Age, growth and mortality of the tropical grouper Epinephelus merra (Pisces, Serranidae) on Reunion Island, SW Indian Ocean

The growth, age, mortality rate and some aspects of the biology of a protogynous fish from the family Serranidae, Epinephelus merra Bloch, 1793, have been studied on the Reunion Island. This key species in the functioning of the Reunion coral reef ecosystem was sampled in a semi-artificial habitat. The log-transformed length-weight relationship was defined by: Ln(W) = 3.015 LT - 1.970 and was not statistically different from that reported by other authors in natural habitats. Comparison between the total length of males and females revealed that sexual change occurred when fish were between 18 and 20 cm (TL), which corresponded to ages ranging from 3 to 5 years. A comparison between otolith reading methods for age assessment was done (in toto observations, assessment on digital photographs, and assessment by image analysis software). The von Bertalanffy growth curve provided low theorical maximum length (Linfinity ranging from 19.8 cm to 25.0 cm TL, depending on the method used) and relatively high growth rate (k ranging from 0.43 to 0.80, depending on the method used). Estimates of natural mortality (M) ranged from 0.90 to 1.67. These results could however be underestimated (Linfinity) or overestimated (k and M), possibly due to Lee's phenomenon linked to sampling which mainly focused on medium and large sizes. Implications for fishery management are discussed.