Quantile-based grading improves the effectiveness of a multimetric index as a tool for communicating estuarine condition

Abstract

Multimetric Fish Community Indices (FCI) were recently developed for assessing the ecological condition of shallow nearshore and deeper offshore waters of the Swan-Canning Estuary, Western Australia. The provisional system for classifying estuarine condition from FCI scores, which divided the possible range of scores (0–100) into four descriptive classes of equal breadth (good, fair, poor, very poor), was shown to be skewed towards producing fair to good grades. An alternative, alphanumeric (A–E) grading system, whose grade boundaries were defined by quantiles of the distribution of historical FCI scores, exhibited greater apparent sensitivity to decreases in ecological condition resulting from a harmful algal bloom than did the provisional classification scheme. These advantages of the quantile-based FCIs have led to their recent implementation as a monitoring and reporting tool by the primary environmental managers of the Swan-Canning Estuary, and their application to other permanently open systems across Western Australia is currently being evaluated.