Scardinius erythrophthalmus Linnaeus, 1758

In addition, rudds may compete with native fishes for food resources like aquatic macroinvertebrates and fish, but rudds are also capable of supplementing their diet with algae and detritus (Garcia-Berthou and Moreno-Amich 2000)—food sources not commonly utilized by native fishes in north temperate North America. The rudd’s ability to shift its diet based on food availability may be a substantial advantage over native species (Kolar and Lodge 2002; Shea and Chesson 2002; Marchetti et al. 2004), which may lead to declines in native species diversity and abundance. (Guinan et al., 2015)

The rudd Scardinius erythrophthalmus is an exotic cyprinid with an intriguing capability: it can hybridize with the native golden shiner Notemigonus crysoleucas. This hybrid is apparently the first known nonsalmonid intergeneric cross of a North American native and an exotic (Taylor et al. 1984). Genetic interchange between native and exotic species can lead to loss of genetic diversity and reduce the fitness of locally adapted populations (Courtenay and Robins 1989; Ferguson 1990). The capability of the rudd to hybridize with the golden shiner may pose a threat to the genetic integrity of the golden shiner genome in areas of potential contact. (Burkhead and Williams (1991)

Rudd were illegally released in widespread areas of the northern half of the North Island of New Zealandaround 1975, and in almost every instance, persistent breeding opulations became established. Frequently, rudd have been associated with macrophyte collapse and a switch from clear to turbid water. Not only do rudd find Nitella spp. and Potamogeton ochreatus, two key species in unmodified lake ecosystems, particularly palatable, but the ecological problems are likely to be compounded by the feeding habit of rudd. Typically rudd concentrate their razing effort on the growing apices of Egeria densa (Lake, 1998), which are the meristems on which plant growth is dependent (Crawley, 1983). ... they may attain high densities without an efficient predator such as pike (Esox lucius L.). Rudd may prove to be a significant additive factor causing macrophyte decline in New Zealand once macrophytes become stressed through perturbations such as decreased water clarity caused by suspended sediment or algal blooms. Also, our study shows that rudd have the capacity to modify plant communities by selectively removing palatable charophytes such as Nitella spp. ..... charophytes have been identified as a key component in the restoration of macrophyte communities attached to lake beds (De Winton et al., 2000). By direct suppression of plant regrowth, rudd in a devegetated lake are likely to inhibit or prevent reestablishment of charophyte vegetation from natural seed banks. (Lake et al., 2002)