Effects of small green sunfish (Lepomis cyanellus) on recruitment of Gila chub (Gila intermedia) in Sabino Creek, Arizona

Young-of-year Gila chub (Gila intermedia) were abundant in upstream reaches of Sabino Creek, Arizona, devoid of green sunfish (Lepomis cyanellus), but were absent in downstream areas occupied by green sunfish. We examined pot ential reasons for this pattern by studying piscivory and habitat use of sm all green sunfish (<75 mm TL), the dominant size-class in Sabino Creek. In one piscivory experiment, we greatly reduced numbers of green sunfish prior to spawning by Gila chub, but there was no recruitment of young-of-year Gi la chub. In a second experiment, three sizes-classes of green sunfish (45-5 5 mm TL, 62-65 mm TL, and 76-84 mm TL) readily consumed two size-classes of young-of-year gila chub (15-20 mm TL and 21-25 mm TL) in instream enclosur es. Green sunfish predation rates were strongly size-dependent, with larger size-classes consuming notably more young-of-year Gila chub than smaller s ize-classes. Shallow habitats may provide refugia for young-of-year Gila ch ub in the presence of large (>150 mm TL) green sunfish. However, even small green sunfish were highly predacious on young-of-year Gila chub, and our h abitat study demonstrated that both taxa occupied similar mesohabitats. Co- occurrence of Gila chub and green sunfish in Sabino Creek seems to be the r esult of periodic downstream movement of adult Gila chub from reaches devoi d of green sunfish. Young life stages of Gila chub apparently do not persis t in sections of Sabino Creek occupied by green sunfish.