| Alosa | |
|---|---|
 | |
| Twaite shad,Alosa fallax | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Clupeiformes |
| Family: | Alosidae |
| Genus: | Alosa H. F. Linck, 1790 |
| Species | |
See text. | |
| Synonyms | |
Caspialosa | |
Alosa[2] is a genus offish, theriver herrings, in thefamilyAlosidae. Along with other genera in the subfamilyAlosinae, they are generally known asshads.[3][4] They are distinct from other herrings by having a deeper body and spawning in rivers. Several species can be found on both sides of theAtlantic Ocean and theMediterranean Sea. Also, several taxa occur in the brackish-waterCaspian Sea and theBlack Sea basin.[5] Many are found in fresh water during spawning and some are only found in landlocked fresh water.
These fishes lived from theEocene toQuaternary (from 55 million years ago to now). Fossils have been found in Canada, the United States, Greece, Kazakhstan, Azerbaijan, Hungary, Romania, and Italy.[6]
Alosa species are generally dark on the back and top of the head, with blue, violet, or greenish tints.[5] Some can be identified as having a grey or green back.[5] Spots are commonly found behind the head, and the fins may vary from species to species or individually.[5] Most species ofAlosa weigh 300 g (11 oz) or less, withA. pontica andA. fallax weighing up to 2 kg, andA. alosa can exceed 3–4 kg.[5]
Shads are thought to be unique among the fishes in having evolved an ability to detectultrasound (at frequencies above 20 kHz, which is the limit of human hearing).[7] This was first discovered by fisheries biologists studying a type of shad known asblueback herring, and was later verified in laboratory studies of hearing in American shad. This ability is thought to help them avoiddolphins that find prey using echolocation.Alosa species are generallypelagic.[8] They are mostlyanadromous or semianadromous with the exception of strictly freshwater landlocked species.[8]Alosa species are generally migratory and schooling fish.[8] Males usually mature about a year before females; they spawn in the late spring to summer.[9][10] Most individuals die shortly after spawning.[9][10]Alosa species seemingly can change readily to adapt to their environments, as species are found in a wide range of temperatures and waters.[10]
AsAlosa species are generally anadromous, they face various obstacles to survival.[11] 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).[11] Estuaries are a major factor in numerousAlosa species' migrations.[11] Estuaries can be highly variable and complex environments contributing to fluctuating biological interactions,[11] with shifts in osmolarity, food sources, predators, etc.[11] Since many adultAlosa species die after spawning, only the young generally migrate to the sea from the spawning grounds.[11] Duration of migration varies among fish, but can greatly affect survival.[11]
Reproduction varies by species.[5] Studies done onAlosa in Iranian waters have shown that spawning varies in time, place, and temperature of the waters they inhabit.[5] Fecundity may also vary.[5] Species are known to spawn as early as April or as late as August.[5] Temperatures range from about 11 to 27 °C.[5] Fecundity can range from 20,000 to 312,000 eggs.[5] Eggs are pelagic.[5] Geography and temperature are important environmental factors in egg and young-of-year development.[12]
The lifespan ofAlosa species can be up to 10 years, but this is generally uncommon, as many die after spawning.[5]
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| Commercial fish |
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| Large predatory |
| Forage |
| Demersal |
| Mixed |
The systematics and distribution ofAlosa shads are complex. The genus inhabits a wide range ofhabitats, and manytaxa aremigratory. A few forms are landlocked, including one fromKillarney inIreland, two from lakes in northern Italy, and two inGreece. Several species are native to the Black and Caspian Seas.Alosa species of the Caspian are systemically characterized by the number ofrakers on the first gill arch.[13] They are classified as being "multirakered", "medium-rakered", or "oligorakered".[13] The multirakered are primarily plankton feeders, the oligorakered have large rakers and are predators, and the medium-rakered generally consume a mixed diet.[13] Most current species of the genusAlosa in North America can be found in Florida, whereas the distribution of most of them is broader.[14]
Morphology is notoriously liable toadapt to changing food availability in these fish. Several taxa seem to haveevolved quite recently, making molecular analyses difficult. In addition,hybridization may be a factor in shadphylogeny.[15] Nonetheless, some trends are emerging. The North American species except theAmerican shadA. sapidissima can probably be separated in asubgenusPomolobus. Conversely, the proposed genus (or subgenus)Caspialosa for theCaspian Sea forms is rejected due toparaphyly.[15]
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| Commercial capture production ofwild shad in tonnes.[16][17] | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1999 | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2008 | 2010 | 2011 | 2012 | 2013 | 2014 |
| 788,770 | 860,346 | 665,284 | 589,692 | 524,800 | 569,160 | 605,548 | 588,978 | 645,977 | 611,371 | 604,842 | 628,622 | 636,678 |
Shad populations have been in decline for years due to spawning areas blocked by dams, habitat destruction, pollution, andoverfishing. Management of shad has called for more conservative regulations, and policies to help the species have lower fishing mortality.[18]
Shad serve a peculiar symbolic role inVirginia state politics. On the year of every gubernatorial election, would-be candidates, lobbyists, campaign workers, and reporters gather in the town ofWakefield, Virginia, forshad planking. American shad served as the focal point ofJohn McPhee's bookThe Founding Fish.[19]
Theroe, or more properly the entire engorged uterus of theAmerican shad—filled with ripening eggs, sautéed in clarified butter and garnished with parsley and a slice of lemon—is considered a great delicacy, and commands high prices when available.[20]
Principal commercialfishery species groups | ||||||||||||||||||||
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| Wild |
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| Farmed | ||||||||||||||||||||
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