Published by and copyright by Public Library of Science (PLoS) ; 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. ; This research was conducted as part of the Tagging of Pacific Predators (TOPP) program and was supported in part by the National Ocean Partnership Program (N00014???02-1???1012); the Office of Naval Research (N00014???00-10880, N00014???03-1???0651, N00014???08-1???1195, and N00014-10-1-0356); the National Science Foundation (NSF) Office of Polar Programs grant ANT-0838937; the National Oceanic and Atmospheric Administration Ocean (NOAA) Exploration Program; the E&P Sound and Marine Life Joint Industry Project of the International Association of Oil and Gas Producers (JIP2207???23); California Sea Grant program; NSF pre-doctoral fellowships to BM, CG, JH, and SM; the University of California Natural Reserve System Mildred E. Mathias Fellowship; the Moore, Packard, and Sloan Foundations; the Ida Benson Lynn Endowed Chair in Ocean Health; Steve Blank; The Myers Oceanographic Trust; Friends of the Long Marine Lab; and the Sooy Graduate Fellowship.