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. W