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er BIOLOGY OF HUMPBACK CHUBS IN THE GRAND CANYON 0.3 C over a reach of this length in the Grand Canyon. Mean daily temperature was computed as a mean of the maximum and minimum read- ings in the 24-hour period starting at midnight. Mean daily temperatures for each month were averaged within six periods; the first five con- sisted of 5 days each and the sixth contained the remaining days of the month. We equated discharge of the Colorado at the gauge with relative water-surface elevation at a reference point in the confluence reach using a relation- ship estimated by the United States Bureau of Reclamation. These data were then summa- rized inthe same manner as were temperatures. Data on Colorado River temperature and dis- charge that predate our study were taken from published reports of measurements made at the gauge. Quantitative fishing gears were minnow traps (45 X 23 cm, 6-mm hardware cloth), seines (9.1 X 1.8 m, 3-mm mesh), and trammel nets (45.7 X 1.8 m, 2.5-cm-mesh wall, 25.4-cm-mesh trammel). The time spent fishing each gear and the area swept by seines were recorded. Total fishing effort by each quantitative gear was about equal among sampling sites within rivers. Qual- itative sampling was performed with fine-mesh hand nets, gill nets of various sizes, and elec- trofishing gear in the Colorado. Fish collected were weighed (g) and measured to total length (mm). Humpback chubs were examined grossly for ectoparasites and external characteristics of seasonal reproductive devel- opment. Dorsal and anal fin-ray counts are among several morphomeristic characters used to taxonomically separate the humpback chub, bonytail and roundtail chub Gila robusta (Miller 1946; Holden and Stalnaker 1970). Later anal- yses were limited to humpback chubs longer than 100 mm, a size that resulted in no dis- agreement over fin-ray counts among workers. Scales used for age estimation were plucked from the caudal peduncle above the lateral line. A numbered Carlin tag was attached to humpback chubs longer than 200 mm before the fish were released. Although external urogenital characteristics almost always proved accurate for selecting fe- malehumpback chubs for later laboratory anal- yses ofgonad development, we selected only fish for which these were pronounced (Suttkus and Clemmer 1977). Sex determinations in the field were sometimes questionable, particularly after 579 the spawning season. Therefore, we made no attempt to separate sexes in analyses other than those of gonad development. Seasonal gonad development of females was followed with gonadosomatic indices (100 • go- nad weight/whole-body weight) and mean ovum diameters from mature fish (longer than 255 mm). Gonads were excised in the field, pre- served in Bouin's fixative, and weighed to the nearest 0.1 g in the laboratory. Maximum di- ameters of 20 of the largest ova were measured with an ocular micrometer. Histological anal- yses ofgonads were made at the Bozeman (Mon- tana) Fish Cultural Development Center of the United States Fish and Wildlife Service (USFWS). Contents of digestive tracts from sac- rificed fish and fish killed accidentally were ex- amined for food items. Tissues from fish killed and microbiological cultures from fish having external symptoms of disease were sent to the Fish Disease Control Center (USFWS), Fort Morgan, Colorado, for routine pathological analyses. Catch per unit of fishing effort (C/f) was cal- culated for humpback chubs in three broad age categories, but no age distinctions were made for other species. Length frequency was effec- tive in identifying age-0 humpback chubs, and juvenile and adult fish were arbitrarily separat- ed at a length of 200 mm. The G/f was calcu- lated as fish per trap-night in minnow traps, fish per 100 m~ swept by seines, and fish per hour in trammel nets. When preliminary analyses re- vealed that catch rate in seines differed signif- icantlybetween daylight and darkness, these data were stratified on that basis. Because trammel nets in the Little Colorado were checked every 1 to 2 hours and fished much of the day and night, but those in the Colorado were fished unattended overnight only, the Little Colorado River data provided resolution of differences in C/f among diet periods not possible with data from the Colorado. After preliminary analyses, trammel-net data from the Little Colorado were stratified into three periods: daylight, sunset (sundown ± 3 hours), and darkness. The C/f was averaged within gear, river reaches, sampling trips, diel periods, and oc- casionally age categories of humpback chubs. s The full tables containing mean C/f data for all species collected during the study can be obtained from the senior author.