During their tender youth, both the endangered fish species totoaba and the commercially important gulf corvina require the brackish water habitat provided by the shrinking Colorado River estuary, report researchers.
Although overfishing has been implicated in the decline of both species, commercial harvesting isn't the only reason for the two species' decline, the finding suggests. Since 1960, diversion of Colorado River water for human uses has greatly reduced the amount of fresh water that reaches the Gulf of California, thereby reducing the brackish-water estuary, the region where river water and ocean water mix.
"It's the first time that we've been able to substantiate that these fish are using Colorado River water," said Kirsten Rowell, the aquatic biologist who led the research team. "We provide evidence that both of these fish need brackish water in their youth, but today the northernmost part of the Gulf of California is more saline than the open ocean."
Rowell, a doctoral candidate in the department of geosciences at the University of Arizona in Tucson, used fish earstones, or otoliths, to decipher where the fish lived during their babyhood.
The chemistry of the almond-sized otoliths documents what type of water the fish lived in during various times in their lives. Otoliths act like the fish version of a flight recorder.
"You are what you swim in," she said, adding "Otoliths are great data loggers."
The team's research shows that when they're young'uns, the fish prefer the brackish water provided by the mixing of fresh and ocean water in the estuary at the mouth of the Colorado.
Team member Karl W. Flessa said, "There are two sources of human impact on these species: one is the direct effect of overfishing; the other is the indirect effect of freshwater diversion." Flessa, a professor in UA's department of geosciences, said, "We don't doubt that overfishing is or has been a threat to these species. We're saying that changes to the nursery area are also a threat to these species."
Rowell will present the team's research at the Gulf of California conference held June 13-16 at the Westward Look Resort in Tucson, Ariz. Her presentation, "Isotopic Logs from the Sea of Cortez: Environmental and Life History Records From Totoaba and Gulf Corvina Otoliths " will be given at 4 p.m. on Monday, June 14, in Colorado River Delta Session. Her coauthors are Flessa and David Dettman, a research scientist in UA's department of geosciences.
The research was funded in part by the National Science Foundation, the Southern Arizona Environmental Management Society, the Chevron Research Fund and T&E, Inc.
Totoaba macdonaldi was the first commercially important fish in the northern Gulf of California. The fish can live 20-something years and grow to six feet in length. "San Felipe used to be just an ephemeral fishing village for totoaba," Rowell said. "They used to ship them overland to San Francisco and San Diego. It was the first time they tried to use refrigerated cars."
The commercial totoaba fishery crashed in 1975. Overfishing has been blamed for totoaba's decline. The fish was listed by the United States as federally endangered in 1979. The current size of the totoaba population is unknown.
Gulf corvina, Cynoscion othonopterus, are still fished commercially, although the American Fisheries Society, the professional society of fisheries biologists, has recently identified the species as "vulnerable," because of habitat changes in the fish's nursery area and heavy fishing pressure in fish's spawning site.
Estuaries, zones where fresh river water and salty ocean water mix to form brackish water, are known to be nursery areas for many species of marine life, including many fishes.
Both totoaba and gulf corvina spawn in the mouth of the Colorado, according to fishermen's reports. Rowell wondered whether otoliths from totoaba and gulf corvina could be used to document the fishes' use of the Colorado River estuary as nursery.
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 affected when we turned off the water upstream."
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