How does water temperature impact the humpback chub's ability to reproduce
Answers
A 1°C increase in water temperature decreased short-term predation vulnerability of Humpback Chub to Rainbow Trout by about 5%, although the relationship is not linear.
Answer:
Introduced rainbow trout and brown trout are considered a threat to the endangered humpback chub in the Colorado River in Grand Canyon. These introduced species eat native fish, but impacts are difficult to assess because predation vulnerability depends on the physical conditions under which predation takes place. We studied how predation vulnerability of juvenile humpback chub changes in response to turbidity. We exposed hatchery-reared juvenile humpback chub and bonytail (a surrogate for humpback chub) to adult rainbow and brown trout at turbidities ranging from 0 (clear water) to 1,000 formazin nephlometric units (FNU). Turbidity as low as 25 FNU reduced predation of bonytail to rainbow trout and led to a 36% increase in survival compared to trials conducted in clear water. Predation vulnerability of bonytail to brown trout at 25 FNU also decreased with increasing turbidity and resulted in a 25% increase in survival. This research suggests that relatively small changes in turbidity may be sufficient to alter predation dynamics of trout on humpback chub in the Colorado River, and that turbidity manipulation may warrant further investigation as a fisheries management tool
Explanation:
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.