Uncertainty in developing fish based multi-metric indices

The process of development of a multi-metric index (MMI) implies a series of decisions to complete the procedure and obtain the formulation of an indicator. We performed an uncertainty and sensitivity analysis to explore the effects of alternative decisions in this process, using the case of fish fauna-based assessment of the ecological status in transitional waters and the Venice lagoon as an example. We formulated the alternative strategies in MMI implementation as levels of three factors affecting this procedure: 1) metrics pre-selection; 2) metrics numerical pre-treatment; 3) and metrics combination rules. The outcome of the uncertainty analysis highlighted that all the decisions impacted the process, potentially leading to indicators with different characteristics, confirming that the process of developing MMIs - even following an empirical data-driven approach - is not a completely objective procedure. However, not all the choices have the same weight in influencing the final index, nor do they affect the same characteristics of the index. The initial choice of candidate metrics influences the number of metrics included in the final index, the correlation of the MMI with anthropogenic pressures and the ecological status classification (i.e. the output of the index itself). However, the most influential decision is the one related to the criterion adopted to select the metrics (i.e. to reduce the number of metrics from the original candidate set) included in the final index. This choice is the one that most likely affects the structure of the index and the ecological status classification, even if the family of indices developed in this work exhibited a certain level of robustness regarding this characteristic.