The effectiveness of spawning habitat creation or enhancement for substrate-spawning temperate fish: a systematic review

Abstract Background Habitat is the foundation for healthy and productive fisheries. For fish that require substrate for spawning, lack of appropriate spawning substrate is inherently limiting and a lack of access to suitable spawning habitat will lead to population collapse. To ensure management resources are being allocated wisely and conservation targets are being achieved, there is an increased need to consider the effectiveness of techniques to enhance or create habitat that has been lost. The aim of this systematic review was to assess the effectiveness of techniques currently used to create or enhance spawning habitat for substrate-spawning (including vegetation-spawning) fish in temperate regions, and to investigate the factors that influence the effectiveness of habitat creation or enhancement. Methods Searches for primary research studies on the effect of spawning habitat creation or enhancement for substrate-spawning fish were conducted in bibliographic databases, on websites and an online search engine, through evidence call-outs, social media, and Advisory Team contacts, and in the bibliographies of relevant reviews. All articles were screened at two stages (title and abstract, and full-text), with consistency checks being performed at each stage. Relevant articles were critically appraised and meta-data and quantitative data were extracted into a database. All included studies were described narratively and studies that met the criteria for meta-analysis were analyzed quantitatively. Review findings A total of 75 studies from 64 articles were included in this systematic review and underwent data extraction and critical appraisal. The majority of these studies were from North America (78.1%) and a large percentage (63.7%) targeted salmonids. We conducted a meta-analysis using data from 22 studies with 53 data sets. Available evidence suggests that the addition or alteration of rock material (e.g., gravel, cobble) was effective in increasing the abundance of substrate-spawning fish compared to controls, with a taxonomic bias towards salmonids (5/6 data sets). The addition of plant material (e.g., large woody debris) with or without physical alterations to the waterbody (e.g., excavation) was also effective in increasing substrate-spawning fish abundance on average compared to controls. Egg life stages (i.e., nests, redds, zygotes or developing embryos) were associated with larger increases in abundance with habitat creation or enhancement than age-0 life stages (i.e., alevin, fry, young-of-the-year). We found no detectable effect of ecosystem type (lotic vs. lentic waterbodies) or time since habitat creation or enhancement on intervention effectiveness for fish abundance. Conclusions The synthesis of available evidence suggests that the addition or alteration of rock material (e.g., addition of gravel, substrate washing) was an effective means of enhancing spawning habitat, but results may only be applicable for salmonids. Furthermore, the synthesis suggests that on average, the addition of plant material with or without waterbody modifications was also effective at increasing fish abundance. Overall, we were limited in our ability to address many of the questions that stakeholders have regarding the circumstances under which spawning habitat creation or enhancement is effective for substrate-spawning fish. Before we can provide recommendations with a higher level of certainty, we need to improve research and reporting, and expand research focus to include a broader range of species and intervention types. We provide several recommendations aimed at researchers and practitioners to improve the quality of evidence being generated.

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