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tended to gradually increase over the duration of the study (60 minutes), however TA of the lazgest fish (36 week-old juveniles; CP4) stabilized very quickly (15 minutes or less). During the day, TA of the largest fish (CP4) was similar to that observed at night, but this was not the case for the smallest larvae (CP1), which were more active during day than night (Figure 2). Differences in TA were not significant among the various life history stages after 60 minutes. There was no directed activity (DA) detected in control trials after 15 minutes (Figure 2), as would be expected at night. However, the largest fish showed a significant displacement toward the "upstream" end of the tank after 60 minutes. The pattern during the day differed slightly in that the youngest larvae displayed a slight "downstream" movement after 15 minutes. Slow Flow The introduction of a small flow rate resulted in an increased TA of small larvae at night (Figure 2). This response was not evident during day conditions, and TA diminished with increasing fish age. In addition, smaller larvae tended to move downstream at night, while larger fish 3.0 2.5 X20 Q 1.5 a i.o os 0.0 3.0 2.5 B Z 2,0 1,5 a i.o 0.5 0.0 CP1 CP2 CP3 CP4 RZ7 Fish Group ^ Control ~ Slow ~ Moderate S aZ a ~ o° -, a' .p -3 it ~_. CP1 CP2 CP3 CP4 RZ1 Fish Group ^Control ^Sbw ^Moderate 3 D z a 00 -, -2 D ~~~~~'n~ CP7 CP2 CP3 CP4 RZ1 Fish Group ~ Control ~ Sbw O Moderate Figure 2. Average activity of fish in each age group and flow level 15 min after trial initiation. A= average activity at night; B= average activity at day; C= average directed activity at night; D= average directed activity at day. The 95% CL shown for each category. CP1 =one week-old, CP2= three week-old, CP3= six week-old, and CP4= 36 week-old Colorado pikeminnow. RZ1= two week-old razorback sucker. Control, Slow and Moderate refer to water velocities in the experimental tank . 530 CP1 CP2 CP3 CP4 R21 Fish Group ^ Control ~ Slow ^ Moderate