The shads or river herrings comprise the genus Alosa, fish related to herring in the family Clupeidae. They are distinct from others in that family by having a deeper body and spawning in rivers. Several species can be found on both sides of the Atlantic Ocean and Mediterranean Sea. Alosa can also be found throughout the Caspian Sea. 2] Many are found in freshwater during spawning and some are only found in landlocked freshwater.
Alosa are generally dark on the back and top of the head with blue, violet, or greenish tints. However, some species can be identified as having a grey or green back. Spots are commonly found behi nd the head and the fins may vary from species to species or individually. Most species of Alosa weigh 300 grams or less with one species, Alosa pontica, weighing up to 2 kilograms.
Shads are thought to be unique among the fishes in having evolved an ability to detect ultrasound (sound 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 are generally pelagic.They are mostly andromous or semiandromous with the exception of strictly freshwater landlocked species. Alosa are generally migratory and schooling fish. Males will usually mature about a year before females and spawn in the late spring to summer months. Most individuals die shortly after spawning.Alosa are seemingly very adaptive vertebrates and can change readily to adapt to their environment as species are found in a variety of temperatures and waters
Life cycle and reproduction
As Alosa are generally anadromous, they are faced with various obstacles to survive.  They may have to pass through numerous barriers and waters to get to either their spawning grounds or normal habitat (the sea in most cases). Estruaries are a major factor in numerous Alosa species migration. Estruaries can be highly variable and complex environments contributing to fluctuating biological interactions. There are shifts in osomolarity, food sources, predators, etc. Since many adult Alosa species die after spawning, it is the young who generally have to migrate to the sea from the spawning grounds. Duration of migration varies among fish but studies have shown that duration of migration can greatly affect survival.
Reproduction varies from species to species. Studies done on Alosa in Iranian waters have shown that spawning in Alosa varies in time, place, and temperature of the waters they inhabit.Fecundity may also vary.It is observed that species will spawn as early as April or as late as August. Temperatures range from approximately 11 to 27 degrees Celsius. Fecundity can range from approximately 20,000 eggs to 312,000 eggs. Eggs are pelagic.
The life span of Alosa can be up to 10 years but is generally uncommon as many die after spawning.
Systematics of shads is complex. The genus inhabits a wide range of habitats, and many taxa are migratory. There are also a few land-locked forms, one from Killarney in Ireland and two from lakes in northern Italy. There are species native to the Black Sea and Caspian Sea, as well as the Persian Gulf. Alosa species of the Caspian are systemically characterized by the number of rakers on the first gill arch by some scientists. They are classified as being "multirakered," "mediumrakered," or "oligorakered." The multirakered are primarily plankton feeders, the oligorakered have large rakers and are predators, and the mediumrakered generally possess a mixed diet. Most current species of the Alosa genus in North America can be found in Florida, although Florida may not be the only place that these species are found. 
Morphology is notoriously liable to adapt to changing food availability in these fish. Several taxa seem to have evolved quite recently, making molecular analyses difficult. In addition, it appears as if hybridization is a factor to be reckoned with when researching shad phylogeny.
- The Founding Fish (2002) ISBN 0-374-10444-1.
Nonetheless, some trends are emerging. The North American species except the Atlantic shad can probably be separated in a subgenus (or even genus) Pomolobus. On the other hand, the proposed genus (or subgenus) Caspialosa for the Caspian Sea forms is rejected due to paraphyly.
- Alosa aestivalis (Mitchill, 1814) (Blueback shad)
- Alosa agone (Scopoli, 1786) (Agone)
- Alosa alabamae D. S. Jordan & Evermann, 1896 (Alabama shad)
- Alosa algeriensis Regan, 1916 (North African shad)
- Alosa alosa (Linnaeus, 1758) (Allis shad)
- Alosa braschnikowi (Borodin, 1904) (Caspian marine shad)
- Alosa caspia (Eichwald, 1838)
- Alosa caspia caspia (Eichwald, 1838) (Caspian shad)
- Alosa caspia knipowitschi (Iljin, 1927) (Enzeli shad)
- Alosa caspia persica (Iljin, 1927) (Astrabad shad)
- Alosa chrysochloris (Rafinesque, 1820) (Skipjack shad)
- Alosa curensis (Suvorov, 1907) (Kura shad)
- Alosa fallax (Lac?p?de, 1803) (Twait shad)
- Alosa immaculata E. T. Bennett, 1835 (Pontic shad)
- Alosa kessleri (Grimm, 1887) (Caspian anadromous shad)
- Alosa killarnensis Regan, 1916 (Killarney shad)
- Alosa macedonica (Vinciguerra, 1921) (Macedonia shad)
- Alosa maeotica (Grimm, 1901) (Black Sea shad)
- Alosa mediocris (Mitchill, 1814) (Hickory shad)
- Alosa pseudoharengus (A. Wilson, 1811) (Alewife)
- Alosa sapidissima (A. Wilson, 1811) (American shad)
- Alosa saposchnikowii (Grimm, 1887) (Saposhnikovi shad)
- Alosa sphaerocephala (L. S. Berg, 1913) (Agrakhan shad)
- Alosa tanaica (Grimm, 1901) (Azov shad)
- Alosa vistonica Economidis & Sinis, 1986 (Thracian shad)
- Alosa volgensis (L. S. Berg, 1913) (Volga shad)
Shad populations have been in decline for years due to spawning areas blocked by dams, habitat destruction, pollution, and overfishing. Management of shad has called for more conservative regulations, as well as policies to help the species obtain a lower fishing mortality.
Shad serve a peculiar symbolic role in Virginia state politics. On the year of every gubernatorial election, would-be candidates, lobbyists, campaign workers, and reporters gather in the town of Wakefield, Virginia for shad planking. American shad served as the focal point of John McPhee's book The Founding Fish.iv class="thumbcaption">roe