Food Partitioning between Young of the Year of Two Sympatric Tui Chub Morphs

The hypothesis that young of the year of two sympatric morphs of tui chub Gila bicolor were trophically indistinguishable was tested in Pyramid Lake, Nevada. Based on gill‐raker counts, young‐of‐the‐year tui chubs longer than 25 mm fork length could be separated into distinct morphs, G. b. obesa (fewer gill rakers) and G. b. pectinifer. Undesignated juveniles shorter than 26 mm, collected from depths 1 m and less, ate primarily zooplankton plus small quantities of macroinvertebrates. Among G. b. obesa, consumption of macroinvertebrates, principally chironomids, rose to 41% for fish 26–50 mm long and to 62% for fish 51–75 mm long. Among G. b. pectinifer, diets of fish 26–50 mm and 51–75 mm long contained 93 and 100% zooplankton, respectively. As young of the two morphs grew larger, their gill‐raker distributions became nonoverlapping. Divergence in food selection accompanied the shift in numbers of gill rakers. Our results suggest that sympatric morphological variants of polytypic fishes like tui chub should be treated as functionally distinct species for fishery‐management considerations. Received October 9, 1982 Accepted April 11, 1983