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1 r 1 1 Table 4. Log-log length-weight regressions for northern pike sampled in the Yampa River by time period and by river reach, from 1986-1991. W =weight in gm; L= total length in mm. Year(s) Regression equation (SE)Coeff. W n 1986-88 log W = 2.985log L - 5.107 0.043 0.979 106 1989 log W = 3.4271og L - 6.358 0.071 0.936 163 1990 log W = 3.264log L - 5.920 0.066 0.951 130 1991 log W = 2.9931og L - 5.122 0.068 0.946 113 Reach Craig log W = 3.124log L - 5.499 0.042 0.961 226 Juniper log W = 3.4451og L - 6.430 0.104 0.932 82 Maybell log W = 3.1881og L - 5.690 0.075 0.944 108 Lily Park log W = 3.068log L - 5.354 0.087 0.930 96 Standard' log W. = 3.059log L - 5.369 ' (Willis 1989) increased length. This changed to a peak value (3.427) indicating greater weight gain with increasing length in 1989. The relationship in 1990 remained similar to 1989, though the weight gain with increased length in 1990 was relatively less in magnitude. In 1991, the coefficient declined further and was similar to 1986-88, again indicating lesser weight gain in proportion to increasing length. The effect of these changing coefficients is shown in Figure 7. The change in northern pike shape was most pronounced in the larger ( > 800 mm), heavier ( > 4 kg) fish. Comparisons between years, using 95 % confidence interval estimates for X coefficients, clearly shows that the slopes of the regression lines for 1986-88 and 1991 are significantly different from the slopes for the 1989 and 1990 lines (Figure 8). These differences in slope correlate with the differences noted in spring flow magnitude for the study years, with greater slope coefficients associated with lesser spring flows. It was previously considered that the large increase of smaller, more slender northern pike in the 350-500 mm size group was responsible for the change in length-weight relationship for 1989, but the lack of statistical difference between the 1989 and 1990 coefficients indicates the influence of this increased number of small northern pike on the length-weight relationship was not very great. Northern pike length-weight relationships by river reach demonstrated a greater weight gain in proportion to increasing length for northern pike samples in all four river reaches, but X coefficients increased from Craig to a peak in the Juniper reach (3.445), then steadily declined ' DRAFT March 12, 1996 24