The fish of the lakes and streams
There is not a great variety of fish species to be found in the streams and watercourses of the Park and the natural population has been altered for some time due to frequent stocking with farmed trout. In recent years, however, the Park has approved the Ecological Water Plan which contains specific management details for fish populations, information aimed at improving the current heritage, both with the protection of the few natural populations still present and by stocking with native strains of brown trout.
Since the 17th century, the waters of the Park Municipalities, have been subject to the exclusive rights. The Municipalities, periodically hold public auctions to administer the river lots. This practice, which has been consolidated for about fifty years, is an important and indispensable source of income for the municipal budgets, but over the years it has led to a qualitative impoverishment of the fish heritage due to continuous introduction of farmed trout, sought after by fishermen but of little value for a stream that would have all the characteristics to host ecologically well-structured biocenosis.
The brown trout (Salmo t. trutta), common in most Italian streams and in those of the Park, comes from a northern European strain, domesticated and bred. The introduction of brown trout into the Park waters has caused the disappearance or genetic contamination of most of the native trout, the macrostigma trout (Salmo trutta macrostigma) and the marble trout (Salmo marmoratus). These trout, Mediterranean strains, native to southern Europe and North Africa, but perfectly adapted to the alpine environment, were once widespread throughout the Tyrrhenian side of Italy, Sicily and Sardinia. Today, however, they are reduced to a few residual populations. In the Gesso basin, the macrostigma trout, locally called "queen's trout ", seems unfortunately extinct. Marble trout occupies a lower range than brown trout and, in the Park territory, its distribution has been disrupted and it is rarely found in pure form, since it can hybridize with brown trout.
The area of the bullhead (Cottus gobio) has also shrunk due mainly to the massive trout populations rather than to a deterioration in water quality which fortunately, in the Park, is excellent. The bullhead has a large head, large and well-developed ventral, dorsal and pectoral fins. It needs clear, fresh water, with an intense to moderate current, good oxygenation and stony and pebbly bottoms. This species is typically linked to the river bed; during the day it remains hidden under the stones or among the aquatic vegetation, activity increases in the twilight hours and takes place mainly during the night. It has territorial habits and, therefore, individuals are always spaced apart. It feeds on benthic invertebrates and on eggs or larvae of other species. There are occasions when it performs cannibalism.
Localized and certainly an introduced species is the American rainbow trout (Oncorhyncus mykiss). We also find the minnow (Phoxinus phoxinus), a European species introduced in some lakes as bait while fishing for trout.