Brief, Protracted or Flexible? Quantitative Versus Qualitative Classification of Spawning for Fish in the Murray-Darling Basin

MDFRC item.

May 2011.

Knowledge of the duration of the spawning period for individual fish species is central to management interventions, including timing of environmental water allocations. A quantitative approach to the classification of fish species into spawning categories is provided based on a dataset resulting from five years of intensive sampling for larvae in the lower River Murray. The six most abundant native fish species were first classified empirically into brief and protracted spawners based on three different 'thresholds' of two, three and four months. A statistical classification was then implemented using receiver operating characteristic (ROC) analysis. A threshold of four months was able to minimise the proportion of incorrect classifications, thereby classifying correctly the spawning categories for the six species. Unspecked hardyhead, carp gudgeons, flathead gudgeon and Australian smelt were classified as flexible spawners, whereas Murray cod and Murray River rainbowfish were classified as brief spawners. Attention is drawn to the fact that a species classified as flexible spawner would actually result from a combination of brief and protracted spawner cases, leading to a 'Sorites paradox'. This would be ultimately reconcilable by group consensus aided by quantitative approaches, such as the one provided in the present study.