Challenges for Diadromous Fishes in a Dynamic Global Environment

<em>Abstract</em>.-Most anadromous fish undergo physiological and behavioral changes that are preparatory and adaptive for seawater entry. In anadromous salmonids, these preparatory changes are collectively known as smolting. Smolt development is regulated by environmental factors such as photoperiod and temperature and mediated by the neuroendocrine system. In this paper, we review evidence that a variety of anthropogenic factors can influence smolt development and affect marine survival. Hatchery rearing can affect the size of smolts and the extent and timing of smolt development. Smolt development is reversible, and the period of peak physiological preparedness in salmon smolts is limited by time and temperature. By influencing temperature and the duration of the migratory period, climate change and dams will have negative effects on smolt survival beyond direct lethal impacts. Contaminants acting on developmental physiology or underlying endocrine control mechanisms can also reduce marine survival. Exposure to estrogenic compounds prior to or during smolt development can reduce seawater tolerance and preference. Acid and aluminum exposure can reverse the development of seawater tolerance and reduce adult return rates. We conclude that environmental conditions in freshwater can affect physiological development, estuarine and ocean behavior, early seawater survival, and long-term seawater growth and homing, thus having influences on adult returns and long-term population sustainability of anadromous fishes.