Table 1 Article inclusion and exclusion criteria, summarized from the protocol [27]

Subject (population)

Any fish species in North (23.5° N to 66.5° N) or South (23.5° S to 66.5° S) temperate regions. Fish can be resident (i.e., non-migratory) or migratory, including diadromous species (e.g., fish that migrate between fresh and salt water), and at any life stage. This can also include species that have been stocked (but are no longer being stocked) or invasive, and are now established in a waterbody. Only studies located in freshwater or estuarine fluvial (i.e., water moving via gravity) ecosystems, including lakes, rivers, and streams were considered. Studies that evaluated the effect of water diversion (into irrigation ponds) i.e., reduced flow on fish from the source stream

Studies that evaluated the effects of altered flows in (1) aquaculture, fish farms, or hatcheries (usually in the form of changes in water velocity), or (2) a completely marine ecosystem or if in a waterbody that does not have moving water via gravity (e.g., irrigation ponds)

Intervention/exposure

Articles that describe a change in, or modification to, a component of flow regime, including: magnitude, frequency, duration, timing (seasonality), rate of change, or surrogate thereof (e.g., water velocity or depth) (see Table 2 for definitions). Articles that (1) do not specify a flow component [e.g., the study compare an unregulated stream (or section of a stream) to a regulated stream (i.e., regulated via a hydro dam)], or (2) report unspecified multiple components affecting flow (i.e., do not report effects of components separately to isolate individual effects of components). Relevant causes of a change in/modification to flow regime include: (1) anthropogenic causes: dams, reservoirs (impoundments), hydroelectric facilities, locks, levees, water withdrawal (abstraction), water diversion, and land-use changes; (2) natural causes: climate change (possible indirect anthropogenic cause as well), floods, droughts, seasonal changes; or (3) restoration activities (i.e., water input into the study waterbody for restoration of flow, dam removal, etc.)

Studies evaluating other environmental drivers in relation to fish and did not also include an evaluation of a component of flow. Other environmental interventions included: (1) changes in physical (hydraulic) habitat (e.g., width–depth ratio, wetted perimeter, pool volume, bed substrate); (2) changes in flow-mediated water quality (e.g., sediment transport, dissolved oxygen, temperature); (3) changes in in-stream cover (e.g., bank undercuts, root masses, woody debris, fallen timber, overhanging vegetation); or (4) changes in the freshwater inflow or input. Studies that focused on effects of altered flows related to road culverts (separate systematic review on this topic, see Rytwinski et al. [29])

Outcome

The reported measured outcome should indicate some change in a component of fish productivity, broadly defined to include any measurement related to: biomass, abundance, yield, diversity, density, growth, survival, individual performance, migration, reproduction, recruitment, sustainability, population viability, persistence, stress, or surrogate thereof. Only studies that evaluated a direct response (outcome) of some aspect of fish productivity listed above. Studies that evaluated fish movement behaviour in relation to flow (e.g., movement in areas of high and low flow) and (1) related this to activity level or energy expenditure, or (2) reported numbers or abundance of fish entering river (i.e., river entry)

Studies that only evaluated an indirect response to altered flow i.e., authors make an indirect link between the measured outcome of altered flow (e.g., growth of aquatic plants) and its “potential” affect on fish (e.g., spawning habitat availability), were excluded. Indirect outcomes included, for example: (1) nutrients (e.g., amount/concentration), food supply (e.g., amount of plankton), infection/disease rate (e.g., parasites). Studies that evaluated: (1) fish habitat preference/use/selection or used habitat suitability indices (HIS); (2) migratory behavior in relation to flow and used timing of river entry as an outcome measure; or (3) stranding (e.g., probability of stranding or numbers of stranded fish)

Comparator

Relevant comparators included: (1) similar sections of the same waterbody with no intervention (i.e., upstream or downstream condition); (2) separate but similar waterbodies with no intervention; (3) before intervention data within same waterbody (i.e., pre-construction/modification); (4) an alternative level of intervention on the same or different study waterbody, or (5) controlled flume study. Studies that look at trends without true comparators: (1) temporal trends that look at the relationship/correlation between fish productivity and changes to flow across time but without a ‘true’ before intervention time period; or (2) spatial trends that do not include “zero-control” sites: (a) across waterbodies [e.g., surveyed fish abundance in 6 different streams (i.e., not all similar in morphology) and related to flow magnitude]; or (b) within a waterbody [e.g., survey fish abundance in different sections of the same stream that differed in morphology (e.g., riffle and run) and related to flow frequency]; were included but subgrouped together (TRENDS). Also, studies that evaluated: (1) > 1 after-treatment time periods but there was no change/modification to flow across time periods [i.e., repeat visits with no before-treatment data or control site; After-only (A-only)]; (2) > 1 impact sites but there was no change/modification to flow across impact sites [multiple impact sites but no control sites or before-treatment data; Impact-only (I-only)]

Studies that evaluated: (1) a single point in time, with no comparison to another site; or (2) a single impact site, with no before-treatment data

Study design

Study designs with appropriate comparators including Before/After (BA), Control/Impact (CI) including a gradient of intervention intensity that included a “zero-control” site (CI-gradient), an alternate level of intervention (ALT-CI), as well as Before/After/Control/Impact (BACI) studies and Randomised Controlled Trials (RCT; e.g., lab or small in field manipulations) were included. Studies that used a reference conditional approach or a normal range design were included and coded as CI designs. Studies that look at temporal or spatial (within or across waterbodies) trends with a change in, or modification to, a component of flow regime but that do not have true comparators (TRENDS). Also, studies that look at multiple after-treatment time periods (A-only) or impact sites (I-only) but there was no change/modification to flow across time periods or impact sites and that do not have true comparators

Theoretical studies, review papers and policy discussions

Language

English at full-text

Any study that is not in English at full-text