Gulf Grunion, Leuresthes sardina
Gulf Grunion, Leuresthes sardina. Fish caught from coastal waters off Puerto Peñasco, Sonora, November 2020. Length: 8.2 cm (3.2 inches). Catch, photograph and identification courtesy of Luke Ovgard, Klamath Falls, Oregon.
Gulf Grunion, Leuresthes sardina. Fish caught from coastal waters off Puerto Peñasco, Sonora, November 2022. Length: 12 cm (4.7 inches). Catch and photographs courtesy of Steve Wozniak, Alamo, California.
Gulf Grunion, Leuresthes sardina. Two fish caught by H.J. Walker, Jr., Scripps Institution of Oceanography, La Jolla, California off the beach in Bahia de Los Angeles (LA Bay), Baja California, October 2005. Length: 20 cm (7.9 inches) each.
The Gulf Grunion, Leuresthes sardina, is a member of the New World Silverside or Atherinopsidae Family, and is known in Mexico as pejerrey sardina. Globally, there are two species in the genus Leuresthes, both found in Mexican waters of the Pacific Ocean.
The Gulf Grunion has a long slender compressed body with a deeply forked tail. They are silvery overall being gray dorsally and silvery ventrally. They have a pale spot between their eyes and a broad silvery stripe on their sides. Their head is compressed and has a straight profile, small eyes, minute teeth, and a wide and strongly extendable top jaw. Their anal fin originates slightly behind their first dorsal fin and has 25 rays; their first dorsal fin has 4 spines and a low profile; the second dorsal fin has 1 spine and 9 rays; and their pectoral fins are short. Their body is covered with large smooth scales.
The Gulf Grunion is a non-migratory coastal fish found inshore in large schools at depths up to 5 m (16 feet). They reach a maximum of 25.0 cm (9.8 inches) in length. They consume plankton. They are preyed upon by various fish and sea lions. Their eggs are a favorite prey of shore birds and various small terrestrial animals.
The Gulf Grunion is a major scientific curiosity as they have a unique breeding strategy. Reproduction is oviparous with breeding occurring between April and August on the highest tides of the second, third, and fourth nights following either a new moon or a full moon. Fish surf the large waves well up onto the beach just above the high tide line and each female digs herself tail first into the sand and lays a clutch of eggs in the wet sand, which are fertilized by one or more males who then quickly retreat to the sea. The complete process requires less than two minutes. The eggs are bright orange but gradually change to sand color; they hatch in about fifteen days following the next new moon or large tidal episode. Juveniles rapidly develop reaching sexual maturity within one year. This reproduction cycle is repeated four to eight times a year. The oceanic lifestyle of the Gulf Grunion is not well understood. They are short-lived and have a maximum lifespan of four years.
The Gulf Grunion is a resident of Mexican waters of the Pacific but has a limited distribution being found only in the northern half of the Sea of Cortez.
The Gulf Grunion is similar to and can be confused with the California Grunion, Leuresthes sardina (non-resident of the Sea of Cortez) and several silversides and smelts, which have their first dorsal fins inserted well before their anal fin.
The Gulf Grunions are of limited interest to most. Due to their decreasing populations, limited range, and shallow water existence, they have been classified as Near Threatened from a conservation perspective. They are strongly affected by beach erosion, coastal construction, foot traffic, artificial lights, and pollution, all of which have a major negative impact on the spawning of the Gulf Grunion. There are no conservation measures currently in place to protect this species. Efforts to introduce the Gulf Grunion to the Sea of Cortez in 1951 failed.