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46 GREAT BASIN NATURALIST MEMOIRS 197f No.3 TABLE 2. Mean size of woundfin and roundtaiJ chub in Virgin River. The 0 indicates collections were made in the area but no individuals of the spe<:ie,~ were taken. The - indicates the area was not collected. Data on woundfin from 197.'3 were provided by Mr. Jerry Lockhart. He probably also took chubs; however. data are not available. or larger fish tended to predominate to a greater extent than was true in either fall 1973 or fall and winter 1978. This suggests (1) that growth of young in 1977 may have been faster than was the case in 1973 and 1978, (2) survivorship may have differentially favored older woundfin during summer 1977, (3) spawning may have occurred earlier in summer 1977 than in 1973 or 1978, or (4) perhaps more probable, the later secondary spawning period was almost entirely unsuc- cessful in 1977, whereas in 1973 and 1978 it was successful and resulted in a significant contribution to the population in the fall (Fig. 1). The effect of the two spawning peri- ods in 1978 is apparent in Figure 2 as two peaks in the length-frequency diagram in the size range below 60 mm. Figure 8 shows dates on first appearance of WoUl Virg tions vals dical by 2: HUTI Cree earH. river ingl two habil Tl in tl was 1977 ance was (Fig. strea WOUl fall ] in th 197.'3 1977 1978 AlIg& 1-8 2.'3-.10 14-1.'5 25-26 12 28 I -yept JlIne AlIg Nov Nov April ~ Nov X N X N X N X N X N X N X N X N Upper river Woundfin .'52.1 60 6'!L'3 172 74..1 70 71.2 416 72..'3 371 76.2 190 46.0 15.1 Chub 158.2 .'5173 I 0 I o 67.6 J()-2 Lower river Woundfin .'50.2 105 44.0 177 5.1.6 202 .'56.6 38.1 58.2 H2 44.4 427 48.6 46 Chub 45.0 I 17.1 I 158 .'3 o 84..1 4 70.0 I - ------- in September 1978. Perhaps even in times of "normal" flows there are relatively few op- ti.mal habitats for the roundtail chub remain- ing in the Virgin River. In addition to the marked differences in re- productive success of woundfin and roundtail chubs in 1977 and 1978, interesting differ- ences in population structures of woundfin above and below the narrows in 1977 were evident. It is apparent from an examination of Figure 6 and Table 2 that young woundfin comprised a far greater proportion of the woundfin population in the lower river in 1977 than was true in the upper river. The effect of the drought on woundfin population structure in the upper river thus appears to have been more severe than was true in the lower river. In the lower river, however, it is apparent that by fall and winter 1977 older 10,000 1000 803 CD Q 70 ::l IJl N 60 CD ~ o -;;; 100 - (.J ~ ~ o ....J l.L 1973 1977 90 z o ~ ....J => a.. o a.. - Q 50 . 40 1978 Fig. 4. Mean monthly flow at Hurricane Gage and mean size of woundfin in upper Virgin River 1973, 1977, 1978. Fig. three i