The Energetic Cost of Migration: Limitations to Shad Restoration
Proceedings of a Workshop on American Shad
Shad do not feed during the freshwater migration. This lack of an exogenous energy source permitted the calculation of caloric expenditures associated with river movements directly from changes in the energy yielding body reserves (fat and protein). This was accomplished for two shad populations (those of the Connecticut and St. Johns Rivers) displaying radically different life history strategies. Determined energy costs per kilometer of upriver displacement varied between rivers and seasonally in the Connecticut River. Florida shad expended from 4.1 to 4.4 kcal per km while expenditures by Connecticut shad ranged between 4.6 and 9.1 kcal per km, with late migrants utilizing body reserves at the highest rate. Despite reduced energy expenditures per kilometer in Florida, increased distance to the spawning grounds resulted in Florida shad utilizing 70-80% of available energy reserves to successfully spawn (after which most adults apparently die). In comparison Connecticut migrants depleted only 40-60% of total reserves to both spawn and return to sea. Water temperature, body size, gonadal maturation in freshwater and possible variations in migratory behavior are discussed as factors contributing to between-river discrepancies in energy utilization. These data suggest Connecticut River shad depletion approaches lethal levels upon completion of the freshwater migration. Range extension in the Connecticut River will increase energy costs and postspawning mortality among shad spawning above Turners Falls. Changes in the age structure and repeat spawning characteristics of the Connecticut population are predicted. Migratory energy costs are sensitive to minor temperature change. Greater reserve depletion and higher mortality among late run Connecticut shad in response to a 4ºC increase in water temperature suggests further calefaction of the Connecticut River could result in increased mortality and changes in reproductive potential.