Waterbird mortality in hypersaline environments: the Wyoming trona ponds

Each year hundreds of salt-encrusted waterbirds, mainly Eared Grebes (Podiceps nigricollis Brehm) die at evaporation ponds of the trona (soda ash) industry in southwestern Wyoming. Clinical investigations attributed the mortality to sodium toxicity because high levels of sodium were found in brains of grebes salvaged from the ponds. This was puzzling because natural history information shows that this species resides in saline and hypersaline environments for most of the year. In addition, field observations gave no indication that grebes at the ponds were ingesting salt or salt crystals. Further, the salt glands were not hypertrophied and gavage experiments showed that healthy birds were able to ingest trona pond water without ill effect. Carcasses immersed in pond water, however, attained brain sodium levels far exceeding those considered toxic within a few hours, indicating that the high levels considered as causal probably resulted from post-mortem events. We attribute the mortality to complications of salt encrustation, notably impaired thermoregulation. Other purported cases of sodium toxicity involving encrusted birds at industrial ponds or hypersaline situations may have a similar etiology.