Adriatic sturgeon

Adriatic sturgeon
Adriatic sturgeon with sea lampreys in Sala maremágnum of Aquarium Finisterrae.
Conservation status
Critically endangered, possibly extinct in the wild  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Acipenseriformes
Family:	Acipenseridae
Genus:	Acipenser
Species:	A. naccarii
Binomial name
Acipenser naccarii Bonaparte, 1836

The Adriatic sturgeon (Acipenser naccarii) is a species of fish in the family Acipenseridae. It is native to the Adriatic Sea and large rivers in Albania, Croatia, Greece, Italy, Montenegro and Slovenia. Specimens can be seen at the Milan Aquarium, Aquarium Finisterrae, Aquarium of the Po, and the protected area of Oasis of Sant'Alessio in Lombardy. It is an elongated fish that can grow to a maximum length of about 2 m (6.6 ft), with an olive-brown back, paler flanks and whitish belly.

This fish is threatened by habitat loss and overfishing, especially the capture of immature fish that have not bred. Populations of the fish have dwindled and it seems to be no longer present in many of its previous habitats. The International Union for Conservation of Nature has rated its conservation status as "critically endangered", and it is likely to be functionally extinct in the wild, as no spawning has been confirmed in recent years. However, it breeds successfully in captivity and survives in commercial fish farms and captive breeding programs. One large specimen was caught, and afterwards released, in the Po River in 2015.

Description

The Adriatic sturgeon reaches a maximum length of about 2 m (6.6 ft) and a maximum weight of 25 kg (55 lb). Like other sturgeons it has an elongated body, a flattened rostrum, a cartilaginous skeleton, distinctive bony scutes, and an elongated upper lobe to its tail. The snout is broad and rounded, the lower lip has a central cleft and the four barbels are closer to the tip of the snout than they are to the mouth. The dorsal fin has no spines and 36 to 48 soft rays, and the anal fin has 24 to 31 soft rays. The dorsal colouring is olive-brown, the flanks are paler and the underside white.^[2][3]

Distribution

Head from above

Like most sturgeon, the Adriatic sturgeon is an anadromous fish and can be found at different periods of its life in freshwater and marine environments, including estuaries and brackish water. Historically it was to be found in the Adriatic Sea and the rivers flowing into it on either side. In 1932 its range in the sea was reported to be from Venice and Trieste to Greece and Corfu. It used to be present in the rivers Adige, Brenta, Bacchiglione, Piave, Livenza and Tagliamento. In the Po and its tributaries, it used to be present as far upstream as Turin. It also traditionally occurred in the Ticino and Adda rivers along the Albanian coasts, and in the rivers of Croatia, Bosnia-Herzegovina and Montenegro, including Lake Skadar. It was reported from Greece in 1977 but is no longer found there, and from Albania in 1997 in the Buna River, but has not been seen there since.^[1]

Ecology

A. naccarii at the Milan Aquarium

Sturgeon are slow-growing, long-lived fish and do not reach sexual maturity until they are fifteen to twenty years old. After the young fish have spent a period of growth in estuaries and coastal waters, they spend most of their lives in large rivers, foraging on the bottom for crustaceans and small fish which they suck up with their toothless, funnel-like mouths. Mature fish move upstream in spring to spawn in shallow, clear-water, gravelly areas. Many of the traditional spawning areas are no longer available to them because of the impoundment of rivers, and the only suitable remaining habitat for spawning is thought to be in the vicinity of the confluence of the River Po with its tributaries.^[1]

Status

The Adriatic sturgeon faces a number of threats such as pollution of rivers by industrial effluent and agricultural runoff and the fragmentation of their habitat by damming, which prevents them from moving upstream to suitable spawning areas. They are also caught legally or illegally and this is particularly harmful when smaller fish are taken before they have reached maturity and reproduced at least once. They also face competition from the wels catfish (Silurus glanis) which has expanded its range into Western Europe. As they have become fewer in number they are affected by the Allee effect which postulates that a fish's growth rate is reduced at low population densities. No spawning has been observed in the wild for a number of years and the International Union for Conservation of Nature has rated the conservation status of this fish as "critically endangered" and possibly "extinct in the wild".^[1] However, a number of organisations are concerned in attempts to preserve this species and a captive breeding scheme has been established, with young fish subsequently being released into the wild.^[1]

The captive broodstock for this programme comprises about 25 individual fish. The Adriatic sturgeon is a tetraploid fish (has four sets of chromosomes) and research based on mitochondrial and microsatellite information is being done on the present breeding stock and other young fish to establish how best to increase the genetic diversity of the fish used in the breeding programme, because the current broodstock has been shown to retain only part of the genetic variation present in the original stock.^[4][5] Despite the release of captive bred fish, no signs of spawning have been observed in the wild.^[1]

In November 2015 a large specimen, estimated to be about 2 m (6.6 ft) long, was caught in the River Po near Ostiglia (Mantova) and later released; it was thought to be an encouraging sign that the water quality of the river was sufficiently high for such a large fish to be living there.^[6]

References