Getting gulf corvina otoliths was fairly easy -- Rowell took them from four fish bought at the market in El Golfo de Santa Clara, Mexico, the little fishing village at the mouth of the Colorado.
Obtaining totoaba otoliths was trickier. Scripps Institute of Oceanography in La Jolla, Calif., loaned Rowell four 1,000-year-old totoaba otoliths that had been collected from an Indian archaeological site near San Felipe, Baja, Mexico.
Otoliths grow in layers, one layer per year of life. Rowell used a dental drill to grind off bits of otolith one layer at a time.
She then analyzed each layer for different forms of oxygen, called isotopes, to see where the fish lived each year of its life. Fresh water has relatively more of the lighter form of oxygen, oxygen-16, so layers with more oxygen-16 represented years that the fish had spent in brackish water. If the layer had more of the other form, oxygen-18, that meant the fish spent that year in the saline ocean water.
Rowell found that the corvina had spent part of their first year of life in brackish water in the mouth of the river, and the totoaba had spent up to their first three years in brackish water.
She said, "It shows these fish chose the brackish water habitat -- and we've taken it away."
"The totoaba are endangered for two reasons," she said. "The primary reason is being overfished. The second reason is that their nursery grounds have shrunk drastically. The Colorado River no longer makes it down to the Gulf except in flood years." She added, "My data say that before the dams, totoaba lived in a brackish water estuary for several years."
Rowell said, "Freshwater rivers are part of a larger system -- they don't just stop at the edge of the continents. Rivers have a large influence on coastal marine ecosystems. These economically important marine fish were af