Effects of migration distance on whole-body and tissue-specific energy usein American shad (Alosa sapidissima)

We examined total and tissue-specific energy content of upstream-migrating American shad (Alosa sapidissima) in the Connecticut River. Total energy de pletion over the course of the 228-km migration ranged from 35 to 60%. The approximate contributions of different tissues to energy use during migrati on were white muscle 57%, subdermal fat 27%, red muscle 8%, viscera 6%, and liver 2%. American shad preferentially use energy stores in the skin and i ts subdermal fat layer (depleted by 63%) while sparing red muscle protein. Both lipid and protein were used as energy sources throughout migration, al though lipids were depleted to a greater extent (e.g., white muscle lipid d ecreased 48% and protein 30%). Large fish expended 2-21% more energy during migration than small fish. Migrating to upriver sites (198-228 km) is 50-1 00% more energetically expensive than to lower river sections for females. This suggests that upriver range expansion may be limited by females in tha t they may have reached a threshold level of energy expenditure in this upr iver area. American shad may possess physiological mechanisms for tissue-sp ecific energy use allowing maintenance of critical tissues necessary for po stspawning survival.