Twaite shad

📖 Overview

Amphihaline species , schooling and strongly migratory, but apparently not penetrating far up rivers . Adults are usually found in open waters along the coast; juveniles are usually found along estuaries and near the shore , possibly making vertical diurnal movements synchronized with the tides; they remain in estuaries for over one year . Several landlocked (lake) non-migratory populations exist . Mostly anadromous, entering river mouths in March (Italy) or early June (northenr European rivers) to spawn in or above the tidal reaches; adults probably return to sea not long after spawning . Eggs are demersal and widely scattered among sand or gravel on river bed . Ichthyophagous, feeds on small fishes and crustaceans, the young taking the fry of herrings, sprats and gobies . Females grow faster and are always larger than males of the same age . Very locally distributed due to pollution and impoundment of large rivers throughout Europe and most populations declined during the first decade of the 20th century, but seem to have stabilized at a low level since then . It has been suggested that members of the genus Alosa are hearing specialists with the American shad (Alosa sapidissima) having been found to detect and respond to sounds up to at least 180 kHz . This may aid in predator avoidance (e.g. cetaceans) . Hybridization between this species and the allis shad (Alosa alosa) has been reported from the Rhine as well as rivers in France and Algeria . There is some evidence that indicates that shad hybrids may reproduce .

🔬 Taxonomy

🐟 Physical Description

Dorsal spines (total): 0; Dorsal soft rays (total): 16 - 22; Anal spines: 0; Anal soft rays: 19 - 26; Vertebrae: 49 - 59. Diagnosis: Body somewhat compressed, moderately deep with depth at pectoral fin less than head length, scutes apparent along belly . Upper jaw notched, lower jaw fitting into it; no teeth on palatine and vomer; gillrakers fairly short and stout, total 30 to 80, shorter than gill filaments . Usually a series of 4-8 black blotches behind gill opening, but sometimes a single blotch, the others faint or absent . Alosa fallax resembles Alosa alosa, which has more and longer gillrakers and at most only 3 dark spots on flank . : fusiform / normal; Cross section: compressed.

🌀 Behaviour

Behavioural data for Twaite shad is currently being reviewed.

🦐 Diet

🥚 Reproduction

Adults in the sea begin to congregate near mouths of estuaries in April. Enter estuaries and ascend rivers in May and June when water temperature is between 10-14 °C. Males begin such movements at 2-3 years, females at 3-4 years . Although movement upstream is usually limited to a few kilometres above the brackish zone, spawning has also been reported in non-tidal freshwater areas up to 400 km upstream . Gametogenesis occurs in the estuaries. Early arrivals in the rivers are mostly males, with the sex ratio becoming more equal with the later arrivals . Spawning movements occur with spring tides and peak when river discharge levels are high. However, when flows are too high, movements upstream become limited. Spawn when water temperature is anywhere between 12-22 °C . Move to riverine spawning grounds at night; spawn in large, very noisy schools near surface and leave these areas before daybreak . Spawning sites consist of sand and gravel areas with flowing water . Spent adults return to the sea and may spawn for 3-4 seasons throughout their lifetime. Most individuals will have lost 22 % of their body weight after spawning . There is some evidence that most individuals return to their natal rivers to spawn. Eggs either drift with the current or sink to the bottom . Eggs hatch after 2-8 days, depending on water temperature (optimal 15-25 °C). Larvae and juveniles move towards the estuaries and river mouths during their first summer and to the sea at the end of their second year . Males mature mainly between the ages of 2-5 years, females between 3-7 years . Length at maturity is between 30-40 cm total length .

🌍 Habitat & Distribution

Marine; freshwater; brackish; pelagic-neritic; anadromous ; depth range 10 - 400 m . Temperate; 66°N - 27°N, 25°W - 42°E Northeast Atlantic: Atlantic coasts from southern and western Iceland to northern Morocco (Ref.

🌿 Ecology

📐 Morphometrics

🌡️ Water Parameters & Tolerance

🛡️ Conservation Assessment

📊 Population Risk & Fisheries

🌐 Names & Taxonomy

🐠 Related Species

🔍 Similar Species

🗺️ Global Occurrence Map

Occurrence records from the Global Biodiversity Information Facility (GBIF) - 51 records indexed .

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