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206 TRANS, AMER. FISH. soe, 1969, NO. 2 " TAJlu; 9,-Number of Colorado chubs in 1957-1966 year-clusses collecled, Grecn Rivcr, 1964-1966 Year-class Year of <:apture Taxon' 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1964 rt 2 3 2 0 0 bl 0 1 9 1 0 c 2 ,I 11 I II JI 257 Jill) 106" rt II 4 .~ II bl 2 12 17 3 c 8 16 22 !l 12 411 605 269 1966 rt I 4 II JO bl 1 i 13 1 c 2 11 19 11 44 102 152 230 ---------------------------------------------------------------------------------------- Tolal nllmber laken rl bl c 2 o 2 1 Tt = rOllndtailj bt ::: bonytaili C = combined. J onez and Sumner ( 1954) described the spawning act of Colorado chubs in Lake Mohave. Approximately 500 bony tails were observed spawning over a gravel shelf up to 9 m deep, in May of 1954, Each female had three to five male "escorts." The adhesive eggs were broadcast on the gravel shelf. A gill net in the spawning area caught 4.2 males and 21 females ranging from 279 to 356 mm (fork length). A 305-mm female conlained an eSlimated 10,000 ripe eggs, DISCUSSION The apparent reduction in growlh rate of the Colorado squilwfish and the Colorado "hllh from pre- 10 posl-im(loulldllH'1I1 years may have been a result of lower or higher waleI' temperatures than those before im- poundment During 4 years (1957-1959; 1962) before dam closure, daily mean tem- perature at Ihe Jensen USGS gaging slation was above 60 F for approximately U5 days of Ihe yeaI'. Numbers of days wilh mean waleI' lemperature above 60 F in 1963, 1964, and 1965 were, respectively, 147, 103, and 33. Thus, the temperatures of the post- impoundment years may have been less favor- able (either 100 cool or 100 warm) for growth Ihun were tempera lures during pre.impound- ment years, Olher possihle explanations of Ibe apparent reduction in growth rate may have been: 1) survival rate of faster.growing fish was higher than that uf slower-growing fish; thus the sample of older fisb would be biased to\\'ard faster growth during earlier calendar years, or 2) earlier annuli on older fish were missed or incorreclly interpreted. 9. 3 12 7 2::! 2U 9 25 34 12 ]6 28 j() I 34 3.11 1096 421 230 No consistent relationship between mean annual discharge and year-class strength of squawfish and chubs was apparent. In 1963, the first year after dam closure when flow was extremely low, a particularly abundant year-class of squawfish was produced. Weak year-classes were produced in 1962 and 1965, years of high discharge. In 1964, a year of relatively high discharge, an abundant year- class of chubs was also produced. No excep- tionally weak year-classes were indicated for chubs, Estimates of year-class strenglhs for these years were based upon seine captures of young-of-the-year and juvenile fish, both of which frequent shallow areas and are vulner- able to this method of eaplure, The very large squawfish specimens thai were found in the Green River in the past have not been reported in recent years, and tbe largest squawfisll taken in this present study weighed 2.3 kg (5 Ibs). It is not known wbether this apparent absence of large fish is due to reduced growth rate or shorter life span. The complete disappearance of these large fish cannot be assumed since a Vernal, Utah, resident reported to have hooked, but failed to land, a squawfish that he estimated to be approximately 4 ft long in the Green River above the tOWI1 of Jensen in 1966. Many local fishermen attributed the dis- appearance of large squawfish to some form of competition with the introduced channel,. catfish. Several reported observing large dead squawfish containing channel catfish with the spines extended and lodged in the pharynx or esophagus, which probably caused suffucation or starvation. Other possible factors contributing to the apparent reduc.