The taxonomy of shads inhabiting the Mediterranean Sea basin has not been fully-resolved.

In particular, the Mediterranean Shad (Alosa agone) was formerly considered endemic to Alpine lakes in Italy and Switzerland, but is currently treated as widespread in northern and eastern parts of the Mediterranean basin (Freyhof pers. comm.).

Published references to A. fallax in the northern and eastern Mediterranean region thus exclusively refer to the current taxonomic concept of A. agone. This requires further investigation, since more than one species may be present (Freyhof pers. comm.).

The Twaite shad's global population has declined significantly since the mid-20^th century, during which period a number of subpopulations have been extirpated. This trend potentially continues over some parts of its extensive range, but there is evidence of partial recovery in the Baltic Sea, northwestern Germany and northeastern Spain. It is currently assessed as Least Concern.

This species is native to the northeastern Atlantic Ocean, where its range extends from Norway to Morocco, including the British Isles, Germany, the Netherlands, Belgium, France, Spain and Portugal, plus coastlines of the southern Baltic Sea in Germany, Poland, Russia (Kaliningrad) and Lithuania. Records from coastal waters of Estonia, Latvia and Finland probably correspond to non-breeding individuals.

It also occurs in the northwestern Mediterranean Sea, from the Guadiaro to the Fluvià rivers in Spain. Vagrant individuals have occasionally been recorded in southern Iceland since the mid-1990s, and in 2010 a single individual was caught off the Azores archipelago.

This species experienced a significant global population decline during the 20^th century, during which period all spawning subpopulations disappeared from Morocco, Mediterranean Spain, the Baltic Sea, Sweden, the Netherlands, Belgium, and Luxembourg. It was extirpated from the Rhine River, Germany, during the mid-20^th century, but spawning subpopulations still exist in the Elbe and Weser rivers, and possibly elsewhere.

Although it was once widespread in the British Isles, recent records suggest it currently spawns only in the Blackwater, Barrow, Nore and Suir rivers in Ireland, plus the Severn, Wye, Usk and Tywi rivers in England and Wales. In France it may no longer spawn in the Seine River, but still enters the Orne, Aulne, Blavet, Vilaine, Loire, Gironde, Adour and Nivelle. It has been extirpated from most rivers along the Atlantic coast of the Iberian Peninsula, with spawning subpopulations remaining in the Ulla and Minho rivers, and potentially in the Lima, Mondego, Tejo, Guadiana and Guadalquivir systems.

In larger rivers some adults historically spawned several hundred kilometres upstream, but this is no longer possible in the majority of cases (see 'Threats'). The respective sizes of most extant subpopulations are therefore likely to have been significantly depleted due to a reduction in available spawning sites. Nonetheless, a gradual recovery has been reported in some areas since the early 2000s, including the lower Ebro River in Spain, lower Elbe River in Germany, and the Curonian Lagoon in the Baltic Sea.

Quantifying the majority of subpopulation trends, and by extension the global trend, has however been complicated by a lack of baseline data, assumed natural fluctuations, and because Twaite shad is not typically separated from the similar-looking Allis shad (Alosa alosa) in fisheries reports.

This euryhaline species is anadromous and mostly iteroparous. It inhabits marine coastal waters and estuaries for most of the year, but migrates to the lower and middle reaches of rivers to spawn in freshwater. It can reach a maximum standard length of 55 cm and weight of approximately 1.5 kg, with males reaching sexual maturity at approximately 3 years of age and females 5 years.

In freshwater, adults feed on invertebrates and smaller fishes, while juveniles prey primarily on insect larvae and zooplankton. At sea, the Twaite shad feeds on mysids and small fishes, and tends to remain on continental shelves at relatively shallow depths.

The spawning migration into rivers begins between April (southern subpopulations) and May (northern subpopulations), and is dependent on water temperature. Reproductive individuals display a high degree of fidelity to their natal rivers and tend to return to them in multiple years. Spawning generally takes place at night in flowing stretches over substrates ranging from mud to gravel at depths of 0.15-9.5 m. Adults return to marine waters post-spawning. 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.

Studies have shown that this species forms natural hybrids with the congeneric Allis shad (Alosa alosa) at some locations, and it has been suggested that this phenomenon has recently escalated (see 'Threats').

This species' global population decline has been attributed to a combination of factors. The main drivers are associated with deteriorating water quality, fluvial channelisation, altered sediment loading, and construction of dams and other barriers in rivers, with the latter impeding the ability of migrating adults to reach their preferred spawning grounds. In some river systems, overfishing is also considered to have played a major role.

Furthermore, genetic introgression via hybridisation with the congeneric Allis shad (Alosa alosa) threatens some 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 due to artificial obstructions, there is strong evidence to suggest that they are forced to share spawning grounds, resulting in higher proportions of hybrids.

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 formerly supported a number of large-scale fisheries, which were timed to coincide with its annual spawning migration. It is now exploited commercially only in France and Lithuania, but remains a regular constituent of bycatch in marine and estuarine fisheries elsewhere.

This species is protected at the national and regional scales throughout the European portion of its range, having been listed under Appendix III of the Bern Convention (as Alosa fallox) plus Annexes II and V of the European Union Habitats Directive. However, at time of writing there appear to be no specific conservation management efforts ongoing anywhere within its range.

Nevertheless, partial stock recoveries have been documented in a handful of populations since the early 2000s (see 'Population'), and it is possible that these have been aided by improved water quality due to implementation of the European Water Framework Directive, coupled with the loss of traditional fisheries. However, it cannot be discounted that changing environmental conditions, such as reduced salinity due to anthropogenic activity in the lower stretches of rivers (e.g., the Elbe River, Germany), or increased water temperatures due to climate change, have favoured an increase in reproductive success.

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 Twaite and Allis shad should ideally be taken into account within the framework of future management efforts. This species occurs within the boundaries of numerous protected areas of differing designation and size.