Investigating Eared Grebe mortality at Mono Lake

Nearly half of all the Eared Grebes in North America visit Mono Lake every autumn by the hundreds of thousands to feed on trillions of brine shrimp (Artemia monica). The bountiful food supply makes it possible for grebes to double their weight and fly to overwintering habitats at the Gulf of California and Salton Sea. But in the last two months, there has been a startling scene of hundreds of dead Eared Grebes on the shores of Mono Lake. These dead birds, according to one ornithologist, are juveniles that starved.

This is not a new occurrence at Mono Lake—hundreds of dead grebes were also found on the shore in 2011 and 2014. While some dead grebes will be found along Mono Lake’s shore every year during the fall migration, in some years there is much higher mortality than in others. Why does this happen every few years? While grebe mortality is poorly understood, we suspect that it is linked to changes in Mono Lake’s ecosystem. In 2014, Mono Lake Committee staff noticed two things about Mono Lake that seemed different: (1) The lake stayed green with algae throughout the summer and fall (figure 1), and (2) shrimp seemed less abundant (figure 2). By August 2014 we began to notice dead grebes on the shore. Also, the traditional mid-October peak of the Eared Grebe migratory population occurred in early September (figure 3), which was a suspected trend that was first documented last year.

So far, 2015 is looking a lot like 2014. Brine shrimp, which feed on algae, provide 90% of the grebe diet. When shrimp density in the water column declines sufficiently, grebes can no longer maintain their weight. When shrimp density is low, any grebes arriving with a low weight (such as juveniles) are more likely to have trouble surviving. So changes in these dynamics are definitely interrelated, but questions like “how” and “why” these changes are occurring can only be answered with scientific study and inquiry.

Part of the 1994 State Water Board decision providing protection to Mono Lake also required annual compliance monitoring and reporting on Mono Lake. This limnology monitoring and reporting has been conducted internally by the Los Angeles Department of Water & Power (DWP) for the last three years, and during that time it has been difficult to get updates or scientific analysis on how or why the Mono Lake ecosystem might be changing. This will be corrected with the issuance of new water licenses to DWP’s with the incorporation of the 2013 Mono Basin Stream Restoration Agreement. Once affirmed by the State Water Board, Mono Lake limnology monitoring will return to the hands of independent scientists with the expertise necessary to determine relationships between the green lake and shifting shrimp populations, which are the underpinnings for understanding Eared Grebe mortality.