Role of winter severity on juvenile Bighead Carp and Silver Carp growth and survival across latitudes

Population dynamics and ecological effects of invasive fishes are affected by offspring survival to sexual maturity (recruitment). Recruitment is oftentimes determined by conditions experienced during early life, and winter conditions become increasingly influential as species invade north in temperate regions. Bighead Carp (Hypophthalmichthys nobilis) and Silver Carp (H. molitrix) have invaded mid-latitudes of North America and threaten to invade farther north. We conducted pond experiments and bioenergetic simulations to: (1) assess the effects of mild winters on juvenile Bighead Carp and Silver Carp at mid-latitudes; (2) determine how conspecific density affects juveniles during mild winters; and, (3) determine effects of increased winter severity at increasing latitudes. Both species displayed high overwinter survival (85%) in pond experiments. Bighead Carp grew in length (mean 2 mm) and maintained initial body mass and percent dry mass (index of energy content). Silver Carp maintained initial length and mass and remained in high condition (dry mass). Conspecific densities tested did not affect growth, condition, or survival. Simulations suggest juvenile Bighead Carp could energetically survive the longest, coldest winter simulated (48° latitude); however, energy reserves at the start of winter became increasingly important for survival as latitude increased. Bighead Carp and Silver Carp recruitment success at mid-latitudes is likely driven by non-winter factors. If these species invade farther north, winter severity may play a stronger role in offspring survival, recruitment, and population dynamics. However, warmer and shorter winters at high latitudes caused by climate change will minimize this potential bottleneck and increase chances of successful recruitment.