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KARP AND TYUS-COLORADO SQUAWFISH INTERACTION 31 TABLE 2. ACTIVITY AND RANGE of Ptychocheilus lucius, Catostomus latipinnis, Pimephales promelas, Notropis l2llrenS%S, R1CharLlSOni2lS baltea.t2ls, LepomiS Cyanell2LS, AND Ameiurus melds IN AQUARIA DURING FEEDING AND NONFEEDING PERIODS.°n is the number of 2 min observations. Activity is the average number of total cell changes per minute. Range is the average number of different cells used per minute. Species n Nonfeeding activity Activity Range n Feeding activity Activity Range Tank 1 Ptychocheilus lucius 18 17.2 6.9 15 22.5 6.5 C. latipinnis 18 15.1 4.3 15 23.8 7.8 Tank 2 P. lucius 18 12.9 4.5 15 16.2 6.7 Pimephales promelas 18 11.3 4.3 15 21.0 6.1 Tank 3 Ptychocheilus lucius 19 16.8 5.7 15 18.7 6.1 N. lutrensis 18 13.8 4.1 15 27.2 6.3 Tank 4 P. lucius 19 19.6 7.8 16 23.9 6.9 R. balteatus 21 24.3 7.6 15 26.7 7.1 Tank 5a P. lucius 30 28.3 6.3 21 18.7 6.1 A, melds 30 10.3 3.5 20 10.8 4.2 Tank 6a P. lucius 26 21.4 5.7 26 26.3 5.8 L. cyanellus 27 9.2 4.7 26 10.4 5.1 `Fish densities in tanks 5 and 6 are half those in tanks 1-4 there was some overlap among all species par- ticularly during feedings. We feel that the sta- tistical differences noted between vertical dis- tributionpatterns ofP. lucius within and between tanks were not indicative of spatial shifts, but rather, supported the observation that young P. lucius exhibit wide ranging and unpatterned movements. Many fishes were observed in the zone between adjacent layers and thus, division of the available vertical space into three layers (rather than two) may have restricted our ability to detect overt spatial shifts. In addition, our observations of fish position only provided dis- crete measurements of a species overall distri- bution and we may not have been able to detect long term changes in space use patterns. The widespread lack of predation (including nipping of fins or other body parts) was sur- prising because of the predatory habits of P. lucius, L. cyanellus, R, balteatus, and N. lutrensis (Scott and Grossman, 1979; Jacobi and Jacobi, unpubl. ms.; Lemly 1985). We attributed this phenomenon, in part, to the lack of a size ad- vantage for most of the fish. Notropis lutrensis, Pimephales promelas, and L. cyanellus exhibited relatively high levels of ag- gressive behavior, and greater amounts of in- terspecific aggression than Ptychocheilus lucius. The wide dispersion of food items in the water column suggests that such aggressive behavior may not have been induced by food shortage. Interactions between L. cyanellus and native fishes have been studied by Lemly (1985), Os- mundson (1987), and Marsh and Langhorst (1988), who showed that predation by L. cy- azzellus can significantly reduce abundance of native fishes. This was supported by the high number of chases initiated by L. cyanellus and the aggressiveness of this species in attacking prey (i.e., razorback sucker larvae). However, mechanisms whereby Pimephales promelas and N. lutrensis affect survival of native fishes are less well understood (Minckley and Deacon, 1968; Gregor and Deacon, 1988). The aggressive behavior noted for N. lutrensis and P. promelas undoubtedly aids their prolif- eration in foreign systems. Territorial aggres- sion by P. promelas during reproductive periods has been noted by others (Carlson, 1967; Sigler and Sigler, 1987), and we assume that most, if