What characteristic allows the crucian carp its renowned hardiness when conditions are poor?

Answer

The duality of their diet

The duality in diet, involving both plant-based matter and animal prey, allows the fish to subsist on detritus or algae when motile prey like insects become scarce, enabling their hardiness.

What characteristic allows the crucian carp its renowned hardiness when conditions are poor?
dietanimalfishcrucian carp