Overriding Effects of Species-Specific Turbidity Thresholds on Hoop-Net Catch Rates of Native Fishes in the Little Colorado River, Arizona

I examined the effects of turbidity, discharge, and temperature on hoop-net catch rates of native humpback chub Gila cypha, speckled dace Rhinichthys osculus, flannelmouth suckers Catostomus latipinnis, and bluehead suckers C. discobolus in the Little Colorado River, Arizona. Results indicated that native fish catch rates were primarily influenced by whether turbidity levels were below or above species-specific thresholds of approximately 545 nephelometric turbidity units (NTU) for humpback chub, 221 NTU for speckled dace, 846 NTU for flannelmouth suckers, and 70 NTU for bluehead suckers. The effects of discharge were negligible, but discharge did dictate the turbidity level. Turbidity levels at hoop-net deployment relative to the turbidity thresholds predetermined much of the catchability of fish. Catch rates were highest in a high-catch zone consisting of the lowermost turbidities, which ranged up to the start of a transition zone for each species (humpback chub, <= 54 NTU; speckled dace, <= 29 NTU; flannelmouth sucker, <= 81 NTU; bluehead sucker, <= 26 NTU), and the secondary effects of temperature were only detectable in this zone. Catch rates within the transition zone decreased at higher turbidities up to the thresholds and thereafter remained consistently low at all higher turbidities within this low-catch zone. The effects of the turbidity thresholds on catch rates decreased for larger fish of all species. Theoretically, the effects of turbidity reflect a behavioral switch by native fishes from relying primarily on structural cover (e. g., hoop nets) to using turbidity as cover to reduce the predation risk as turbidity increases from below to above the thresholds. Moreover, turbidities within the low-catch zone could be totally incapacitating the fishes' visual capabilities such that fish perceive these turbidity levels as unlimited visual cover. The turbidity threshold concept probably applies to the hoop-net catch rates of other fishes in other riverine systems.