Mediterranean trout

 

Mediterranean trout (Salmo Macrostigma)

Class: Actinopterygii
Order: Salmoniformes
Family: Salmonidae
Genus: Salmo

The trout is a wide-ranging species complex native to Eurasia and North Africa: the northernmost populations are in Iceland, Northern Scandinavia and Russia, while the southern limit is represented by the mountain streams of Northern Morocco; the species has also been introduced in several other locations around the world (North and South - America, Australia, etc).

Macrostigma trout has been considered a vulnerable species in Europe, critically endangered in Italy and it is included in the Red List category Near Threatened due to water abstraction, overfishing and stocking of non-native trouts (resulting in hybridization and competition). It is suspected that in the last 10 - 12 years the population has declined by almost 30%, and it will continue to do so as water abstraction will most likely increase.

Description of the species

Mediterranean trout is a medium size fish, which maximum dimensions rarely exceed 45-50 cm length and 1.2-1.5 kg weight.

This specie, that originally lives in Appennine watercourses, can be recognized on the basis of the following characters: 9-13 large ellipsoidal grayish spots along the middle region of each side, that are sometimes splitted or fragmented; roundish black spots on the sides, some of which feature a clear halo; large black spot in the opercular region; coloration of the adipose fin and small grayish spots or brown, never red. The populations living in the Adriatic region and the Nera river have a very large number of small black and red dots spread along both sides. There is no sexual dimorphism between males and females.

Feeding

The diet is mainly based on benthos; because of low trophic level of the Mediterranean environments the diet is composed mostly of terrestrial insects. There is a negative selection for some species of Ephemeroptera and Diptera and positive selection for species of Trichoptera. Moreover, it was also found that ephemeropteran species dominated spring and summer diets, while trichopteran species prevailed in winter. With age fish consumed significantly different prey taxa: plecopteran nymphs tended to increase in percentage as the individuals became older, trichopteran larvae were the most abundant prey in trout younger than 2+, while their percentage decreased considerably in older fish; terrestrial prey were consumed more by older individuals.

Reproduction

During the breeding season, which extends from December to April, trouts spawn in shallow and current waters characterized by a bottom with gravel and clear of submerged vegetation.

The spawning of gametes has been observed in December and January in Sicily; and in February and March in Lazio. 

Males are sexually mature when measuring 17-19 cm in length (average weight about 80g), females 28-30 cm (average weight 300 g); Sicily’s females are rather mature when measuring just over 20 cm in length and being 3 years old.

Habitat

This species requires clear waters and moderate currents with rocky and stony riverbeds. The macrostigma prefers water temperature ranging between 10° and 17° C and adapts to different environmental conditions.

The main threats

Macrostigma trout has been considered a vulnerable species in Europe, critically endangered in Italy and it is included in the Red List category Near Threatened due to water abstraction, overfishing and stocking of non-native trouts (resulting in hybridization and competition). It is suspected that in the last 10 - 12 years the population has declined by almost 30%, and it will continue to do so as water abstraction will most likely increase.

Loss of the species pureness features, hybridization and genic introgression. This issue has been highlighted for a long time.

In particular, during the 20th century massive stockings of brown trout with specimens of Atlantic origin were performed throughout European rivers. This practice has profoundly altered the genetic structure of the autochthonous populations of this species complex making the identification of native indigenous strains or the natural distributional range extremely difficult. The loss of the original genetic diversity is to be considered a serious threat. Indeed, the autochthonous gene pool, being the result of a long evolutionary history of this species, makes S. macrostigma populations fit to the local environmental conditions.