Fish Identification Photos: Pacific Thread Herring, Opisthonema libertate: The Pacific Thread Herring has a deep body, which has a depth that is 38 percent of its length. It is grey-green on the back with silver white sides. It has a long, filamentous ray originating from the last dorsal fin ray that almost reaches the caudal fin base.

The Pacific Thread Herring has long pectoral fins that reach the origin of the dorsal fin. It has a series of black dots that run along the middle of the first two-thirds of the body. The inner half of the caudal and dorsal fins is pale yellow, and the caudal fin is dark.

The Pacific Thread Herring can be easily confused with three other members of the Clupeidae Family found in Mexican waters: the Middling Thread Herring, Opisthonema medirastre (yellow stripe along flank, no spots along flank); the Slender Thread Herring, Opisthonema bulleri (body depth 31 percent of body length, short pectoral fins, yellow stripe along flank, no spots along flank); and the Threadfin Shad, Dorosoma peteneuse (black spot behind upper edge of gill cover, no spots along flank).

The Pacific Thread Herring reaches a length of 20 inches. It is normally found in the first 150 feet of the water column in small schools over sandy bottoms. The Pacific Thread Herring is a coastal pelagic species, found in Mexican waters south of Guerrero Negro along the Pacific side of the Baja California peninsula, throughout the Sea of Cortez, and south along the coast of the Mexican mainland to Guatemala. The Pacific Thread Herring appears to be absent from around the Mexican oceanic islands.

The Pacific Thread Herring is a member of the Clupeidae Family, which consists of Herrings, Menhadens, Pilchards, Piquitingas, Sardines, and Shads.

Pacific Thread Herring, Opisthonema libertate: Fish courtesy of Soriana's Mercado, Cabo San Lucas, Baja California Sur, Mexico, June 2008. Size approximately 14 cm. Identification courtesy of H.J. Walker, Jr., Scripps Institute of Oceanography, La Jolla, Calif. Description and photo courtesy of John Snow.