The Impact of Enhancement Initiatives on the Structure and Dynamics of the Connecticut River Population of American Shad

During the interval 1975 to 1981, improvements to existing fishways and new fishway construction at high-head mainstream dams located at river km (rkm) 139, 198, and 228 on the Connecticut River, combined with the breaching of a low head dam at rkm 110, created access by American shad to their historical spawning areas from which they had been excluded since the late 18th century. This resulted in a significant upriver relocation of the main spawning activity by shad, but, to date, has failed to produce the anticipated increase in total population size from the increase in total spawning habitat available. This upriver displacement of spawning resulted in an increase in the length of the spawning migration and a corresponding increase in the energy expended to reach the spawning grounds. The result was higher adult mortality and dramatic reduction in the repeat spawning component of the population and in the mean size and age of adult fish. An increase in the variance in annual population abundance followed this reduction in population age structure. The loss of larger repeat spawning females is estimated to have resulted in a 20% reduction in mean population fecundity and could account for a 14% reduction in annual recruitment to the population. These outcomes are consistent with predictions founded on knowledge of the population dynamics and migration energetics of the species available at the time the enhancement initiatives were begun. Unrelated, but very successful restoration efforts directed to Atlantic Coast populations of striped bass also appear to have negatively impacted the shad population of the Connecticut River through increased predation. The implications of these findings for future management are discussed