Dispersal pattern of glass eel stage of Anguilla australis revealed by otolith growth increments

Abstract

Glass eels of the Australasian shortfin eel Anguilla australis (Richardson 1841) were collected from 6 Australian and 2 New Zealand estuaries. Their ages were estimated by counting daily growth increments in the otoliths. According to mean size and age at capture, the glass eels were classified into a northern Australia group or a southern Australia and New Zealand group. The mean length of glass eels increased from 47.6 ± 2.11 mm in northern estuaries to 59.2 ± 3.61 mm in the south. The mean age of glass eels at capture was significantly lower in the north (214 ± 14.6 to 223 ± 17.7 d) than in the south (243 ± 19.7 to 261 ± 22.4 d) (p < 0.05). In contrast, the otolith growth rate was greater in the north than in the south. The dramatic increase in increment width and the decline of the Sr:Ca ratios in otoliths were used to determine the timing of metamorphosis from leptocephalus to glass eel stage. The mean ages of leptocephali at metamorphosis were significantly lower in the north (160 ± 14.2 to 161 ± 12.6 d) than in the south (168 ± 14.5 to 189 ± 16.9 d), indicating that faster-growing and earlier-metamorphosed leptocephali recruited to northern Australia and slow-growing and late-metamorphosed leptocephali recruited to southern Australia and New Zealand. In addition, based on current direction and the similarity in age of leptocephali at metamorphosis, age at capture and the period between metamorphosis and estuarine arrival, New Zealand glass eels are unlikely to be transported across Tasman Sea from southern Australia by the East Australian Current, and must reach their destination via a different route(s).