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

Authors: Rylan Morton-Starner, David L. Ward, Ben Vaage

Year: 2014 (xlvi)

Abstract

Predation on 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, in the Colorado River in Grand Canyon. Diet studies of rainbow and brown trout collected from the confluence of the Little Colorado and Colorado Rivers indicate the incidence of piscivory for trout in this area changes with turbidity but these relationships are not well understood. In overnight laboratory trials, we exposed hatchery-reared juvenile bonytail, Gila elegans, 60-70 mm TL, as a surrogate for humpback chub, to adult rainbow and brown trout, 190-400 mm TL, at turbidities ranging from 0 to 150 formazin nephlometric units (FNU) mixed from either Little Colorado River or Paria River mud. Our results indicate that turbidity as low as 50 FNU reduced predation vulnerability of bonytail to rainbow trout by 63 % (95% confidence interval = 43% - 82%). Predation vulnerability of bonytail to brown trout did not change significantly with turbidity up to 150 FNU. Relatively small changes in turbidity may be sufficient to alter predation dynamics of rainbow trout on humpback chub in the mainstem Colorado River and warrants further investigation as a fisheries management tool.

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