Recent seabird mass morbidity events have highlighted the need to elucidate the effects of freshwater and hypertonic saline use for fluid therapy and housing in captive aquatic birds. Serum electrolyte concentrations of northern fulmars (Fulmarus glacialis; n = 9), western grebes (Aechmophorus occidentalis; n = 6), and common murres (Uria aalge; n = 25) housed on freshwater while undergoing rehabilitation at the San Francisco Bay Oiled Wildlife Care and Education Center in Fairfield, California, were compared to reference intervals from free-ranging populations. Additionally, northern fulmars were given intermittent oral boluses of hypertonic saline. In birds housed on freshwater, there were significant decreases in serum sodium and chloride, with 44% of northern fulmars and 72% of common murres falling below the established reference interval. All of the western grebes were able to maintain serum sodium and chloride within the reference intervals. The significance of these findings reflect the behavior and natural history of the species studied. The physiologic regulation of salt by the salt glands, gastrointestinal tract, and renal systems of seabirds, along with their behavior and natural history, should be considered when working with these birds in a rehabilitation or captive setting. Salt supplementation is necessary for some species of seabirds. Monitoring of serum electrolytes should be used for individual animals or salt supplementation should be considered in holopelagic species.