Why are larger fish farther upstream? Testing multiple hypotheses using Silver Chub in two Midwestern United States riverscapes

Objective

Three competing hypotheses might explain the widely documented intrapopulation larger-fish-upstream phenomenon. The age-phased recruitment hypothesis posits that fish spawn downstream and move upstream as they age and grow, the static population with growth and mortality gradients hypothesis posits that fish spawn throughout a riverscape and growth is greater upstream while recruitment is greater downstream, and the colonization cycle hypothesis posits that fish spawn upstream, larvae drift downstream, and individuals move upstream as they age and grow.

Methods

We tested for the larger-fish-upstream pattern using populations of Silver Chub Macrhybopsis storeriana in the Arkansas and Ohio rivers, as well as investigated longitudinal variation in reproductive investment (Arkansas River), age structure for adult fish (Arkansas River), and number and occurrence of age-0 fish (Ohio River).

Result

The larger-fish-upstream pattern was temporally persistent in both riverscapes. In the Arkansas River, reproductive investment was greatest upstream, where initiation of spawning likely occurred based on gonadosomatic indices. Adult fish were most numerous in the Arkansas River 125–175 km upstream from Kaw Reservoir, with age-2 fish numbers peaking farther upstream compared with age-1 fish. In the Ohio River, age-0 fish counts increased downstream and were rare among the shortest river fragments (