Alosa

Alosa is a genus of fish, the river herrings, in the family Clupeidae. Along with other genera in the subfamily Alosinae, they are generally known as shads. They are distinct from other herrings by having a deeper body and spawning in rivers. Several species can be found on both sides of the Atlantic Ocean and Mediterranean Sea. Also, several taxa occur in the brackish-water Caspian Sea and the Black Sea basin. Many are found in fresh water during spawning and some are only found in landlocked fresh water. These fishes lived from the Eocene to Quaternary (from 55 million years ago to now). Fossils have been found in Canada, the United States, Greece, Kazakhstan, Azerbaijan, Hungary, Romania, and Italy. Alosa species are generally dark on the back and top of the head, with blue, violet, or greenish tints. However, some can be identified as having a grey or green back. Spots are commonly found behind the head, and the fins may vary from species to species or individually. Most species of Alosa weigh 300 grams (11 oz) or less, with one species, A. pontica, weighing up to 2 kg. Shads are thought to be unique among the fishes in having evolved an ability to detect ultrasound (at frequencies above 20 kHz, which is the limit of human hearing). This was first discovered by fisheries biologists studying a type of shad known as blueback herring, and was later verified in laboratory studies of hearing in American shad. This ability is thought to help them avoid dolphins that find prey using echolocation. Alosa species are generally pelagic. They are mostly anadromous or semianadromous with the exception of strictly freshwater landlocked species. Alosa species are generally migratory and schooling fish. Males usually mature about a year before females; they spawn in the late spring to summer. Most individuals die shortly after spawning. Alosa species are seemingly very adaptive vertebrates, so can change readily to adapt to their environments, as species are found in a variety of temperatures and waters. As Alosa species are generally anadromous, they are faced with various obstacles to survival. They may have to pass through numerous barriers and waters to get to either their spawning grounds or normal habitats (the sea in most cases). Estuaries are a major factor in numerous Alosa species' migration. Estuaries can be highly variable and complex environments contributing to fluctuating biological interactions, with shifts in osomolarity, food sources, predators, etc. Since many adult Alosa species die after spawning, the young generally have to migrate to the sea from the spawning grounds. Duration of migration varies among fish, but it can greatly affect survival. Reproduction varies by species. Studies done on Alosa in Iranian waters have shown that spawning varies in time, place, and temperature of the waters they inhabit. Fecundity may also vary. Species are known to spawn as early as April or as late as August. Temperatures range from about 11 to 27 °C. Fecundity can range from 20,000 to 312,000 eggs. Eggs are pelagic. The lifespan of Alosa species can be up to 10 years, but this is generally uncommon, as many die after spawning. The systematics and distribution of Alosa shads are complex. The genus inhabits a wide range of habitats, and many taxa are migratory. A few forms are landlocked, including one from Killarney in Ireland, two from lakes in northern Italy, and two in Greece. Several species are native to the Black Sea and Caspian Sea. Alosa species of the Caspian are systemically characterized by the number of rakers on the first gill arch. They are classified as being 'multirakered', 'medium-rakered', or 'oligorakered'. The multirakered are primarily plankton feeders, the oligorakered have large rakers and are predators, and the medium-rakered generally consume a mixed diet. Most current species of the genus Alosa in North America can be found in Florida, whereas the distribution of most of them is broader.