Hypophthalmichthys nobilis
The bighead carp (*Hypophthalmichthys nobilis*), also known as "spotted busa", is the giant of the group of Asian fish introduced to Romania. It is an impressive pelagic fish that reaches considerable weights in record time. Unlike the silver carp, the bighead carp is specialised in consuming zooplankton, having high economic value due to the quality of its flesh and the large dimensions it attains.
It is distinguished from the silver carp by its much more voluminous head (hence the English name "bighead") and by its colouration: the body is dotted with numerous irregular spots of dark grey or blackish colour on a silver-marbled background. The eyes are positioned low, as in the silver carp, but the abdominal keel is present only between the ventral fins and anus (in the silver carp it starts from the throat). The mouth is large and directed upwards.
It is present in all large lowland waters, in the arms of the Danube and in the deep lakes of the Danube Delta. It prefers pelagic zones (open water), where zooplankton concentrations are densest. It is a hardy species, capable of tolerating high water temperatures and low oxygen levels better than many native fish.
The bighead carp is a zooplankton consumer (small crustaceans, daphnia, rotifers). Its filtering apparatus is coarser than that of the silver carp, being adapted to retain microscopic animal organisms. Although it has "monster" dimensions, the bighead carp is a peaceful fish. It cannot be caught with traditional baits (worms, corn), as it feeds by filtration, and is usually fished with special techniques based on groundbait clouds (the "tablet" technique).
Sexual maturity is reached at 5-7 years. Natural reproduction follows the same pattern as the silver carp: it requires spring floods and rapid river current for egg development. In the Danube Delta, specimens over 40-50 kg are not rare, demonstrating the enormous biological potential of this species in nutrient-rich waters.
Globally it is Least Concern (LC). In Romania it is an essential species for aquaculture. Ecologically, its massive introduction can put pressure on native species that also feed on zooplankton; however, in the Danube Delta, due to the huge volume of water and food, the bighead carp coexists relatively balanced with local fauna.
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