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Figure 72. Striped mulletllisa, 38 cm in total length, from Estuario de Santa Clara, Sonora. This predominately marine species enters the lower parts of major rivers throughout most of the tropics worldwide; it was not considered by the American Fish- eries Society. their young move into estuaries to feed and grow. Young individuals (to 15 cm total length) swarm in the Colorado River estuary; larger (presumably older) fish tend to move farther upstream when water is available. When adult (longer than 35 cm) they disappear from freshwater, moving back to the sea to reproduce. ~ Fishes of Desert Oases Springs rising from subsurface sources sometimes form special aquatic habitats in deserts, which may be inhabited by equally special fishes. Springs occur where fractures in the earth allow deep waters to rise to the surface, or as points where water accumulatinl! at higher altitude and percolating through porous strata emerges. Water from deep sources often is hot and charged with gasses and salts otherwise uncommon in a region. Others represent a simple intersection of a local water table with the land surface. Spring water is typically clear and constant in volume. Fishes living in such places have few environmental cues other than daylength-there are no spring floods or summer droughts, seasonal temperature changes, or changes in substrate or turbidity with which to deal. They may thus reproduce throughout the year and grow at a constant rate dependent on population size rather than physical factors. Figure 73. Pacific tenpounderl machete, 15.0 cm in total length, from Estuario de Santa Clara, Sonora. This relative of the tarpon was not considered by the American Fisheries Society. 37