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c ~ 7
<br />Of 72 fish sampled in both years, 13 GI tracts (18%) were to be empty. The frequency of empty
<br />tracts was more than four times higher for evening (33%) than for night (7%) samples (Fig. 3), but
<br />the gross composition of GI contents was similar between the two feeding times (Fig. 4).
<br />AOM consisted predominantly of nondescript, brownish material or "grutch." This might have
<br />included stomach lining, mucous, or ingested materials in advanced stages of digestion (beyond
<br />identification). Inorganic material consisted of pebbles, rocks, grains of sand, and insect larval
<br />cases that were composed of sand grains and pebbles (e.g., trichopterans including
<br />hydroptilidae). Plant matter consisted of various aquatic macrophytes including Najas sp.,
<br />Potamogeton sp., and Chara sp. Fish matter consisted of any fish part or whole including scales,
<br />bones, and flesh. Invertebrate matter consisted of a variety of groups including microcrustaceans
<br />(copepods, ostracods, and Daphnia), crayfish, corbiculidae, tapeworms, dipteran larvae and
<br />adults, notonectidae, and odonate nymphs and adults. Asian tapeworm Bothriocephalus
<br />acheilognathi was positively identified in one specimen and tapeworm proglotids, presumably
<br />Asian tapeworm, were found in 8 of 72 (11 %) samples representing ail available sizes of bonytail.
<br />Other matter included both identifiable (bull frog Rana catesbiana) and unidentifiable vertebrate
<br />remains.
<br />For invertebrate, fish, and plant matter, composition varied by fish size (TL): plant matter
<br />decreased while invertebrate matter increased with increased fish size (Figs. 5 and 6). Fish parts
<br />were observed in 8% of GI samples (6 of 72), and were restricted to fish longer than 425 mm.
<br />GI sample weights showed little linear relationship to fish body weight or total length (Figs. 7 and
<br />8). Mean stomach sample wet weight was 1.544 g, the nonzero range was 0.061 to 13.970 g,
<br />and standard deviation was 2.667. Fish length and weight ranges are provided in Fig. 1.
<br />Discussion
<br />Telemetry studies at Cibola High Levee Pond indicate that adult bonytail are active during
<br />nighttime and spend the daylight hours dormant and hidden under cover amongst large boulders.
<br />This is consistent with the volume and composition of stomach contents and proportion of empty
<br />guts, which indicated the most intense feeding occurred at night.
<br />Asian tapeworm was reported in humpback chub Gila cypha from the Little Colorado River in
<br />Grand Canyon (Clarkson et al. 1997), but this represents the first record of Asian tapeworm in
<br />bonytail from "wild" habitat on the lower Colorado River, and may signal future occurrences of this
<br />pest in other species and in other places. It is unknown if the tapeworm was introduced
<br />accidentally with hatchery stocks of bonytail or razorback sucker, or with other species that were
<br />stocked illegally by unknown persons. Researchers, managers and other should be aware of its
<br />potential presence and provide interested parties with incident reports as they occur.
<br />The few available data from other studies indicate that bonytail feed on benthic and drifting
<br />aquatic invertebrates and terrestrial insects under natural stream conditions (Kirsch 1889). A
<br />composite sample of sub-adult bonytail and roundtail (Gila robusta) chubs from Green River,
<br />Utah, ate mostly chironomid dipteran larvae and mayfly (ephemeroptera) nymphs when small,
<br />shifting to floating items (e.g., terrestrial insects) as they grew (Vanicek and Kramer 1969). Adult
<br />bonytail in Green River fed mostly on terrestrial insects, presumably taken from the surface, but
<br />there was no evidence of piscivory. In contrast, bonytail in Lake Mohave were found to prey on
<br />small (64 mm TL), newsy stocked rainbow trout (Wagner 1955). Jonez and Sumner (1954) found
<br />plankton, insects, algae, and organic debris in bonytail from Lake Mead, and a few specimens
<br />from lakes Mohave and Havasu contained zooplankton (Minckley 1973). Our results contribute
<br />substantial hew detail to our understanding of bonytail feeding ecology, but add little new
<br />qualitative information about their food utilization.
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