Longfin Silverside, Atherinella eriarcha

Longfin Silverside, Atherinella eriarcha. Fish collected by hand off the beach, Cabo Real, Baja California Sur, December 2014. Length: 2.5 cm (1.0 inch). Can also be found on occasion in the bait tanks of commercial bait salesmen.

Longfin Silverside, Atherinella eriarcha. Fish collected by hand off the beach, Cabo Real, Baja California Sur, December 2017. Length 2.6 cm (1.0 inch).

Longfin Silverside, Atherinella eriarcha. Fish caught from coastal water off Mazatlán, Sinaloa, October 2017. Length: 6.0 cm (2.4 inches). Catch and photograph courtesy of George Brinkman, Guelph, Ontario, Canada. An impressive catch via hook and line on size 27 micro-hooks.

Longfin Silverside, Atherinella eriarcha. Fish caught from coastal water off Mazatlán, Sinaloa, October 2017. Length: 7.8 cm (3.1 inches). Catch, photograph, and identification courtesy of Josh Leisen (joshadventures.com), Gaylord, Michigan.

Longfin Silverside, Atherinella eriarcha. Fish caught from coastal waters off Mazatlán, Sinaloa, October 2021. Length: 7.8 cm (3.1 inches). Catch, photograph and identification courtesy of Bart, The Netherlands  (worldangler.eu).

The Longfin Silverside, Atherinella eriarcha, is a member of the New World Silverside or Atherinopsidae Family, and are known in Mexico as plateadito plateado. Globally, there are thirty-six species in the genus Atherinella, of which seven are found in Mexican waters, two in the Atlantic and five in the Pacific Ocean.

The Longfin Silversides has an elongated and relatively deep body with a rounded cross section. They are translucent and pale silvery-olive in color with narrow scale outlines and a broad silver stripe on their sides that narrows into a large diamond on their caudal fin base. Their head is large with disproportionately large eyes and a fairly large terminal mouth that opens in the front. Their anal fin has 1 spine and 27 rays; their caudal fin is forked; their first dorsal fin have 4 spines and originates over the anal fin; their second dorsal fin has 1 spine and 11 or 12 rays; and, their pectoral fins are short and reach the middle of the pelvic fins. They are covered with smooth scales.

The Longfin Silverside is a schooling pelagic species found in shallow intertidal waters, coastal bays, and near coastal reefs at depths up to 11 m (35 feet). They reach a maximum length of 8.0 cm (3.2 inches). They are active daytime feeders that consume algae and small crustaceans. Reproduction is oviparous with large masses of sticky eggs released by females several times per year. These eggs attach themselves to shallow water seaweed via long filaments and are then fertilized by males. The larvae are planktonic. The Longfin Silverside  is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.

The Longfin Silverside is a resident of Mexican water of the Pacific Ocean but has a limited distribution being found from Guerrero Negro, Baja California, southward along the central and southwest coasts of Baja, in the lower half of the Sea of Cortez, and along the coast of the mainland south to Guatemala.

The Longfin Silverside is similar to, and can be confused with, the Broadstripe Silverside, Atherinella nesiotes (stripe wider than eye) and the Pitcher Silverside, Atherinella nepenthe (last dorsal fin ray over last anal fin ray).

From a conservation perspective the Longfin Silverside is currently considered to be of Least Concern with stable, widely distributed populations. They are very small in stature and  are of limited interest to most, however, they are an important food source for a wide variety of fish, marine mammals, and sea birds.