Linking ecosystem processes to consumer growth rates: gross primary productivity as a driver of freshwater fish somatic growth in a resource-limited river

Individual growth can exert strong control on population dynamics but is constrained by resource acquisition rates. Difficulty in accurately quantifying resource availability over large spatial extents and at high temporal frequencies often limits attempts to understand the extent to which resources limit individual growth. Daily estimates of stream metabolism, including gross primary productivity (GPP), are increasingly available but have not, to our knowledge, been linked to fish growth. Here we examine how environmental variables such as GPP, water temperature, turbidity, and high-flow releases from a dam are linked to spatiotemporal variation in the growth of flannelmouth sucker (Catostomus latipinnis) in the Colorado River within the Grand Canyon. We fit state-space growth models to 6 years of mark-recapture data collected in four river reaches spanning 300 river kilometers. Consistent with past research in this system, we find that all four environmental variables influence growth in length of a native primary consumer fish. GPP and temperature have a positive influence on growth, while turbidity and high-flow events have a negative influence. Water temperature is the dominant driver of spatiotemporal variation in growth, while the link between high-frequency GPP and fish growth is relatively novel. Fish growth is likely to be linked to stream metabolism in other systems where overall productivity, not the quality of primary producers, limits the food webs that support fish growth.