Latitudinal Differences in Energy Allocation and Use During the Freshwater Migrations of American Shad (Alosa sapidissima) and Their Life History Consequences

Abstract

We studied the relationships between tissue dynamics and bioenergetics of the anadromous American shad (Alosa sapidissima) homing to the St. Johns (Fla.), York (Va.), and Connecticut (Conn.) Rivers and the life history characteristics of these populations. Shad in the three populations studied differed in the degree of development of the gonads at river entry, in the absolute and relative energy allocation to reproductive products vs. migration, and in the extent of total energy depletion during the migration. St. Johns River fish consumed 70–80% of their total energy reserves to reach the spawning grounds and spawn. In this population all shad die following spawning. York River shad consumed ~30% and Connecticut River shad 40–60% of their energy reserves to migrate to the spawning grounds, spawn, and return to the sea. In these populations 25 and 35%, respectively, of the spawning adults survive to spawn again. The principal determinants of energy use were migration distance and river gradient. We rejected the hypothesis that the latitudinal cline in adult survival results from differences in energy use during migration. We concluded that interpopulation differences in energy allocation to migration vs. reproduction are a consequence, rather than a cause, of the different life history strategies exhibited by populations of shad over its Atlantic Coast range. A similar pattern is apparent in other anadromous species: obligate semelparous species use > 70% of their energy reserves to reach the spawning areas and spawn; iteroparous species allocate more energy to postreproductive reserves at the expense of reproductive products thereby ensuring a successful return migration to the sea. Freshwater swimming speeds also appear to differ in a consistent way between semelparous and iteroparous species. Semelparous species swim at close to maximum sustained speed thereby minimizing the duration of the migration. Iteroparous species swim at speeds yielding near optimum energy efficiency (J∙kg−1∙km−1) thereby minimizing the energy cost of migraiton.Key words: American shad, (Alosa sapidissima); migration, bioenergetics, life history tactics, latitudinal clines, swimming speeds, energy allocation, anadromy