Background & Importance
Closure of Glen Canyon dam in 1963 resulted in significant changes to the physical environment of the Colorado River in Grand Canyon, including decreased water temperature and reduced turbidity. These changes allowed introduced fish such as rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) to flourish, whereas native species such as humpback chub (Gila cypha) declined in abundance and distribution. Humpback chub was listed by the U.S. Fish and Wildlife Service in 1967 and was given full protection as an endangered species pursuant to the U.S. Endangered Species Act. As Lake Powell filled with water, an estimated 95% of the sediment supply to the Colorado River through Grand Canyon was cut off and deposited into Lake Powell. These changes in turbidity likely contributed to changes in the fish community.
Predation on juvenile humpback chub by rainbow trout and brown trout within the mainstem Colorado River has been identified as a potential cause for humpback chub population declines in Grand Canyon. Investigations of trout diets within the Colorado River indicate that these species eat juvenile native fish, but impacts of trout predation on native fish populations is difficult to predict because predation vulnerability changes with environmental conditions. Turbidity changes the way visual predators such as trout detect prey. As water becomes more turbid and light decreases, the ability of trout to use contrast to successfully identify and react to prey decreases.
Turbidity varies on both a seasonal and annual basis leading to highly variable incidence of piscivory for trout inhabiting this area of the Colorado River. It has been hypothesized that management actions that increase turbidity within the Colorado River could lead to increases in recruitment of native Colorado River fishes by reducing predation mortality. Understanding the relationship between water clarity and predation vulnerability is therefore critical when evaluating management actions designed to benefit native fish species such as humpback chub within the Colorado River.
General Methods
We conducted laboratory experiments to evaluate how short-term predation vulnerability of humpback chub changes in response to turbidity using juvenile captive-reared humpback chub and bonytail (Gila elegans) as prey and wild caught adult rainbow trout and brown trout as predators. Bonytail were included in the experiment because limited numbers of humpback chub are available for research purposes. Bonytail are closely related to humpback chub, native to the Colorado River and its tributaries, and make good surrogates for humpback chub because of similarities in morphology and life history, especially as juveniles. All fish were maintained in separate holding tanks in a temperature controlled greenhouse at the U.S. Forest Service Rocky Mountain Research Station in Flagstaff, AZ.
Predation trials were conducted in 12 replicate 568-liter (~150 gallons) fiberglass tanks that measured 1.2 m (~4’) long by 0.9 m (~3') wide with a water depth of 30 cm (~1’). Four trout of a given species were placed into each tank and 12 humpback chub or bonytail were placed into a cylinder-shaped mesh basket within each tank at the beginning of the trial. All fish were allowed to recover from capture and handling and to acclimate to the predation tanks for a period of 48 hours. Fine sediments were gathered from the lower Paria River near the confluence with the Colorado River and the Little Colorado River directly above Grand Falls. This sediment was sifted through a 63 micron sieve and blended using a standard kitchen blender to obtain a suspension of fine silt and clay particles. This suspension was added gradually at the beginning of the acclimation period and tested using a Hach® 201 turbidimeter until the turbidity stabilized at each target level.
After the 48-hour acclimation period, turbidity was measured again and additional sediment was added if needed to reach target turbidity levels. Baskets containing prey fish were then tipped over and removed from the tanks with chub being carefully released into each tank, initiating the predation experiment. Tanks were left undisturbed for 24 hours under natural light. After 24 hours, water in each tank was drained and all surviving fish were captured, counted and measured for total length (TL). Experiments took place from October 2013 to May of 2014 and typically began between 8 AM and 10 AM. A total of 124 individual overnight trials were conducted utilizing 256 rainbow trout (223 – 330 mm (~9-13”) TL) and 208 brown trout (193-399 mm (~8-16”) TL). Prey fish consisted of two sizes of juvenile chub, 120 individual humpback chub (41-64 mm (~1.5-2.5”) TL) and 1008 bonytail (60-73 mm (92.5-3”) TL).
Important Results
Initial trials with a wide range of turbidity (0 – 1000 formazin nephlometric units (FNU)) using humpback chub revealed that the greatest changes in predation vulnerability occurred at turbidities < 200 FNU. Subsequent trials using bonytail, conducted at 0 – 150 FNU, indicated that increased turbidity significantly reduced predation vulnerability of bonytail to both rainbow and brown trout, although increases in survival with increasing turbidity were much more pronounced for rainbow trout than for brown trout. An increase in turbidity from clear water to 150 FNU (with all other factors held constant) resulted in a more than 4 fold increase in juvenile chub survival to predation by both rainbow trout (21% to 91%) and brown trout (7% to 35%).
Abiotic factors like turbidity can have large impacts on biotic interactions and play an important role in structuring animal communities. Our study suggests that rainbow and brown trout are less effective predators on native chub species as turbidity increases, with turbidity as low as 25 FNU reducing predation vulnerability of bonytail to rainbow and brown trout by 36% and 25%, respectively. Our observed decrease in predation vulnerability associated with increases in turbidity is consistent with other published studies conducted in both laboratory and natural environments. This information may be useful in evaluating the ecological implications of turbidity changes caused by Glen Canyon Dam and in evaluating potential management actions aimed at benefitting humpback chub within Grand Canyon. Relatively small changes in turbidity may be sufficient to alter predation dynamics of rainbow trout on humpback chub in the Grand Canyon. Management actions aimed at augmenting sediment below Glen Canyon Dam have been considered, but not implemented because of high cost. Evaluations of low level silt and clay augmentation only, for the purpose of producing turbidity in the 25-50 FNU range, to reduce predation vulnerability of native fishes may warrant further evaluation and field testing.
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