Advances in electronic tagging and genetic research are making it possible to discern population structure for pelagic marine predators once thought to be panmictic. However, reconciling migration patterns and gene flow to define the resolution of discrete population management units remains a major challenge, and a vital conservation priority for threatened species such as oceanic sharks. Many such species have been flagged for international protection, yet effective population assessments and management actions are hindered by lack of knowledge about the geographical extent and size of distinct populations. Combining satellite tagging, passive acoustic monitoring and genetics, we reveal how eastern Pacific white sharks ( Carcharodon carcharias ) adhere to a highly predictable migratory cycle. Individuals persistently return to the same network of coastal hotspots following distant oceanic migrations and comprise a population genetically distinct from previously identified phylogenetic clades. We hypothesize that this strong homing behaviour has maintained the separation of a northeastern Pacific population following a historical introduction from Australia/New Zealand migrants during the Late Pleistocene. Concordance between contemporary movement and genetic divergence based on mitochondrial DNA demonstrates a demographically independent management unit not previously recognized. This population's fidelity to discrete and predictable locations offers clear population assessment, monitoring and management options.

The white shark (Carcharodon carcharias) is a wide-ranging apex predator in the northeastern Pacific (NEP). Electronic tagging has demonstrated that white sharks exhibit a regular migratory pattern, occurring at coastal sites during the late summer, autumn and early winter and moving offshore to oceanic habitats during the remainder of the year, although the purpose of these migrations remains unclear. The purpose of this study was to use stable isotope analysis (SIA) to provide insight into the trophic ecology and migratory behaviors of white sharks in the NEP. Between 2006 and 2009, 53 white sharks were biopsied in central California to obtain dermal and muscle tissues, which were analyzed for stable isotope values of carbon (δ13C) and nitrogen (δ15N). We developed a mixing model that directly incorporates movement data and tissue incorporation (turnover) rates to better estimate the relative importance of different focal areas to white shark diet and elucidate their migratory behavior. Mixing model results for muscle showed a relatively equal dietary contribution from coastal and offshore regions, indicating that white sharks forage in both areas. However, model results indicated that sharks foraged at a higher relative rate in coastal habitats. There was a negative relationship between shark length and muscle δ13C and δ15N values, which may indicate ontogenetic changes in habitat use related to onset of maturity. The isotopic composition of dermal tissue was consistent with a more rapid incorporation rate than muscle and may represent more recent foraging. Low offshore consumption rates suggest that it is unlikely that foraging is the primary purpose of the offshore migrations. These results demonstrate how SIA can provide insight into the trophic ecology and migratory behavior of marine predators, especially when coupled with electronic tagging data.

Knowledge of how animals move through heterogeneous environments is essential to understanding the ecological functions they fulfill in each habitat and their responses to environmental change. Upper trophic level organisms exert structural influences through the food web, so information on their range, migration and foraging strategy is necessary to understanding ecosystem function. Recent technological advances have enabled researchers to follow individual animals over seasonal and multi-year timescales, revealing long-distance migrations in a variety of taxa. We used satellite telemetry to monitor female salmon sharks Lamna ditropis and remote sensing to characterize their environment. Salmon sharks ranged throughout the entire eastern North Pacific Ocean during a seasonal migration cycle. During long-distance migrations, quantitative movement analyses of speed, path straightness and first passage time (FPT) revealed area-restricted search (ARS) behaviors in northern and southern regions, with transiting behaviors at mid-latitudes. Individuals migrating to a highly productive southern region displayed more ARS behaviors than those moving to a low productivity region. The combination of multi-year time-series of animal behavior with synoptic environmental data reveals factors influencing migration and indicates that different life history functions are fulfilled in each habitat.KEY WORDS: Migration · Behavior · Habitat selection · Landscape ecology · Oceanography · Foraging behavior · Reproduction · Elasmobranch · Lamna ditropis Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 372: [253][254][255][256][257][258][259][260][261][262][263][264] 2008 has been used as a feeding proxy for a variety of marine vertebrates (Robinson et al. 2007), and studies combining both movement and feeding data have validated ARS in fish and bird species (Hill et al. 2000, Nolet & Mooij 2002.Like all proxies, the use of ARS behavior to indicate foraging is not perfect. Animals may move for reasons other than foraging (Dingle 1996), so ARS behavior could be foraging, mating, parturition or selection of environment. By combining measurements of both behavior and the environment we may gain clues to the ecology of organisms that cannot be directly observed (Pinaud & Weimerskirch 2005). ARS can be measured with analyses of speed, turning and straightness, or with more complex analyses (Robinson et al. 2007). First passage time (FPT) is a useful indicator of ARS behavior (Fauchald & Tveraa 2003) and is defined as the time an organism takes to cross a circle of a given radius. It is more robust to gaps in track records than speed or straightness.Top-down processes can have structural impacts on ecosystems (Hunter & Price 1992), so knowledge of upper trophic levels is essential to understanding ecosystem function. The salmon shark Lamna ditropis Hubbs et Follet, 1947 is an important component of the ecosystem due to its upper trophic level, abundance and high forage requirement...