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The largest population of endangered humpback chub Gila cypha inhabits the Colorado River below Glen Canyon Dam and the lower 14 km of the Little Colorado River (LCR), Arizona. Currently, adults from both rivers spawn and their progenies grow and recruit to adulthood primarily within the LCR, where we studied G. cypha's life history using hoop net capture data. Humpback chub undergo an ontogenesis from diurnally active, vulnerable, nearshore-reliant young-of-the-year (YOY; 30-90 mm total length) into nocturnally active, large-bodied adults ([greater than or equal to] 180 mm TL). During the day, adults primarily resided in deep midchannel pools; however, at night they dispersed inshore amongst the higher densities of YOY conspecifics. Many YOY G. cypha shifted to nocturnal habitats that provided greater cover, possibly, to avoid inshore invading adults. These findings mirror predator-prey scenarios described in other freshwater assemblages, but do not refute other plausible hypotheses. Gila cypha piscivorous activity may escalate in hoop nets, which can confine fish of disparate sizes together; adults were significantly associated with YOY conspecifics and small dead fish in hoop nets at night and eight G. cypha (156-372 mm TL) regurgitated and/or defecated other fish body parts during handling following capture. Gila cypha can definitely be piscivorous given the opportunity, but the magnitude of their piscivorous activity in the wild is debatable.

Two nonlethal alimentary tract lavage techniques were conducted on ten bonytail chub (Gila elegans, 211-241 mm TL) that had previously ingested 2-4 rainbow trout (Onchorynchus mykiss, ∼40 mm TL) to evaluate their capabilities of detecting piscivory in the field. Each fish was initially lavaged with water down its buccal cavity and out its anal vent using the Wasowicz and Valdez (W-V) technique and then immediately flushed in the opposite direction with the Baker and Fraser (B-F) method. Microscopic examination of evacuated material revealed only three identifiable rainbow trout scales and other assorted clumps of presumably well-digested body material. Overall, 30% of bonytail chub expelled rainbow trout scales and/or other body material during the initial W-V lavage, and 60% expelled material during the B-F flush. Dissections of four bonytail, seven humpback (G. cypha), and four roundtail chubs (G. robusta) revealed that their intestines were similarly curved and tapered to le;50% of the anterior widths. Whereas the B-F method pushes ingested items backwards through the increasingly larger intestinal circumferences in these Gila fishes, it is more apt to retrieve greater quantities of food items. Food preference studies based on findings from a highly size-selective lavage technique will likely be biased in favor of smaller, less-digestible prey items, such as invertebrates with chitinous exoskeletons. Use of the B-F rather than W-V technique should lessen this bias. [ABSTRACT FROM AUTHOR]