The importance of a holistic ecohydraulics approach in advancing fish passage design

Recognition that monitoring the effectiveness of fish passes has tended to use non-standardized methods that make it difficult to directly compare between sites and structures on which generalized conclusions, e.g. using meta-analyses, can be formed has led to current efforts to develop a European Standard guidance document (currently a draft) for assessing fish passage efficiency. It is argued that only systematic, reproducible monitoring studies assessing the performance of fish passes will enable us to improve and develop current fish pass design. Despite describing the need for a multi-method approach, e.g. involving traps, observation, and laboratory studies, the focus of the draft standard is solely directed towards telemetry, a bias that is clear in other recent publications (e.g. Silva et al. 2018). While telemetry (e.g. acoustic, radio, and PIT tagging techniques) undoubtedly has an important role to play as part of a suite of methods that may be employed by researchers to better understand the key factors that influence fish passage efficiency, reliance only on this tool is problematic for a number of reasons. First, telemetry studies often provide valuable information, but are frequently constrained by the sitespecific context on which the case studies are based at the time they are conducted. Second, in situ field studies are unable to control for multiple confounding variables that have the potential to influence results on which conclusions are based. Third, even the most high tech and fine-scale telemetry techniques available are unable to obtain the detailed information on fish behaviour that may be achieved through direct observation. Fourth, telemetry is unsuitable for small bodied adult fish or juveniles of many species, a critical limitation in many understudied regions, such as in the temperate South (e.g. Chile and New Zealand; see Knapp et al. 2019 in a future issue of the Journal of Ecohydraulics). Finally, despite the high standard of current surgical techniques used by those proficient in telemetry, there remains the potential for tag effects that may influence behavioural and other responses (e.g. Jadot et al. 2005; Thorstad et al. 2013). Thus, in contradiction to statements made in the draft standard document, telemetry may have disadvantages, as well as “major advantages”, when compared to the other methods available, and is not the only means by which efficiency estimates can be formed as implied. Laboratory studies enable a variety of observation methods to be used (Knapp et al. 2019), ranging from tracking of fish trajectories to visualisation of flow patterns using techniques such as Particle Imaging Velocimetry (PIV) that are difficult to apply in the field (but see Biggs et al. 2019 in a future issue of the Journal of Ecohydraulics). In the current issue, Vowles et al. (2019) use a large open channel flume to assess the potential for spoiler baffles to improve passage of upstream migrating adult river lamprey (Lampetra fluviatilis) through a full-scale culvert. Adopting the classical manipulative experimental approach, this study was able to quantify using videography fine-scale behaviours exhibited by the fish in response to the manipulation of factors of interest, while controlling confounding variables. By focusing on observations of fine-scale behaviour we gain insight to, not only why fish passes work, but and as in this case, why they may fail to perform as predicted, emphasising the importance of publishing studies that present negative as well as positive findings. Ascertaining the mechanisms that underpin the results gained are not always possible when using telemetry alone. This does not mean experimentation is without drawbacks. Such studies represent a simplification of reality, a problem which is often exacerbated through spatial constraints imposed by the domain relative to the size of the subject species (Rice et al. 2010). Although the fish are not usually tagged in such studies (if they are we are adding a further confounding variable), as is the case for telemetry, the capture and handling of fish may impact various metrics, including measures of behaviour or swimming performance. Such effects will apply also to other approaches, such as trapping and mark-and-recapture techniques. Observations of fish or other aquatic animals are of course one part of ecohydraulic research; physical observations are the other. A variety of methods are also used for data collection of physical variables in the laboratory or the field, also with different strengths and weaknesses. It is essential that in selecting biological and physical observation methods, their limitations and compatibility are thoroughly examined. For example, mean velocity measurements may be insufficient to elucidate fish behaviour based on detailed fish movement observations through a fishway, which may depend on flow turbulence. Striving for compatibility and symmetry between biological and hydraulic observations in study design, data collection techniques, and CFD simulations allows for full ecohydraulic integration in the pursuit of improved fish passage (Amaral et al. 2019; Katopodis et al. 2019; Quaranta et al. 2019). This editorial presents an opinion that is not intended to criticize one approach in support of any other, as all biological and physical methods have