| Summary: | 博士 === 國立臺灣大學 === 動物學研究所 === 90 === The otoliths of the glass eels Anguilla australis, A. reinhardtii and A. dieffenbachii specimens collected from the estuaries of eastern Australian and New Zealand were examined to clarify their early life history and migrating routes from the spawning grounds to the nursing grounds. The ages of the glass eels were estimated by counting daily growth increments in otoliths. 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. One to several discontinuous rings (elver check) appeared at the edge of otoltihs after the glass eels entered the estuaries from the ocean. The elver check was an indicator to distinguish the estuarine duration from oceanic duration in glass eel stage. The mean ages of leptocephali at metamorphosis of A. australis (n=183)、A. reinhardtii (n=176) and A. dieffenbachii (n=96) were 171.2±20.0d, 144.5±12.2d and 225.4±15.1d, respectively. The age from metamorphosis to estuarine arrival of A. australis, A. reinhardtii and A. dieffenbachii glass eels were 53.8±15.2d, 38.2±9.4d and 62.4±11.7d, respectively. The mean ages of A. australis, A. reinhardtii and A. dieffenbachii glass eels at estuarine arrival were 225.0±28.9d, 182.7±16.3d and 287.9±19.4d, respectively. The larval duration of glass eel was longest in A. dieffenbachii, middle in A. australis and shortest in A. reinhardtii but vice verse in growth rate. Among the 3 species, the ages at metamorphosis were linearly related with ages at estuarine arrival. This indicates that the leptocephalus metamorphosing at a younger age will arrive at the estuary as a younger glass eel. The main spwaning seasons of A. australis、A. reinhardtii and A. dieffenbachii, back-calculated from daily increments, were between December to March、June to September and October to January. The presumed spawning grounds of A. australis、A. reinhardtii and A. dieffenbachii were between Fiji and Samoa, west of Fiji and between Cook Islands and French Polynesia, respectively. Therefore, there were temporal and spatial isolation of reproduction among the 3 species. The total length and daily age of A. australis showed geographical declined from north to south, indicating that the glass eels were transported from norther to southern Australia by South Equatorial Current and East Australian Current. In addition, based on the current direction and the similarity in age of leptocephali at metamorphosis, age at catch and the time between metamorphosis and estuarine arrival, the New Zealand glass eels were unlikely to be transported across Tasman Sea from southern Australia by the East Australian Current, and they must reach their destination via different routes. Another possible migration route is transportation by the southwest flowing portion of the SEC. The differences in geographical distribution between Anguilla reinhardtii and A. australis on the eastern coast of Australia can be understood by comparing, by otolith growth increments and microchemistry, the ages between species of the eels at metamorphosis from leptocephalus to glass eels and the ages of glass eels at estuarine arrival. The shorter duration of the marine larval period and faster growth rate may make A. reinhardtii occur in tropical-subtropical waters while the longer marine larval duration and slower growth rate make A. australis dominate in more temperate waters. Consequently, the oceanic current, spawning grounds, growth rate and larval duration play an important role in determining the geographical distribution of the 3 eels.
Key words, Anguilla australis, A. reinhardtii, A. dieffenbachii, otolith, early life history, migration.
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