Influence of sampling effort on metrics of fish-based indices for the assessment of estuarine ecological quality

► Study of the influence of sampling effort on metrics of a fish-based index. ► Pseudo-random samples were generated from data of four Portuguese estuaries. ► For metrics on percentages, the means levelled off with less than 20 hauls. ► Metrics on species richness required much larger samples ► solutions alternative to increasing the sampling effort are suggested.