fishes

The good water quality of resurgences, the good food supply and the variety of these microenvironments support the presence of numerous species of fishes.
These are distributed along the water courses according to their needs for shelter, food and reproduction.
Six fishes are included in the Annexes of the Habitats Directive as species of Community interest and ten in the IUCN Red List of Italy as vulnerable or low-risk because of the destruction of riverine and resurgence habitats.

Species of Community interest included in the Annexes of the Habitats Directive:
• bullhead (Cottus gobio)
• italian soufie (Leuciscus souffia)
• italian barbel (Barbus plebejus)
• Po brook lamprey (Lethenteron zanandreai)
• marble trout (Salmo marmoratus)
• spined loach (Cobitis taenia)

Species included in the IUCN Red List of Italy:
• bullhead (Cottus gobio)
• italian soufie (Leuciscus souffia)
• italian barbel (Barbus plebejus)
• Po brook lamprey (Lethenteron zanandreai)
• marble trout (Salmo marmoratus)
• spined loach (Cobitis taenia)
• minnow (Phoxinus phoxinus)
• goby (Padogobius martensi)
• Po spring goby (Knipowitschia punctatissima)
• pike (Esox lucius)

In medium/fast stream waters, with gravel or sand bottom and with few vegetation, live the bullhead (Cottus gobio) and the italian soufie (Leuciscus souffia),two fishes that eat mostly invertebrates.
There are also two omnivorous species: italian barbel (Barbus plebejus) and minnow (Phoxinus phoxinus).
All these species lay their eggs under rocks and gravel to prevent the stream carries it away. The male of the bullhead nests under rocks and guards the eggs until they hatch. Barbel may become very large, others are of medium size fish, up to 15 cm long.
Except for the minnow, the other three species are included in the Annexes of the Habitats Directive and all included in the IUCN Red List of animals in Italy.

In this waters also live the Po brook lamprey (Lethenteron zanandreai).
In this environment were once common even the marble trout (Salmo marmoratus) and the grayling (Thymallus thymallus),.the first is a species endemic to the Po-Venetian plain included in the Annexes of the Habitats Directive and the Italian Red List of animals as endangered species.
Now, in the resurgences area, it has been replaced by brown trout (Salmo trutta), a species which, however, is in competition for food with the trout fed artificially for fishing purposes. The two species are crossed with each other so hybrids may found locally, a phenomena considered very negative and referred as "genetic pollution" by biologist because it causes the loss of the genetic heritage of the species.
The grayling is a species distinctly sensitive to water quality and pollution, this fact, combined with the lack of suitable areas for breeding has contributed to its depletion.

In the range of waters with slower stream and denser vegetation living omnivorous speciessuch as rudd (Scardinius erythrophthalmus), tench (Tinca tinca) and spined loach (Cobitis taenia).
These species lay their eggs by attaching to vegetation or the bottom. Among these fishes the tench reaches the larger dimension (no more than 30 cm), while the common loach does not exceed 12 cm. The loach is included both in the Annexes of the Habitats Directive and in the Red List of animals in Italy. 
Notable two other omnivorous species: chub (Leuciscus cephalus) and carp (Cyprinus carpio). Among the fishes that feed mainly on animal substances are the bleak (Alburnus Alburnus arborella), the european eel (Anguilla anguilla), the goby (Padogobius Martens), the Po spring goby (Knipowitschia punctatissima), the pike (Esox lucius) and the perch (Perca fluviatilis).
With the exception of eel, which reproduces the Atlantic Ocean, all other species spawn in these environments. Goby andPo spring goby nests are guarded by males. Goby, Po spring goby and pike are included in the IUCN Red List of animals in Italy.

Alien species
In the resurgences waters are also some fishes that were imported from other countries or continents for several reasons: for fishing purpose, in the waters have been released the brown trout, the rainbow trout (Oncorhynchus mykiss) and the catfish (Ictalurus melas); the sunfish (Lepomis gibbosus) has been imported for ornamental purposes from North America and released in natural waters.
Two other very small fish can be found in the waters of springs: the pseudorasbora (Pseudorasbora parva) from Asia, and Gambusia (Gambusia holbroki), introduced by the United States to fight malaria by eliminating the larvae of mosquitoes, of which feeds.
Alien species have a negative effect on populations of native fishes. They can become new predators of their eggs and juveniles, they may occupy the space or consume and destroy the food source.
Some species can interbreed with local ones and may be carriers of new diseases and new pests not previously present. That is why today the introduction of non-native species is forbidden in Italy.