Physiological and Performance Consequences of Landlocking in Alewives

Anthropogenic disturbance has resulted in reduced connectivity among aquatic habitats, presenting challenges for anadromous fish such as river herring.  Dams in coastal Connecticut block the anadromous migrations of alewives, and have resulted in multiple, independently-derived landlocked populations.  The functional consequences of land-locking among alewives have not been fully characterized.  Through a series of field and laboratory salinity challenge experiments, we show that landlocked alewife populations are more tolerant of freshwater than anadromous alewives, but have lost seawater tolerance and hypo-osmoregulatory function.  Divergence in salinity function between landlocked and anadromous alewives is driven in part by changes in the expression and activity of gill ion transport proteins.  Furthermore, we show that landlocked alewives have lower growth rates in seawater and reduced swimming performance at any salinity relative to anadromous alewives.  Our results suggest a trade-off in salinity function; landlocked alewives are adapting to an exclusively freshwater life cycle, which has resulted in dramatic reductions in seawater function and performance.  Increasing anthropogenic disturbance and land-use change may further reduce the connectivity between freshwater and seawater.  Anadromous alewives that become restricted to freshwater will lose key functions related to anadromous migrations, potentially contributing to population declines.