questions, we collected diving, tracking, foraging success, and natality data for 297 adult female northern elephant seal migrations from 2004 to 2010. During the longer post-molting migration, individual energy gain rates were significant predictors of pregnancy. At sea, seals focused their foraging effort along a narrow band corresponding to the boundary between the sub-arctic and sub-tropical gyres. In contrast to shallow-diving predators, elephant seals target the gyre-gyre boundary throughout the year rather than follow the southward winter migration of surface features, such as the Transition Zone Chlorophyll Front. We also assessed the impact of added transit costs by studying seals at a colony near the southern extent of the species’ range, 1,150 km to the south. A much larger proportion of seals foraged locally, implying plasticity in foraging strategies and possibly prey type. While these findings are derived from a single species, the results may provide insight to the foraging patterns of many other meso-pelagic predators in the northeast Pacific Ocean.

Stressors associated with human activities interact in complex ways to affect marine ecosystems, yet we lack spatially explicit assessments of cumulative impacts on ecologically and economically key components such as marine predators. Here we develop a metric of cumulative utilization and impact (CUI) on marine predators by combining electronic tracking data of eight protected predator species (n ¼ 685 individuals) in the California Current Ecosystem with data on 24 anthropogenic stressors. We show significant variation in CUI with some of the highest impacts within US National Marine Sanctuaries. High variation in underlying species and cumulative impact distributions means that neither alone is sufficient for effective spatial management. Instead, comprehensive management approaches accounting for both cumulative human impacts and trade-offs among multiple stressors must be applied in planning the use of marine resources.

SUMMARYThe range of foraging behaviors available to deep-diving, air-breathing marine vertebrates is constrained by their physiological capacity to breath-hold dive. We measured body oxygen stores (blood volume and muscle myoglobin) and diving behavior in adult female northern elephant seals, Mirounga angustirostris, to investigate age-related effects on diving performance. Blood volume averaged 74.4±17.0liters in female elephant seals or 20.2±2.0% of body mass. Plasma volume averaged 32.2±7.8liters or 8.7±0.7% of body mass. Absolute plasma volume and blood volume increased independently with mass and age. Hematocrit decreased weakly with mass but did not vary with age. Muscle myoglobin concentration, while higher than previously reported (7.4±0.7g%), did not vary with mass or age. Pregnancy status did not influence blood volume. Mean dive duration, a proxy for physiological demand, increased as a function of how long seals had been at sea, followed by mass and hematocrit. Strong effects of female body mass (range, 218-600kg) on dive duration, which were independent of oxygen stores, suggest that larger females had lower diving metabolic rates. A tendency for dives to exceed calculated aerobic limits occurred more frequently later in the at-sea migration. Our data suggest that individual physiological state variables and condition interact to determine breathhold ability and that both should be considered in life-history studies of foraging behavior.Key words: age, dive duration, elephant seal, oxygen stores. THE JOURNAL OF EXPERIMENTAL BIOLOGY 586 MATERIALS AND METHODS Field site and study sampleWe conducted this study at Año Nuevo State Reserve, San Mateo County, California, USA during semiannual female elephant seal haul-outs. For seven consecutive molt and breeding cycles between 2004 and 2008, we collected data on diving behavior in sampled animals. Individual seals were sampled 1-6times over the study period. We collected 257 measurements of mass, plasma volume and Hct in 112 known-age individual females at instrument deployment and recovery, prior to departure and after arrival from sea. Muscle myoglobin concentration was measured from 128 samples in 59 individuals.For each breeding and molt haul-out between 2004 and 2008, 19-25 females between 4 and 17years old were sampled and instrumented. Age of females was determined from identification codes on plastic flipper tags (Jumbo roto-tags, Dalton Company, Oxon, UK) that were attached shortly after weaning. Instruments were deployed on breeding females at least 22days post-partum. Females returning to breed were observed for 5days post-partum to ensure that the mother-pup bond was established before they were immobilized, instruments were recovered and samples were taken. We are not aware of data for whether blood volume changes after parturition in marine mammals but it does not change in ewes or cows and it has been suggested that maternal blood volume is maintained throughout lactation (Metcalfe and Parer, 1966;Reynolds, 1953). The effect of pr...

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