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The Delta Smelt, Hypomesus transpacificus, 8-10 cm (3-4 inches) long, short-lived, almost translucent, and smelling of cucumber, is one of a number of fish species endemic to the enormous tidal marsh of the Sacramento-San Joaquin River Delta. Living mostly in the mixing zone between fresh water and the salt water, where it feeds on copepods and other zooplankton, it moves into fresh waters in the spring months (March-May) to spawn. Historically occurring from San Pablo Bay upstream to Sacramento on the Sacramento River and to Mossdale on the San Joaquin River, the Delta Smelt until 30 years ago was considered the most common pelagic fish in the delta and was even commercially fished.
However, since the 1980s, population numbers of the delta smelt have plummeted. Federal and state projects to control and pump water from the delta to other parts of California especially for agriculture have intensified, significantly changing the salinity, temperature and oxygenation of water in the Delta Smelt’s habitat. The introduction of non-native predators and multiple effects of pollution have also contributed to extreme degradation of this ecosystem. In 1996 the IUCN listed H. transpacificus as endangered and in 2014 upgraded its status to critically endangered. Monthly surveys carried out by the California Department of Fish and Wildlife, which usually identify hundreds of individuals, now recover too few individuals to calculate population sizes; in May 2015 they found just one fish.
Pressured even further since the start of historic drought in California starting in 2012, the Delta Smelt population has a predicted 50% chance of extinction in the wild in the next 20 years, according to analyses. Peter Moyle, researcher and associate director of the Center for Watershed Sciences at University of California Davis says, “The probability of the delta smelt surviving in the next three years is relatively low.” The UC Davis Fish Conservation and Cultural Laboratory have started a captive bred refuge population to harbor the genetic diversity of the Delta Smelt in a sophisticated facility until perhaps at some point their environment will be more hospitable and the species can be reintroduced, although the possibility looms large that this population will not make it back to the wild again.
Protection of the Delta Smelt has stirred up controversy in California as dwindling water resources are demanded for human use, while the US Fish and Wildlife Service, with mandate to protect this species under the Federal Endangered Species Act, restricts pumping of water to farms and cities in Southern California. Water export projects were found in court to jeopardize the fish and its environment.
With their one-year life span, Delta Smelt are considered an immediate indicator of the well-being of the 29 other endemic fish of the Bay-Delta (12 of which are now extinct or endangered), in addition to the whole ecosystem itself. The failure of this species has scientists desperately concerned for the insufficient protection of this fragile ecosystem, and imperiled species such as longfin smelt, Sacramento splittail, Sacramento perch, river lamprey, green sturgeon, Central Valley steelhead trout and spring and winter runs of chinook salmon.
(Kay 2015; Krieger 2015; Moyle et al. 1992; NatureServe 2014)