Relative importance of fishing, bycatch, and dam passage mortality to status of northeastern US American shad populations

American shad of the northeastern US are affected by bycatch of immature fish, directed fishing on adults, and upstream and downstream mortality of mature fish at migratory barriers. We evaluated the relative importance of these mortalities, independently and in combination, by modeling predicted life time spawning biomass of an age one female recruit (SSBR).  Bycatch losses had the greatest impact on SSBR followed by directed fishing and mortality caused by upstream passage, and then by either upstream passage failure (did not spawn but survived) or downriver passage mortality. Fishery managers strive to keep mortality rates below those that reduce SSBR to less than 30 % of a stock with no anthropogenic mortality.  In our modeling, SSBR dropped below that benchmark when bycatch rates exceeded u = 0.21, directed fishing or upstream passage mortality exceeded u = 0.45, or when upriver passage failure without mortality exceeded u = 0.70. Since downriver passage mortality occurred after spawning, SSBR did not decline below the benchmark even at 100% downriver passage loss.  Impacts of upstream passage mortality always exceeded those from comparable downstream passage mortalities.  Fishing pressure and bycatch loss seriously reduced any gain in SSBR from improved fish passage.