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21 12r..i f t sa fiP- 1 iJJ.q mf?t.b..9.rt9_LQ.9.i~.L~_q.LQ.r.:.E.f.:Ll;L_31 u.:::\!.NfLs 11. - Re f el~ en c: E'S; hay e bee n made previously and in Haynes et 211. (1985) to catch/effort (C/f) as an inde:< of relative abundance and reproductive success of the Colorado squawfish. Analyses of the drift-net catches of larval Colorado squawfish for 1983 - 1985 demonstrated some potential sources of error that are of an unpredictable magnitude, but may affect the accuracy of Clf indices. In 1983, drift sampling started June 14-15 at Box Elder and no Colorado squawfish were collected on this date or during the subsequent effort on July 12-13. Colorado squawfish larvae were first collected during the next sampling effort on July 22. Given our 1985 experience, the question arises concerning the large, 26 and 8-day gaps between consecutive sampling efforts, and what may have been missed during those intervals? In 1984, drift sampling started on July 7-9, and Colorado squawfish were first collected during the subsequent effort on July 15-16. The question arises here whether or not sampling was started soon enough, as well as concerns over the 5-day interval between sampling efforts. It is possible that a large pulse of drifting Colorado squawfish could have been missed between sampling intervals, especially in 1983. However, examination of predicted spawning dates according to seine and drift net samples in Haynes et aI. C1985i indicates that seine sampling in the 1983 and 1984 seasons around the time intervals of drift net sampling probably made the sample of drifting larvae and the resultant predictions of spawning activity more representative. In 1985, the largest collection of Colorado squawfish larvae occurred on the last 24 hr cycle of the July 9-12 sampling effort. A significant drop in the number of larvae collected occurred during the subsequent sampling effort starting July 16. Even with only three or four day intervals between samples in 1985, evidence suggests that large numbers of Colorado squawfish could have been missed sometime during the July 13-15 interval. These questions of potential bias suggest that caution be exercised when using C/f indices as indicators of larval Colorado squawfish abundance or adult reproductive success until they can be satisfactorily correlated with other indices of relative abundance or year class strength for the species. Currently, recruitment and year class strength are poorly understood for Colorado squawfish. The experience above suggests that representative sampling of drifting larval Colorado squawfish may be best achieved by combined seine/drift net sampling, or reducing intervals between drift net samples to two days or less. With regard to catch, elf indices for 1983-1984 seine data in Haynes et 211. (1985) suggest the reproductive success of Colorado squawfish in the Yampa River from 1983-1985 has been relatively similar. Given the negative aspect of the bias described above, the Clf indices reported here may be considered underestimates. l!lt er J} r eta t j.J?P ~_..~~.i._].:...D..r v 0L[p.lpr ~3. q..fL..?-9.1lill1f i ~tL1.J~D..9.. t I"LJ!E1.:.!& -. The 1 en 9 t h data for larval Colorado squawfish collected by drift-nets from 1983-1985 on the Yampa River indicates that the maximum length of drifting larvae is 10mm. Length data for larval Colorado squawfish collected downstream from the Yampa at the Stateline site on the Green River supports this conclusion. These results and seine data in Haynes et al. (1985) suggest that larger Colorado squawfish have greater control of their position in the river current, thereby reducing the occurrence of passive drift and their vulnerability to drift-net sampling. Haynes et al. (1985)