Inland Silverside

The inland silverside, Menidia beryllina, is a neotropical silverside native to eastern North America, and introduced into California. It is a fish of estuaries and freshwater environments.

Inland silversides are quite elongate even for silverside, with lengths 6 to 7 times depth. They have large eyes, a considerably upturned mouth, and a head noticeably flattened on top. Of the two widely separated dorsal fins, the anterior fin is small and has 4-5 weak spines, while the posterior fin is larger, with one spine and 8 or 9 rays. The lengthy anal fin is somewhat sickle-shaped, has one spine and 16 to 18 rays. As befits the name, they are silvery on the sides; the back is somewhat yellowish, and the underside is a translucent greenish. These are small fish, with 15 cm recorded, but most adults 10 cm or less.

They primarily feed on zooplankton, moving in enormous schools capable of depleting populations of the small arthropods and crustaceans they favor. In turn, they are prey for a variety of fish and birds. The silversides congregate in the shallows, generally over sand or gravel bottoms with overhead cover if possible, but then move out to open water in search of additional food, which increases predation risk. They are often observed in a sort of daily migration pattern as a result.

The exact native range of the inland silverside is not known; they are widespread along the Atlantic coast from Maine to Florida, and along the Gulf of Mexico. In the Mississippi River they can be found in backwaters and reservoirs as far north as Missouri and Illinois, hundreds of miles inland.

They were introduced into Clear Lake and the Blue Lakes of California in 1967, in order to control the Clear Lake gnat Chaoborus asticopus and midges, and in lakes and reservoirs of Alameda County and Santa Clara County the following year. From there they spread into the San Francisco Bay and Central Valley, and have since become widespread in central California. In some areas, they are the most abundant fish of any species. Moyle suggests that this fish may have contributed to the demise of the Clear Lake splittail, although the effect of the silversides' introduction on California ecosystems not been much studied.

Inland Silversides are currently an EPA approved indicator species for acute marine aquatic toxicity testing and short-term chronic toxicity estimating of marine and estuarine organisms.