Does drifting passage need to be linked to fish habitat assessment? Assessing environmental flow for multiple fish species with different spawning patterns with a framework integrating habitat connectivity

The use of environmental flow (e-flow) regimes has been widely implemented to improve fish habitat quality in river restoration efforts. However, e-flow designs focusing only on one key life stage (e.g., spawning) without considering potential bottlenecks in other stages (e.g., hatching) can result in little to no improvement, especially when targeting the restoration of fish with drifting eggs. Few e-flow assessments are available that closely link the spatial-temporal dependence of the hydrodynamic effect of drifting eggs on hatching habitats. Moreover, an understanding of how to allocate e-flows to achieve the best possible outcomes for biological diversity conservation is still lacking. In this study, a new framework was developed to assess e-flows, aiming to satisfy the requirements of multiple fish species with different spawning patterns concerning streamflow requirements during spawning and hatching periods. In this framework, the final weighted usable area (FWUA) was proposed by linking the spawning demand to the hydrodynamic effect of drifting eggs to assess habitat quality for fish that produce drifting eggs, and water temperature was used to guide when and how to shift from fixed to moving protection targets in allocating e-flows. Here, we used the Xiangjiaba Reservoir, located in the lower Jinsha River, as a case study to design e-flows for the conservation of multiple fish species with different spawning patterns, which was beneficial for increasing the probability of restoration success. By testing scenarios with an absence of drifting passage, the ecological base flow considering only spawning habitat appears to be lower than that considering both quality of spawning habitat and hatching passage. The ecological benefits (FWUA) generated from the ecological base flow identified by traditional models represent only 64.91% of our framework and are thus anticipated to have cascading deviant effects on ecological patterns and processes in riverine ecosystems. This underlying difference in FWUA generated due to different ecological base flows determined from traditional models and our framework, however, has been overshadowed in previous research. We highlight that the highest fish population density recovery potential will be reached at only certain ratios for both sets of habitat benefits. This work provides a tool that can help managers evaluate e-flows and compare different river restoration scenarios to protect degraded rivers or develop strategies to build resilience to climate change.

Automated summary

The use of environmental flow regimes, known as e-flows, in river restoration projects to improve fish habitat quality is widespread. However, focusing solely on one key life stage, such as spawning, without considering other stages like hatching, may not lead to significant improvement, especially for fish with drifting eggs. Little research has been done to study the spatial-temporal dependency of hydrodynamic effects on drifting eggs and how to allocate e-flows for the best outcome in biodiversity conservation. This study proposes a new framework that takes into account the requirements of multiple fish species with different spawning patterns during spawning and hatching periods. The framework introduces the concept of final weighted usable area (FWUA) to assess habitat quality for fish that produce drifting eggs and suggests using water temperature as a guide for shifting from fixed to moving protection targets when allocating e-flows. The case study conducted in the Xiangjiaba Reservoir demonstrates the benefits of designing e-flows based on different spawning patterns for multiple fish species, increasing the likelihood of successful restoration. The study highlights that considering both spawning habitat quality and hatching passage is crucial for ecological base flow, as traditional models that only consider spawning habitat may underestimate the ecological benefits. This research provides a valuable tool for managers to evaluate e-flows and compare different river restoration scenarios in order to protect degraded rivers and build resilience to climate change.