Alosa fallax (La Cepède, 1803), Twaite shad (Clupeidae)

Author: Ronald Fricke, Germany

1. Description of the habitat/autecology of the species

The twaite shad Alosa fallax is a species that spends much of its life pelagically in the open water of coastal oceans, and anadromously migrates into large rivers for spawning. Adult fishes enter estuaries annually, and then from April to June they migrate upstream to the inland extent of the brackish water zone. First year adults just train the migration, but second year or older specimens spawn above sand and mud bottoms. Spawning occurs repeatedly during several nights, from May to June; the adults usually survive spawning and return in autumn to the sea. When hatching, larvae measure 4.25-6 mm, but juveniles have grown to 7-8 cm when they start their downstream migration into the sea in autumn. Twaite shad does not grow any further in winter; growth periods are in summer in their river habitat. They feed on aquatic insects, crustacea, fish larvae and young fishes (Fricke, 1987: 62; 2004: 25; Quignard & Douchement, 2004). Maximum total length 43 cm male), 5 cm (female); maximum body weight 1.5 kg; maximum individual age 8 years.

2. Distribution (past and present)

Distributed throughout the HELCOM area including adjacent rivers and streams. It has occurred only occasionally in the Baltic Sea since the 1800´s at least (Fries et al 1895). Outside the HELCOM area, twaite shad occurs from southern Norway to Morocco (now to south to Portugal). Local populations are genetically distinct.

3. Importance (sub-regional, Baltic Sea-wide, global)

Due to their genetic differentiation, Baltic Sea populations of twaite shad are considered of global importance in the HELCOM area according to the HELCOM (2007) definition.

4. Status of threat/decline

This species is threatened throughout its distribution range in the HELCOM area and beyond in other European waters, and has been regionally declining; some populations have started to slowly recover, but numbers are still far from the former abundance of the species. In a HELCOM assessment this species is classified as endangered (EN) according to IUCN criteria in the HELCOM area, and as a HELCOM high priority species (HELCOM, 2007). It was listed as critically endangered (CR) by Germany, as endangered (EN) by Denmark, Poland and Russia, as data deficient (DD) by Lithuania, and it was considered not applicable (NA) by Sweden and Finland since no reproduction occurs in these countries. The species is also included on Annexes II and V of the EU Habitats Directive. It is listed as data deficient on the IUCN global red list.

5. Threat/decline factors

Threatened by a variety of factors: it is a target species of fisheries in some areas, it is caught as by-catch in pelagic and river fisheries, and construction of river dams and weirs inhibit its spawning migration. The species is considered as rare in the northern parts of the HELCOM area and it is sensitive to human activities. Twaite shad is considered as a keystone species where it is still common.

6. Options for improvement

In the HELCOM area, twaite shad would benefit from a restoration of potential spawning sites and reduction of eutrophication in spawning rivers. No new obstacles for migration (weirs etc.) should be constructed in spawning areas of twaite shad. It may also be important to manage and restrict pelagic fisheries (including herring fisheries). In order to conserve the genetic differences of local populations, each population should be addressed separately when conservation measures are considered.

7. References

Fricke R., 2004.  Der Maifisch (Alosa alosa).  Offenbach am Main (Verband Deutscher Sportfischer), 39 pp.

HELCOM 2007. HELCOM Red list of threatened and declining species of lampreys and fish of the Baltic Sea. Baltic Sea Environmental Proceedings, No. 109, 40 pp.  Available in: http://www.helcom.fi/stc/files/Publications/Proceedings/bsep109.pdf

Quignard J.P. & Douchement C. 1991. Alosa fallax fallax  (Lacepède, 1803). Pp. 212-253.  In: Hoestlandt, H.: The freshwater fishes of Europe. Vol. 2. Clupeidae, Anguillidae.  Wiesbaden (Aula-Verlag), 447 pp.