All Fish, All the Time: A Good General Objective for Fish Passage Projects?

Abstract

Better understanding of the degree of “passability” needed to sustain robust sub‐populations upstream of barriers could help address and perhaps eliminate perceived trade‐offs between fish passage and other resource management goals. We used spatially explicit, individual‐based modeling to explore the population‐level consequences of various levels of upstream passability by (1) adult resident trout, and (2) juvenile salmon. In simulations of a stream network with widespread barriers, adult trout allowed to move upstream during only the highest 4% of streamflows sustained the distribution and abundance of the population to the same extent as in a no‐barrier scenario. In simulations of salmon production in a reach fully accessible to spawning adults, the number of barriers to upstream movement by juvenile salmon did not affect the production of large outmigrants. Passage objectives less ambitious than “all the fish, all the time” may sometimes suffice to maintain populations of resident trout and anadromous salmon.