How do changes in flow magnitude due to hydropower operations affect fish abundance and biomass in temperate regions? A systematic review

Abstract

Background Altering the natural flow regime, an essential component of healthy fluvial systems, through hydropower operations has the potential to negatively impact freshwater fish populations. Establishing improved management of flow regimes requires better understanding of how fish respond to altered flow components, such as flow magnitude. Based on the results of a recent systematic map on the impacts of flow regime changes on direct outcomes of freshwater or estuarine fish productivity, evidence clusters on fish abundance and biomass responses were identified for full systematic review. The primary goal of this systematic review is to address one of those evidence clusters, with the following research question: how do changes in flow magnitude due to hydropower operations affect fish abundance and biomass? Methods This review follows the guidelines of the Collaboration for Environmental Evidence. It examined commercially published and grey literature originally identified during the systematic map process and a systematic search update. All articles were screened using an a priori eligibility criteria at two stages (title and abstract, and full-text) and consistency checks were performed at all stages. All eligible articles were assessed for study validity and specifically designed data extraction and study validity tools were used. A narrative synthesis included all available evidence and meta-analysis using the standardized mean difference (Hedges’ g) was conducted where appropriate. Review findings A total of 133 studies from 103 articles were included in this systematic review for data extraction and critical appraisal. Most studies were from North America (60%) and were conducted at 146 different hydropower dams/facilities. Meta-analysis included 268 datasets from 58 studies, separated into three analyses based on replication type [temporal (within or between year replication) or spatial]. Fish abundance (226 datasets) and biomass (30 datasets) had variable responses to changes in flow magnitude with estimated overall mean effect sizes ranging from positive to negative and varying by study design and taxa. In studies with temporal replication, we found a detectable effect of alterations to the direction of flow magnitude, the presence of other flow components, sampling methods, season, and fish life stage. However, we found no detectable effect of these moderators for studies with spatial replication. Taxonomic analyses indicated variable responses to changes in flow magnitude and a bias towards salmonid species. Conclusions This synthesis did not find consistent patterns in fish abundance or biomass responses to alterations or changes in flow magnitude. Fish responses to flow magnitude alterations or changes were highly variable and context dependent. Our synthesis suggests that biotic responses may not be generalizable across systems impacted by hydroelectric power production and operations, where specific features of the system may be highly influential. Site-specific and adaptive management may be necessary. To improve study validity and interpretability, studies with long-term continuous monitoring, and both temporal and spatial replication are needed. When this gold standard is unfeasible, studies should strive, at minimum, to maximize replication within both intervention and comparator groups for either temporal or spatial designs. To further address knowledge gaps, studies are needed that focus on non-salmonids, multiple seasons, and systems outside of North America.

How do natural changes in flow magnitude affect fish abundance and biomass in temperate regions? A systematic review

“…Most datasets for quantitative synthesis were from North America (41%), of which 94% were from the United States, potentially limiting interpretation of review results to other geographic regions. Similar geographical biases were identified in the systematic map (Rytwinski et al, 2020) and in reviews on anthropogenic impacts of flow magnitude alterations (Harper et al, 2022). Taxonomic bias, however, was less evident.…”

“…The issue of insufficient reporting of flow data has also been identified in previous reviews on the general topic of flow-fish relationships (e.g. Harper et al, 2022;Piniewski et al, 2017). We felt our qualitative descriptions of flow changes based on author descriptions allowed us to adequately categorize flow changes and thus capture more studies for quantitative analyses than would otherwise have been possible.…”

“…Relevant studies identified during full‐text screening or during data extraction, were assessed for internal study validity using a review‐specific critical appraisal tool (see Supporting Information 3 for further details). This appraisal tool was made in consultation with the Advisory Team to ensure that it incorporated all components of well‐designed studies and was informed by previous reviews (Harper et al, 2022; Macura et al, 2019; Martin et al, 2020). Study validity assessment took place at the same time as data extraction and was performed by a single reviewer following a consistency check (see Supporting Information 3).…”

“…spawning). Furthermore, as noted by Harper et al (2022), efforts should be made to minimize gaps between sampling years, and to ensure sampling occurs in multiple seasons. From a quantitative perspective, a minimum of two years before and two years after are needed to calculate a standardized effect size measure such as Hedges' g for use in a more formal meta-analysis.…”

Determination of ecological flow thresholds for rainfall-recharging rivers based on multiple hydrological methods

“…With the population growth as well as economic and social development, human intervention in regional soil and water resource and the surrounding ecological environment has been continuously intensified, among which human activities such as damming, sand panning, and shipping have significantly changed the hydrological situation of rivers, leading to increasingly prominent problems such as river drying, aquatic species extinction and ecological environment degradation (Angus Webb et al, 2013;Brown et al, 2015;Ren et al, 2018;Harper et al, 2022). As the main living water source for most urban and village residents, the reasonable and accurate measurement of ecological flow within inland river channels has become an important part of protecting the ecological security of rivers and lakes and promoting regional coordinated sustainable development (Wu and Chen, 2018;Hairan et al, 2021).…”