At the time of its Federal and State listing as endangered in 1970, there were estimated to be about 600 breeding pairs of the California least tern, Sterna antillarum browni, nesting in California. In the first decade of recovery efforts, emphasis was on protection and/or establishment of designated nesting areas. Statewide annual monitoring of the breeding sites and estimates of the breeding population began in 1973, with estimates of fledgling production beginning in 1978. By 1980, the tern's breeding population had doubled to 1160 pairs with 16 nesting sites supporting 20 or more pairs and a total of 31 sites used in that year. In 1982, the severe oceanographic phenomenon, involving the northerly extension of tropically warmed surface waters and declines in some southern California fishery resources, known as El Nifio, contributed to a decline in the least tern breeding population to a low of 944 pairs in 1987. The population recovery emphasis since 1980 bas been nesting site management and reduction of predation impacts in order to increase reproductive success. By 1990, the Statewide breeding population was estimated to be 1708 pairs with 20 nesting sites supporting 20 or more breeding pairs and a total of28 sites used in that year.
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