Archived and recent harbor seal tissues have been used to determine food web structure and trophic dynamics of seals with Prince William Sound (PWS) and the adjacent Gulf of Alaska. Within the sound, isotope ratios confirm that most harbor seals are at the top of food chains that are based on in situ primary and secondary productivity and not on allochthonous production from outside the Sound. Carbon isotope ratios also indicate that benthic prey are a large component of harbor seal diets. Isotope ratios along wild seal whiskers indicate, however, that some individuals migrate into areas (presumably in the Gulf) wherein food web structures are different and isotope ratios of prey are considerably lower than within the sound. Experiments with captive seals to determine whisker growth rates showed that vibrissal growth is highly seasonal and occurs primarily in early spring. Sea lions and fur seals have relatively constant vibrissal growth. Data on isotope ratios of potential prey species from PWS and from other sites in the Gulf of Alaska indicate that a geographic isotopic gradient in both carbon and nitrogen exists between onshelf and deep pelapic waters. The detailed patterns of these isotopic regimes have not yet been fully defined.