The Cabo Rojo salt flats are an important wintering area for migratory shorebirds. Their quality is intimately related to prey availability, as prey are needed to meet energetic requirements. Understanding prey dynamics is, therefore, a key element of shorebird conservation plans. To this end, we monitored the density and distribution of water-boatmen (Trichocorixa spp.) and brine shrimp (Artemia spp.) in relation to water salinity from September to November of 1994 and 1995. Salinity ranged from 4 to 292 ppt, and gradients were related to hydrological alterations (e.g., salt extraction) and connection to the ocean. Brine shrimp were restricted to areas of highest salinity (??? 106 ppt), whereas water-boatmen to areas of lowest salinity (< 65 ppt). We used aquaria experiments to discern potential mechanisms influencing density and distribution of water boatmen. We focused on this species because its caloric value is similar to the brine shrimp's, but it occurs in areas of lower salinity where shorebirds are less prone to hyperosmotic stress. We hypothesized that areas devoid of water boatmen exceeded their tolerance limit, and that these limits could hamper survival as individuals move among areas. Experiments showed that an increase of 8.5 ?? 2.1 ppt, when the base salinity was 40 ppt, induced a 50% mortality rate. From a base salinity of 55 ppt, median survival time decreased curvilinearly across salinity concentrations of 65 to 195 ppt. Median survival was lowest > 100 ppt. Lowering water salinity did not result in osmolal related mortality. Results underscored the sensitivity of water boatmen to high salinity, particularly when the difference in salinity between the 'source' and 'destination' localities widened. Water boatmen density increased in one lagoon as salinity decreased from 65 to 47 ppt. On the basis of our experiments, local adult survivorship improved and immigration and subsequent survival of adults, if any, was not hindered. The density of nymphs also suggested that hatching occurred concurrently. The foraging value of the salt flats can be enhanced by maintaining salinity at < 65 ppt in selected management units and minimizing differences in salinity concentrations among them.