The Adriatic Sturgeon (Acipenser naccarii)

 

Geographic Distribution

The species is endemic in the basin of the Adriatic Sea. Historically the species was present in the sea from Venice and Trieste, to Greece and Corfù. It was recorded in the rivers Adige, Brenta, Bacchiglione, Livenza, Piave, Tagliamento, and Po (and its affluents). Currently the species is restricted to the Adriatic Sea area and its tributaries between Po (Italy) and Buna (Albania) drainages. In particular specimens have been recorded in the Po River and its inflow rivers (Ticino, Adda, Oglio, Mincio), and in the rivers Adige, Livenza, Piave, Tagliamento.

 Habitat

The Adriatic Sturgeon is a long-lived, anadromous species.

The Adriatic Sturgeon lives in large rivers where, in the past, reproduction occurred from May to July. It spawns in freshwater after a marine period of growth during which it remains near the shore (at the mouths of the rivers), of preference on muddy and sandy bottoms at a depth of 10–40 m., not entering pure marine waters.

The upstream migration into Italian rivers occurs during the first months of the year.

Some time the sturgeon is able to adapt to an entire life cycle in fresh water, giving rise to landlocked populations. Its ecological value in inland waters appears to be discreet.

 Biology

The Adriatic Sturgeon is a fish of great size: the maximum length is 200 cm, though rarely exceed 140-150 cm and weight 25-30 kg, as well as for the size, it is distinguished from congenerico Acipenser sturio for the different profile of the head, viewed dorsally or ventrally, has more rounded profile.

In Acipenseridi there are three different modes of carrying out the life history: species or populations that perform the entire cycle in freshwater; species or populations that breed in freshwater and lingers long in inland waters (accrescendosi in the brackish waters of estuaries); species or populations that breed in fresh water and quickly reach the sea after spawning of gametes. Some species, such as Huso huso, adopt one of the three alternatives. Other species, such as various kinds of Acipenser, take two or all three of the possible alternatives, being composed of populations living in different ways in different parts of the territory. The sturgeon belongs to the second group, but some people might show trends and the ability to live like Acipenseridi the first group. The presence of specimens in the Po upstream of the dam at Isola Serafini, difficult to overcome by individuals in the process of recovery, seems to be an argument in favor of this hypothesis.

The diet of specimens analyzed in the Rivers Po and Ticino, with a length between 30 and 130 cm, includes exclusively benthic invertebrates: gammarids crustaceans (43%), larvae of Diptera (24%, mainly chironomids) oligochaetes (21%), asellidi (5%). The larger specimens also feed on fish.

The growth, considered in the same river environments on specimens of total length between 16 and 164 cm (0.016 to 26.8 kg), it is very fast: 3 years is reached, the length of 90-110 cm, 5 years the length of 120-150 cm. Different results are known for the final stretch of the Po, where it seems that the length of a meter (8-9 kg) is reached only about 10 years of age. In each population the females are in greater numbers than males. Sexual maturity appears to be reached in males at 6 years (longer than 80 cm) and in females at the age of 9 (length greater than 90 cm). There is no sexual dimorphism.

The reproduction takes place from May to July, but may also extend to early summer. The deposition of gametes occurs in still water or moderately currents near the banks of gravel substrates with discrete oxygenation. Each female ovulates every 2-4 years and the complete deposition of the eggs takes place over a period of twelve hours, with a series of subsequent emission. Each female can lay a quantity of eggs that varies between a few hundred thousand to 5 million.

In the '50s it had been suggested that occasional depositions may also occur in brackish waters not far from the sea, but it was rejected in the light of the results of studies on the adaptation to brackish water and its effects on growth, on the respiratory metabolism and swimming ability of young specimens, carried out by McKenzie et al. (2001), which show only a partial adaptation of the subjects to hyperosmotic environments. Studies carried out by several authors have demonstrated that the maximum tolerable salinity of young sturgeon seem not exceed the value of 20 per thousand (Cataldi et al., 1995; Cataldi et al., 1998). Studies carried out, however, in Spain in 2000 on young fish, have shown the ability to survive in environments with salinity of 35 per thousand thanks to a series of compensatory mechanisms of osmoregulation (Martinez-Alvarez et al., 2002).

 Conservation Status

The species is assessed as Critically Endangered (Possibly Extinct), on the basis of an estimated population decline of greater than 80% (possibly 100%) in the past three generations (60 years). This population decline is based on overfishing (both legal and illegal), and in particular of pre-reproductive sized fish. It is also threatened by the creation of barriers to its migratory routes, which reduce its reproductive success; and fragmentation of populations, particularly by dams for hydropower on the Po River (Isola Serafini's Dam, Piacenza). Water pollution, the Allee effect, and competition for habitats with allochthonous species (Silurus glanis) also threaten the future survival of this sturgeon..

The remaining potential suitable spawning grounds are restricted to very few areas in the Po River. There may still be some wild individuals left, but it is unknown how many there may be; potentially there are less than 250. Without continuous re-stocking the survival of this species is doubtful as continued successful reproduction in the wild can no longer be confirmed.

Recent surveys on the Ticino River have revealed the existence of a rarefied population and currently in decline, though in a good state of health (Graia Ltd., 2004) and adapted to complete their life cycle without having increased in the sea. Urgent concrete measures have recently been approved by LR and prepared as a result of two separate projects aimed Life Nature conservation of the species. In all regions of the Po basin fishing for this species is prohibited throughout the year.

 

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