Effects of water temperature and turbidity on predation vulnerability of juvenile humpback chub to rainbow trout and brown trout

Authors: David L. Ward, Rylan Morton-Starner

Year: 2013 (xlv)

Abstract

Predation of juvenile native fish by introduced rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta is considered a significant threat to the persistence of endangered humpback chub Gila cypha and other native fishes in Grand Canyon. Diet studies of rainbow trout and brown trout in Grand Canyon indicate that these species do eat native fish, but population level impacts are difficult to assess because predation vulnerability changes with turbidity, fish size, and water temperature. We conducted laboratory experiments to evaluate how short term predation vulnerability of humpback chub changes in response to each of these factors. Hatchery raised, juvenile humpback chub 50 to 90 mm TL were exposed to rainbow and brown trout at 10, 15, and 20 °C and at turbidities ranging from 0 to 1000 formazin nephelometric units (FNU). Brown trout were highly piscivorous at all sizes and water temperatures. A 1° C increase in water temperature decreased short term predation vulnerability of humpback chub to rainbow trout by about 5 percent. Increases in turbidity reduced predation vulnerability but only when turbidities approached 1000 FNU. Understanding the effects of predation by trout on endangered humpback chub is critical in evaluating management options aimed at preservation of native fishes in Grand Canyon.