Facultative habitat selection in Pacific tarpon Megalops cyprinoides as revealed by otolith Sr:Ca ratios

The Pacific tarpon Megalops cyprinoides is a euryhaline fish that is widely distributed in offshore waters of the tropical and subtropical Indo-Pacific Ocean. It spawns in offshore waters and the larvae drift with tidal currents before recruiting to estuarine nursery areas at similar to 25 to 30 d posthatching. Young Pacific tarpon are found in coastal waters, rivers, estuaries, inner bays, and mangrove areas. However, their age-specific migratory behavior and habitat selection are still unclear. The strontium (Sr) concentration in seawater is similar to 100x higher than in fresh water, and the Sr:Ca ratios in fish otoliths are positively correlated with ambient salinity and are thus widely used in studying fish migration between seawater and freshwater environments. To understand the migratory environmental history of the Pacific tarpon, otolith Sr:Ca ratios of leptocephali collected from the estuary and sub-adults collected from both Tadu Creek and offshore waters on the west coast of Taiwan were examined using an electron probe microanalyzer (EPMA). Fish were aged by counting otolith annuli. The Sr:Ca ratios in leptocephalus otoliths were high (7.4 +/- 0.03 x 10(-3)). However, the patterns of otolith Sr:Ca ratios in 1 to 3 yr old sub-adults were diverse and can be divided into 3 types: largely brackishwater residents, largely freshwater residents, and vagrants between brackish and fresh waters. Fish older than 4 to 5 yr all return to the marine environment. This study demonstrates that habitat selection in the Pacific tarpon after the leptocephalus stage is facultative and that their migratory environmental history can be reconstructed from otolith Sr:Ca ratios.