The review team has conducted a scoping exercise to test alternative search strings. The exercise resulted in the selection of the following search terms:
Subject: lake*, reservoir*, pond*, fresh$water
Intervention: *manipulat*, remov*, restor*, stock*, introduc*, reduc*, addition
Target: *planktivor*, *benthivor*, cyprinid*, piscivor*, “predatory fish*”, Rutilus, Abramis, Esox, Perca, Stizostedion, Micropterus, Dorosoma, Coregonus, Oncorhynchus, Salmo, roach, bream, pike, muskellunge, perch, pike$perch, zander, sander, “*mouth bass”, whitefish, cisco, minnow, “gizzard shad”
The terms within each of the categories ‘subject’, ‘intervention’ and ‘target’ will be combined using the Boolean operator ‘OR’. The three categories will then be combined using the Boolean operator ‘AND’. An asterisk (*) is a ‘wildcard’ that represents any group of characters, including no character, while a dollar sign ($) represents zero or one character.
Searches will also be made for Danish, Dutch and Swedish counterparts to the above terms. The following search strings will be used (although they in some cases will have to be simplified):
English: (lake* OR reservoir* OR pond* OR fresh$water) AND (*manipulat* OR remov* OR restor* OR stock* OR introduc* OR reduc* OR addition) AND (*planktivor* OR *benthivor* OR cyprinid* OR piscivor* OR “predatory fish*” OR Rutilus OR Abramis OR Esox OR Perca OR Stizostedion OR Micropterus OR Dorosoma OR Coregonus OR Oncorhynchus OR Salmo OR roach OR bream OR pike OR muskellunge OR perch OR pike$perch OR zander OR sander OR “*mouth bass” OR whitefish OR cisco OR minnow OR “gizzard shad”)
Danish: (sø* OR dam OR mose* OR ferskvand*) AND (*manipulat* OR opfisk* OR restau* OR udsæt* OR introduk* OR reduk*) AND (*planktivor* OR *benthivor* OR cyprinid* OR piscivor* OR rovfisk* OR fredfisk* OR skidtfisk* OR Rutilus OR Abramis OR Esox OR Perca OR Stizostedion OR Coregonus OR Oncorhynchus OR Salmo OR skalle OR brasen OR gedde OR sandart OR aborre OR *ørred OR helt)
Dutch: (meer* OR plas* OR zoetwater*) AND (biomanipul* OR “actief biologisch beheer” OR afvissen OR restauratie* OR uitzetten*) AND (*planktivor* OR *benthivor* OR planktoneten* OR bodemomwoel* OR piscivor* OR visetende* OR roofvis* OR Rutilus OR Abramis OR Esox OR Perca OR Stizostedion OR brasem OR snoek OR ruisvoorn OR snoekbaars OR karper)
Swedish: (sjö* OR insjö* OR *magasin* OR *damm* OR sötvatten* OR färskvatten*) AND (biomanipul* OR utfisk* OR reduktionsfisk* OR reducer* OR *restaurer* OR inplanter* OR utplanter* OR utsättning*) AND (*planktivor* OR *planktonäta* OR bent$ivor* OR bottenäta* OR bottendjursäta* OR cyprinid* OR karpfisk* OR piscivor* OR rovfisk* OR Rutilus OR Abramis OR Esox OR Perca OR Stizostedion OR Coregonus OR Oncorhynchus OR Salmo OR mört OR brax* OR gädda OR abborre OR gös OR sik OR *lax OR *öring OR regnbåge)
No time, language or document type restrictions will be applied.
In addition to the main search described above, a complementary search will be made in a few of the sources mentioned below. The complementary search will focus on potential mechanisms and outcomes of biomanipulation, using the following set of search terms:
Subject: lake*, reservoir*, pond*, fresh$water
Mechanisms: trophic, cascad*, food$web, top$down, bottom$up, resuspen*, “stable state*”, bistable, “regime shift*”
Outcomes: water$quality, transparency, clarity, turbid*, secchi, “suspended solids”, phosph*, nitrogen, oxygen, chlorophyll, phytoplankton
The search aims to include the following online databases:
Aquatic Sciences and Fisheries Abstracts
Directory of Open-Access Journals
Other data sources
Relevant literature will also be searched for in bibliographies of literature reviews such as those mentioned in the background section.
In addition, unpublished data may be available from e.g. consultants or local authorities involved in biomanipulation projects. Stakeholders will be asked to suggest suitable contacts.
Study inclusion/exclusion criteria
Articles found by searches in databases will be evaluated for inclusion at three successive levels. First they will be assessed by title. In cases of uncertainty, the reviewer will tend towards inclusion. As a check of consistency, a subset of 100 articles will be assessed by all members of the review team.
Next, each article found to be potentially relevant on the basis of title will be judged for inclusion on the basis of abstract. The reviewer will tend towards inclusion in cases of uncertainty. A subset consisting of at least 10% of the articles will be assessed by at least two reviewers. A kappa statistic relating to the assessments will be calculated. If this statistic indicates that the reviewers are inconsistent in their assessment (κ < 0.5), discrepancies will be discussed and the inclusion criteria will be clarified or modified.
Finally, each article found to be potentially relevant on the basis of abstract will be judged for inclusion by reviewers studying the full text. Again, the reviewers will tend towards inclusion in cases of uncertainty.
