The trophic ecology of a desert river fish assemblage: influence of season and hydrologic variability

Abstract

Citation: Behn, K. E., and C. V. Baxter. 2019. The trophic ecology of a desert river fish assemblage: influence of season and hydrologic variability.Abstract. Foodweb analyses can provide insight as to how species persist in naturally dynamic environments and how these communities may respond to anthropogenic alteration. In the remaining undammed rivers of the arid southwestern United States, it is thought that plasticity in feeding behavior allows native fishes to thrive in the context of natural fluctuations in temperature, sediment, and resource availability associated with extreme swings in season and hydrology. There have been few evaluations of this hypothesis, in part owing to the rarity of rivers that retain their natural environmental dynamics and complement of native species. We investigated the feeding ecology of the fish assemblage of one such river, the Little Colorado River, Arizona, focusing on four native species: bluehead sucker (Catostomus discobolus), flannelmouth sucker (Catostomus latipinnis), speckled dace (Rhinichthys osculus), and endangered humpback chub (Gila cypha). Over two years, we sampled fish diets in July, September, January, March, and June, encompassing turbid and clear-water conditions across seasons. We quantified food resource contributions to fish diets in fish-centered food webs, comparing diversity of trophic linkages and linkage-strength distribution across time. We used ordination analyses to investigate differences in diets among species and changes in diet composition across seasons and conducted seasonal comparisons of gut fullness. We observed a high diversity of dietary items (as absolute numbers and consumer-resource ratios) within a fish assemblage that was apparently resistant to extreme and recurrent environmental variability. Fish diet diversity was 29 that of the nearby dammed Colorado River's tailwater, and over 50% of fish-resource linkages were weak (<1% of species' diet). Native fishes consumed a mix of terrestrial and aquatic resources. Notably, the water column-feeding humpback chub consumed the largest percentage of terrestrially derived food items and appeared to capitalize on food resources made available by a summer flash flood, consuming twice as much during this flood than during other seasons (based on gut fullness, corrected by estimates of food quality). Other fishes exhibited similar dietary variation. Such opportunist/generalist strategies, coupled with diverse food resources, may be important to the persistence of these and other native fishes in dynamic environments.

Trophic niches of native and nonnative fishes along a river-reservoir continuum

“…Although we observed more overlap among species in the two habitats downstream of the waterfall, we do not expect competition is a strong driver of assemblage dynamics as river-reservoir inflows are not likely resource-limited 26 , 75 , 76 . Because native fishes in the Colorado River basin tend to be trophic generalists 77 , they might be less susceptible to competitive exclusion by nonnative species despite overlap in isotopic niche space. Fishes in the Colorado River basin likely evolved to capitalize on resource availability that varied across space and time, including across lotic-lentic habitat gradients such as the Colorado River Delta 78 , 79 , expansive reaches of river impounded by lava dams 80 , and lentic habitats created by high water events 81 .…”

Non‐random dispersal in sympatric stream fishes: Influences of natural disturbance and body size

“…The second explanation is that our results reflect spatial tracking of temporarily dynamic resources. It is well known that flood disturbance significantly alters aquatic habitats through redistribution of substrate and reformulation of channel geomorphology (Behn & Baxter, 2019; Death et al., 2015; Larson et al., 2018; Nakamura et al., 2000; Robertson et al., 2015). Our previous study documented that high flows, even those of smaller magnitudes than defined and modelled in this study, mobilised and redistributed nest substrate of bluehead chub during the reproductive period (Kim et al., 2020).…”

Effects of Disparate Water Temperatures and Food Bases on Humpback Chub Growth Rates within the Little Colorado River, Arizona

“…For fish to grow, they must consume more food than is needed to meet the basic energy costs for body maintenance (Wootton ). During base flow, the diet of Humpback Chub is primarily restricted to autochthonous aquatic food items (Behn and Baxter ), which may limit their growth in the lower reach compared with the translocation–Atomizer reaches. However, Behn and Baxter () found that Humpback Chub consume large amounts of diverse food items during floods in the LCR, which deliver terrestrially derived food items and expand the available foraging habitat into adjacent floodplains.…”

“…During base flow, the diet of Humpback Chub is primarily restricted to autochthonous aquatic food items (Behn and Baxter ), which may limit their growth in the lower reach compared with the translocation–Atomizer reaches. However, Behn and Baxter () found that Humpback Chub consume large amounts of diverse food items during floods in the LCR, which deliver terrestrially derived food items and expand the available foraging habitat into adjacent floodplains. Therefore, the growth rates of Humpback Chub, with respect to invertebrate food items, are apt to be higher in the upper than in the lower reaches during base flow and uniform across reaches during floods.…”