Environmental and anthropogenic influences in the marine environment are primary drivers of behavior and demographic outcomes for marine birds. We examined factors influencing the foraging patterns of the Westland Petrel (Procellaria westlandica), a highly threatened, endemic petrel that inhabits subtropical water masses primarily in the Tasman Sea, with a poorly known at-sea distribution. Risk assessments place the species at moderate risk of population impacts from fisheries-related mortality. Studies in the 1990s indicated that trawl fisheries would have an important influence on the Westland Petrel's foraging behavior. We investigated the influence of climatic conditions, marine productivity, bathymetry, the core fishery zone, concurrent fishing activity, light conditions, sex, and breeding stage on Westland Petrel foraging patterns. We analyzed the stable isotopes of carbon and nitrogen from blood sampled during the incubation period and examined changes in isotopic niche width over a 6-yr period. We found that the Westland Petrel's foraging zone varied only slightly between years and that the location of intensively used areas was strongly influenced by bathymetric slope and latitude, and negatively influenced by chlorophyll-a. The core fishery zone had a secondary influence, suggesting that these petrels co-occur with fisheries, but are not dependent on waste for food. Trophic niche width was significantly wider during strong El Niño conditions, indicating that food type, rather than location, was most affected by climatic variation. Consistent use of one marine area across varying times and conditions increases the risk of adverse effects of climate or human-induced impacts on the species. However, marine spatial management tools become viable in these conditions. Further, with rapid increases in sea surface temperatures and extreme values recorded in the region in recent periods, changes to fisheries zones and distributions of natural prey of the species are likely to occur and may change the population's sustainability. Direct all requests to reproduce journal content to the AOS Publications Office at pubs@americanornithology.org que les pétrels cohabitent avec les activités de pêche mais ne sont pas dépendants des rejets de pêche pour s'alimenter. Les niches trophiquesétaient significativement plus larges lorsque fortes dans des conditions El Niño, ce qui indique que le type de nourriture, plutôt que la localisation,était le plus affecté par la variation du climat. Une utilisation continue d'une zone marineà différents moments et dans diverses conditions augmente le risque d'impacts climatiques ou anthropiques négatifs sur l'espèce. Cependant, les outils de gestion spatiale du milieu marin deviennent viables dans ces conditions. De plus, avec de rapides augmentations de la SST et des valeurs extrêmes enregistrées dans la région récemment, des changements aux zones de pêches et dans la répartition naturelle des proies de l'espèce sont susceptibles de se produire et peuvent affecter la pérennité de la po...