Fish recruitment in rivers with modified discharge depends on the interacting effects of flow and thermal regimes

Abstract

Summary Alteration of flow and thermal regimes is a key consequence of human use of river and floodplain ecosystems, and these impacts result from a range of interacting ecological mechanisms. Environmental flow regimes are a management strategy to restore or maintain ecologically important aspects of river hydrology. However, inadequate understanding of the processes that determine the effects of flow on population dynamics of biota hinders the maximum benefits of environmental flows. Spawning and recruitment of riverine fish is mediated by temperature, access to nursery habitat and the timing and availability of resources. Differences in population sizes between regulated and unregulated rivers are often attributed to the effect of flow and thermal regime change on population dynamics. Dams and extraction of water have altered the flow and thermal regimes of rivers in the northern Murray–Darling Basin, Australia. Combining three studies, we (i) examined differences in the recruitment of fish and patterns of potential prey availability between a regulated and unregulated river, (ii) studied the relationships between patterns of flow and temperature on spawning of fish in two rivers with contrasting regulated flow regimes and (iii) experimentally tested the effects of environmental flow releases in a regulated river on fish spawning and recruitment by comparing patterns over time against two unregulated rivers. Comparisons between regulated and unregulated rivers indicated that the significantly lower recruitment of Macquaria ambigua in the regulated river was linked with large differences in available prey. Species‐specific differences in the abundance of fish larvae between two regulated rivers indicated that different reproductive strategies determine population‐level responses to flow regulation. Finally, the experimental flow releases in a regulated reach during late spring did not result in a change in spawning and recruitment of fish, although influences of antecedent flow and temperature on larval and juvenile assemblage composition were significant. These findings indicate that the responses of fish spawning and recruitment to flow regime change and restoration are dependent on the conditions that determine the success of these critical life‐history processes. Specifically, key effects of flow regulation are reduced summer water temperatures due to hypolimnetic discharge from dams, and the reduced frequency and duration of low flows. Reduced water temperatures limit opportunities for fish to spawn, and sustained base flows were associated with lower densities of potential prey sources in conjunction with potential flushing of larvae and juveniles, thereby influencing recruitment. The benefits of environmental flow programmes can be enhanced if flow regimes are modified to ensure that abiotic and biotic conditions that support persistence of biota are provided. In conjunction with flow pulses and floods, low flows are a natural and ecologically important component of flow regimes, particularly in drylan...

Macroinvertebrate responses to flow and stream temperature variability across regulated and non‐regulated rivers

“…The strength of associations between abiotic indices and macroinvertebrate community responses typically improved when seasonal models were considered, with no thermal indices comprising the most highly correlated models. This is in contrast to research highlighting that comparable ecological variance could be explained by flow and stream temperature variability within some regulated systems (Jackson et al, ; Rolls et al, ). This study found that hydrological indices relating to the timing of extreme flows were of high ecological significance across both regulated and non‐regulated sites.…”

Seasonal temperature and precipitation regulate brook trout young‐of‐the‐year abundance and population dynamics

“…YOY abundance is regulated by abiotic and biotic conditions in freshwater fishes. Spawning success, egg survival and post‐hatching survival have been linked to water temperature (Warren et al ., ; Rolls et al ., ) and stream flow (Fausch et al ., ; Smith, Odenkirk & Reeser, ; Dutterer et al ., ). This indicates that climate change is a mechanism that will likely affect YOY abundance dynamics.…”

Importance of antecedent environmental conditions in modeling species distributions

“…Temporal variations in environmental forces often result in time‐lagged ecological responses and the effects persist for certain periods of time (the so‐called antecedent effect, hysteresis, and ecological memory). Such antecedent effects have been observed in freshwaters and other realms at all ecological scales: physiological and behavioral changes of organisms (Tonkin et al , Amorim et al ), population dynamics (Magalhães et al , Rolls et al , George et al ), distributional ranges of species, and resistance and resilience at the community and ecosystem levels (Kinzig et al , Tockner et al ). The inclusion of antecedent effects at appropriate time scales in models generally better explains variations in ecological responses (Ogle et al ).…”