Studies or datasets found by other means than database searches may be entered at any of the two latter stages of this screening process.
A list of studies rejected on the basis of full-text assessment will be provided in an appendix together with the reasons for exclusion.
Each study must pass each of the following criteria in order to be included, either by providing all the required data itself or by referring to other studies where supplementary information is presented.
Relevant subjects: Temperate lakes or reservoirs (with an area equal to or larger than 1 hectare) characterised by study authors as eutrophic (or hypertrophic) and/or having summer concentrations of total phosphorus exceeding 30 μg/l before biomanipulation.
Relevant types of intervention: Removal (mainly by fishing) of planktivorous or benthivorous fish, stocking of piscivorous fish and any combination of such interventions.
Relevant type of comparator: No intervention.
Relevant types of outcome: Change of Secchi depth, of concentrations of oxygen, nutrients, suspended solids and chlorophyll, and of phytoplankton abundance.
Relevant types of study: Any primary field study of water quality in lakes or reservoirs (or in artificially separated compartments with areas ≥ 1 ha in such water bodies) that have been subjected to large-scale biomanipulation of the kinds described above. The study should be based on before/after comparisons or site comparisons or both (see Study quality assessment below).
Potential effect modifiers and reasons for heterogeneity
To the extent that data are available, the following potential effect modifiers will be considered and recorded:
Mean and maximum lake depth
Lake connectivity (presence of tributaries and/or connections that allow fish migration into the lake)
Lake salinity and conductivity
Water colour or concentration of dissolved organic carbon (DOC)
Occurrence of stratification in the lake
Annual and monthly water temperature
Special weather conditions (e.g. droughts, heat waves, storms)
Land use in the surrounding area before, during and after biomanipulation (including attempts to reduce nutrient losses by modifying the use of fertilisers, establishing buffer zones with permanent vegetation between fields and watercourses etc.)
History of external supplies of nutrients (and other pollutants) from point sources and runoff, of internal nutrient loading and of any experimental nutrient additions to the lake. It may also be relevant to consider the removal of nutrients with fish that are caught in the lake.
History of fisheries and stocking
History of damming, lake lowering and other hydrological disturbances
Presence of introduced species
Presence of grazing or piscivorous birds
History of disturbances (algal blooms, fish kills etc.) caused by eutrophication
Earlier or contemporary attempts to mitigate eutrophication problems (using other approaches than biomanipulation) and to improve recruitment habitats for predatory fish
Dominating fish species before manipulation
Methods for fish removal
Study duration and seasonality
Intervention duration and seasonality
Further modifiers and causes of heterogeneity will be identified in an iterative process.
Study quality assessment
Most studies that may be relevant in the present context report on how the water quality of a lake (or a number of lakes) has responded to some kind of biomanipulation. Thus, they are usually ‘BA’ (Before/After) studies comparing data that have been collected prior to and following the intervention. Some of them present data from a single sampling occasion after the intervention, while others are based on repeated data collection over several years. Long-term studies are particularly valuable in this context, since they may clarify whether biomanipulation has lasting effects or not.
Alternatively, a study may be based on comparison between a manipulated lake and a similar lake where no such intervention has taken place (or between different parts of a single lake that has been artificially separated into two or more compartments, at least one of which has not been manipulated). These may be termed as ‘CI’ (Comparator/Intervention) studies, or ‘BACI’ (Before/After/Comparator/Intervention) if they present data collected both before and after the intervention.
The BACI study design is generally to be regarded as more reliable than the BA and CI designs. Similarly, studies that describe potential effect modifiers are more valuable than studies that do not report on the local environment in detail.
The following factors will be assessed and used to categorise studies as having high, medium, or low susceptibility to bias:
Study design (BA/CI/BACI)
Temporal extent of study
Methodological detail Accounting for potential effect modifiers
Use of statistics and statistical analysis
Detailed reasoning concerning these factors will be recorded in a transparent manner. In general, the quality of a study will be assessed by one reviewer. As a check of consistency, however, a subset of the studies will be appraised by all reviewers involved in the quality appraisal.
A list of studies rejected on the basis of quality assessment will be provided in an appendix together with the reasons for exclusion.
Data extraction strategy
Means and measures of variation (standard deviation, standard error, confidence intervals) will be extracted from tables and graphs, using image analysis software when necessary. If only raw data are provided, summary statistics will be calculated. Data on potential effect modifiers will also be extracted.
It may in some cases be useful to ask authors of relevant articles for access to unpublished primary data.
Data synthesis and presentation
A narrative synthesis of data from all studies included in the review will describe the quality of the results along with the findings of studies of sufficient quality. Tables will be produced to summarise these results. Where studies report similar outcomes, meta-analysis may be possible. In these cases effect sizes will be standardised (using standardised mean effect size) and weighted appropriately. Details of the quantitative analysis will only be known when full texts have been assessed for their contents and quality.
Meta-analysis of effect sizes will take the form of random-effects models, and meta-regression will be performed where effect modifiers cause significant heterogeneity between studies. Subgroup analysis of categories of studies will also be performed where sufficient studies report common sources of heterogeneity. Publication-bias and sensitivity analysis will also be carried out where possible. Overall effects of biomanipulation will be presented visually in plots of mean effect sizes and variance.