Growth of a threatened desert fish becomes resource dependent when released from temperature limitation in a regulated river
1. Growth of ectotherms typically positively correlates with increasing temperature towards an optimal temperature range, followed by a decline in growth when temperatures exceed the optimal temperature range for that organism. When temperature is within or close to the optimal range, food availability and other environmental factors can play important roles in modifying growth rates. River regulation often alters both environmental conditions (e.g., flow, turbidity and temperature) and aquatic food webs, modifying the quality and quantity of resources available to consumers. Here we aim to understand how food availability, water temperature and other environmental factors are associated with fish growth (defined here as a change in length) in a large, regulated desert river. 2. We measured growth of humpback chub (Gila cypha) at two river reaches located 240 km apart in the Colorado River within the Grand Canyon, Arizona, USA. We then fit Bayesian state-space models of growth that account for variable time at large to test environmental predictors such as water temperature, flow, turbidity (which increases during tributary flooding and can be a proxy for allochthonous inputs) and gross primary productivity (a proxy for autochthonous food) using an inclusion parameter approach. Both river reaches are located below Glen Canyon Dam, which regulates flow, alters seasonal water temperatures and traps sediment, nutrients, and organic carbon; however, merging tributaries and a greater distance from the dam lead to differences in environmental conditions. 3. In the upriver reach, where warm season water temperatures are below the optimal range for humpback chub most of the time, temperature was the primary driver of variation in growth according to inclusion parameter values and the strength of the standardised effect size. Turbidity was also included in the final model for the upper reach, but the inclusion parameter values and standardised effect size were smaller than for temperature. In the downriver, warmer reach, temperature still had the highest inclusion parameter value, but GPP had the highest standardised effect and nearly as high an inclusion parameter value. 4. We conclude that temperature is the primary limiting factor on growth of humpback chub in the Grand Canyon; however, as water temperatures increase, other factors, especially GPP, are associated with temporal variation in growth rates. Over the past two decades, declines in reservoir elevations and the volume of summer releases have increased downstream water temperatures. If warm temperatures persist into the future, other factors may have increasing roles in regulating interannual variability in chub growth. 5. Field studies of ectotherm growth often emphasise the role of water temperature; however, the role of resource availability is also increasingly recognised, especially in bioenergetics studies. Quantifying resource availability directly can be difficult, especially in large, remote rivers. Here, we show here that using proxies for resource availability (i.e., GPP as a proxy for autochthonous inputs) can yield useful insights regarding resource limitations with implications for management of federally listed fish species in a larger regulated river system.