The Department of Fish and Wildlife has been conducting the Fall Midwater Trawl survey since 1967, collecting data and determining the abundance of the Delta’s pelagic species, such as the Delta smelt, longfin smelt, splitttail, American shad, and threadfin shad. The populations of pelagic fishes has shown extreme annual variability in the past, given the variable nature of estuaries, and historically, the lowest populations have been in dry years.
However, the decline to record or near-record lows of the abundance indices for the four pelagic species, raised concerns, especially since flows were moderate during this period. The species abundance indices further deteriorated over the next several years. By 2004, the declines had been recognized, and it came to be known as the Pelagic Organism Decline.