The native Mediterranean trout – Salmo macrostigma
The Mediterranean trout S. macrostigma is a medium-sized fish and, despite its maximum size rarely exceeding 45-50 cm in length and 1.2-1.5 kg in weight, its size is closely linked to environment in which it grows. In mountain environments and small streams it can reach reproductive size already at less than 18 cm, while in the main course of larger and more productive rivers such as the Volturno and Biferno it can reach and exceed 60 cm.
The livery of Mediterranean trout can be very different depending on the population. In general the species is recognized thanks to:
- 9-13 large ellipsoid blue-greyish spots (parr spots) along the median region of each side, sometimes divided or fragmented and/or zebra-striped on the side of the fish consisting of the alternation of four darker bands and three lighter bands (in some populations the parr spots are the most evident, in others, with marked zebras, they are almost not noticed);
- a large blue-black spot in the opercular region;
- no spots on the back;
- absence of obvious light halos around the speckling;
- absence of an evident black and white band at the anterior margin of the dorsal and anal fins.
In the project area, above all belonging to the Volturno river basin, many individuals have a very evident typical tiger.
Pitting can be mainly black or red, depending on whether the population lives in mountain streams (usually predominantly red) or on the valley floor (usually predominantly black). However, several environmental characteristics are involved, including temperatures, the slope of the river bed, water turbulence, the presence of aquatic plants, type of substrate and others.
The sexual dimorphism between males and females is usually less pronounced than in the North European trout populations of Atlantic origin.
The original area of the native Mediterranean trout in Italy is represented by almost all the island and Apennine peninsular hydrographic basins and by the mountain streams of the western sector of the Alps. Its current distribution area is very limited due to habitat degradation and above all from hybridization with the S. trutta of northern European origin introduced widely throughout Italy for several decades.
Generally the Mediterranean trout feeds on macrozoobenthos and terrestrial insects. Older individuals can turn to eating habits that are mainly ichthyophagous.
Habitat and reproduction
The S. macrostigma lays the eggs in running and shallow waters characterized by a gravel bottom. The deposition of gametes does not occur anywhere in the same period: in Sicily it was observed in December and January, in Lazio in February and March, while in Molise and Campania from December to March.
This species, able to adapt to different environmental conditions, requires clear and oxygenated water, preferably with a temperature below 17 ° C, on rocky or stony substrates.
The species in the Nat.Sal.Mo project
Due to a repeated and abundant introduction of trout of Atlantic origin, in Italian territory today there is only a small portion of salmonid populations with recoverable genetic integrity levels. In the guidelines for the protection of salmonids it is believed that, where possible, it is necessary to protect genetic variability by recovering indigenous populations rather than reintroducing the species with specimens from other basins.
Specifically, in Molise preliminary investigations in project areas have recently highlighted the presence of recoverable populations that show different degrees of genetic introgression (between 10% and 40%).
In the project area there are at least two main distinct populations, which inhabit the opposite basins of the Apennine watershed:
– a population that inhabits the main course of the Biferno river and that, like other populations of the Adriatic peninsular basins, shares the same haplotype (ie the mitochondrial DNA control region) of the species S. marmoratus, a taxon originating from Alpine rivers at the bottom of the valley;
– a particular population that lives in the Volturno river basin (Tyrrhenian side), characterized by a marked stripes along its sides.
Both populations have a migratory behavior, but we still know little about migration routes: preliminary studies suggest that during the breeding season most adults leave the trophic area to reach, after several km of migration, the water from the Bojano and Rio Santa Maria springs.
Also in the main course of the Volturno river a similar behavior is observed, and if in this case the migration route is less clear, the reproducers probably make an even longer route. The purest populations seem to be those of the main courses, while in minor tributaries the introductions seem to have totally compromised the native populations. During the course of the project we will discover if, in addition to the heart of the native populations of Molise, constituted by the two main courses, some more or less isolated trout nucleuses still survive that preserve the genetics and the original characteristics.