The Allis Shad (Alosa alosa) remains extant throughout much of its natural range. However, the global population has declined significantly since the mid-20th century (including by c.80% in the last 20 years), and some subpopulations have already been extirpated. It continues to spawn in at least 30 river systems, but is currently abundant only in the French Atlantic basin where it has declined sharply since the early 2000s. There are a series of ongoing threats, and no evidence that the overall decline has been abated despite conservation efforts leading to partial recovery in some countries. It is therefore assessed as Critically Endangered under Criterion A.

This species is native to the northeastern Atlantic Ocean, where its range extends from Norway to Morocco, including the Atlantic coastlines of the British Isles, Germany, the Netherlands, Belgium, France, Spain and Portugal. It is also known to enter the southern Baltic Sea and western Mediterranean Sea at least until the Ebro River in Spain.

Historic records from the Mediterranean coasts of France and northwestern Italy are considered doubtful. Vagrant individuals have been recorded regularly in southern Iceland since the mid-1990s.

It is currently rare within most of this range, with the largest remaining subpopulations located in southern France (Seine, Loire and Gironde rivers) and Portugal (Minho, Lima and Tagus rivers). Recent records from the British Isles suggest it now migrates only to the Blackwater, Suir, Barrow and Slaney rivers in Ireland, plus the Tamar and lower Severn rivers in England and Wales.

This species has experienced a significant global population decline since the early 20^th century, and it has been extirpated from much of its former range.

The largest remaining subpopulations in France, which reproduce in the Dordogne, Garonne and Loire rivers, have plummeted by at least 80% since 2003 due to overfishing in estuaries and a possible demographic Allee effect on reproductive dynamics. Conversely, there is rudimentary evidence of a limited population recovery in the Seine River. In Germany it had virtually disappeared from the Rhine River, up which it formerly ascended at least 850 km to Basel, Switzerland, by the late 1940s (but see 'Conservation'). In the Iberian Peninsula it is estimated to have declined by c. 90% since the mid-20^th century, with the last stable subpopulation restricted to an 80 km stretch of the lower Minho River in northern Portugal and western Spain. It no longer breeds in Morocco or Mediterranean Spain.

Smaller subpopulations throughout the species' range are also reported to be in decline. Many of these are tiny and it is suspected that some might only remain extant due to recruitment of vagrants from larger populations, although studies suggest that gene flow primarily takes place between neighbouring subpopulations. However, quantifying subpopulation trends, and by extension the global trend, has generally been complicated by a lack of baseline data, assumed natural fluctuations, and because Allis shad is not typically separated from the similar-looking Twaite shad (Alosa fallax) in fisheries reports.

The Allis Shad is euryhaline, anadromous and semelparous, typically inhabiting offshore coastal waters and estuaries but migrating up rivers to spawn, with most individuals dying after a single reproductive episode. A landlocked subpopulation exists in at least one man-made lake in Portugal.

At sea it feeds predominantly on plankton, while in freshwater adults do not feed and juveniles prey on insect larvae. While offshore this species is pelagic and occurs in deep water down to 300 m. Juveniles remain close to the shore and often frequent estuaries.

Adults migrate into freshwater rivers and spawn in main channels or large tributaries, often near a confluence. Males first migrate at 3-9 years of age, and females 1-3 years later, with average generation length estimated to be 7 years.

Adults start approaching coastlines at the end of February and enter rivers when temperatures reach 10-12°C, from February (southern subpopulations) to April (northern subpopulations). Males tend to migrate slightly earlier than females. Spawning occurs in shallow, fast-flowing freshwater over substrates of cobbles, gravel or pebbles, and commences when temperature reaches 15°C, with a range of 22-24°C optimal. Breeding adults form large, often noisy, schools near the surface at night. Eggs drift in the current until they become caught in the substrate. Juveniles move downstream to river mouths and estuaries during summer and autumn, where young-of-the-year remain for several months before migrating to sea.

Mature individuals are believed to return to their natal spawning sites, and genetic studies have revealed significant differences between populations. Studies have shown that this species forms natural hybrids with the congeneric Twaite Shad (Alosa fallax) at some locations, and it has been suggested that this phenomenon has recently escalated.

The Allis shad's decline has primarily been driven by overfishing, pollution, altered sediment loading, and construction of dams and other barriers. In particular, the latter restricts access to spawning sites because the species lacks the physical ability to to pass obstacles exceeding c. 1 m in height.

Extraction of gravel from river beds has also reduced the extent and quality of its preferred spawning sites.

Furthermore, genetic introgression via hybridisation with the congeneric Twaite shad (Alosa fallax) threatens a few subpopulations as an additional outcome of barrier construction. Under natural conditions the two species are reproductively isolated on a spatial basis, since Allis shad spawns much further upstream than Twaite shad. However, when migration is blocked by artificial obstructions there is strong evidence to suggest that they are forced to share spawning grounds, resulting in a greater proportion of hybrid offspring.

Shifting water temperatures due to climate change may in the future render some river systems unsuitable for spawning in the south of this species' range, while increasing its success further north.

This species was formerly exploited as a food fish throughout its range, but most fisheries have collapsed. It is now harvested commercially only in France, Portugal and Galicia (Spain), where it remains among the most economically-valuable species targeted by inland fisheries. In most other countries within its range, it is a regular component of bycatch in marine and estuarine fisheries, especially during the annual spawning migration. Small numbers are also caught by trawls and purse seines in the Eastern Central Atlantic.

The Allis shad is listed under Appendix III of the Bern Convention, Annexes II and V of the European Union Habitats Directive, and included on the Commission for the Protection of the Marine Environment of the North-East Atlantic's list of threatened/declining species.

Specially-designed fish passes and elevators in France permit access to spawning sites, where a moratorium to prevent overfishing in the Gironde drainage has been in place since 2008. Two European Union-funded LIFE projects to restock the Rhine River ran from 2007-2015, and involved the collection and artificial (hormone-induced) spawning of adult individuals from the Gironde at an aquaculture facility in France, with the larvae released into backwaters and tributaries of the Lower and Middle Rhine, mostly in western Germany. This came after significant efforts were made to improve accessibility for migratory fishes, including the construction of fish passages at a number of weirs. A total of 10.66 million larvae with marked otoliths were released between 2008-2014. In spring 2014 small numbers of adults bearing the marks were observed migrating upstream and young-of-the-year individuals were later recorded 100 km upstream of any of the stocking locations. However, the declining numbers of fishes suggesting the project was potentially not successful. Long-term monitoring is reportedly in place to estimate the size of the restocked population as it evolves.

Elsewhere, it is suggested that the status of individual populations should be assessed against targets established from baseline surveys of each of the four main life-stages, i.e., spawning distribution, spawning success, density of juveniles and adult run size, although the practical considerations and costs associated with such extensive sampling approaches could prove prohibitive. Interspecific interactions and gene flow between Allis and Twaite shad should ideally be taken into account within the framework of future management efforts.