Effects of water temperature, salinity and feeding regimes on metamorphosis, growth and otolith Sr:Ca ratios of Megalops cyprinoides leptocephali

We examined the effects of water temperature, salinity and feeding regime on leptocephalus metamorphosis, and on daily growth increment deposition and strontium:calcium (Sr:Ca) ratios of otoliths in the Pacific tarpon Megalops cyprinoides. The tarpons at the pre-metamorphic leptocephalus stage (SI) were collected in the estuary, marked with tetracycline and then reared for 18 and 30 d in 2 independent experiments with different temperatures (20, 25 and 30C), salinities (0, 10 and 35) and feeding regimes (fed and starved)-environmental conditions that the fish may experience in the wild. Temporal changes in Sr:Ca ratios from the primordium to the otolith edge of the reared tarpon were examined with an electron probe microanalyzer. At the optimal temperature (25C) in Expt I, the leptocephalus completed metamorphosis (SII and SIII) after ca. 2 wk irrespective of being fed or starved and reared in low (10) or high (35) salinity, although both somatic and otolith growth rates were lower in starved than in fed groups. However, in Expt II, metamorphosis was delayed at SII when the larvae were reared close to winter water temperature (20C), and the duration of metamorphosis was reduced to less than 12 d when the leptocephalus was reared at 30C. This indicated that the rate of metamorphosis was significantly influenced by water temperature. Meanwhile, the drastic change in Sr:Ca ratios and increment width in the otolith of the leptocephalus during metamorphosis was also significantly different among different temperatures, salinities and feeding regimes in different degrees. The present study indicated that these factors significantly influenced the metamorphosis rate of the leptocephalus and subsequently their daily growth increment deposition and Sr:Ca ratios in otolith.