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

Abstract

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 (<100 km) between lock-and-dam structures. These findings are inconsistent with the age-phased recruitment hypothesis based on upstream spawning in the Arkansas River and inconsistent with the static population with growth and mortality gradients hypothesis based on virtual absence of age-2 fish downstream (Arkansas River) and age-0 fish upstream (Ohio River). The most likely explanation for longitudinal variation in Silver Chub size distribution is downstream drift of ichthyoplankton followed by net upstream movement (i.e., colonization cycle hypothesis), but formal assessments of movement and ova characteristics require more research. Conclusion Managing multidimensional riverscapes requires insight into the mechanisms that regulate upstream-to-downstream patterns in fish populations, and our work underscores a potential size-related benefit to maintaining broadscale longitudinal connectivity.