Temperature preference and heat resistance of grass carp, Ctenopharyngodon idella (Valenciennes), bighead carp, Hypophthalmichthys nobilis (Gray), and their F₁ hybrid

Acute temperature preferenda and critical thermal maxima were determined for 23°C‐acclimated stocks of grass carp, bighead carp and their F, triploid hybrid. Preferred temperature and critical thermal maximum of the hybrid were significantly higher than those of the grass carp (female parent), which is consistent with prior field observations. The hybrid's values for both parameters also were significantly higher than those of the bighead carp. These results comprise experimental evidence for substantial heterosis in the thermal physiology of fishes.