ðɪ ˈoʊnli mˈɛθəd tə dɪtˈɜːmɪn lˈɔŋfɪn ˈiːlz sˈɛks ɪz θɹuː ɪntˈɜːnəl ɛɡzˌæmᵻnˈeɪʃən ænd ˈoʊnli bɪkˌʌmz ˈiːzi tə dɪstˈɪŋɡwɪʃ wɛn ðɪ ˈiːlz mətʃˈʊɹ ænd mˈaɪɡɹeɪt.
The only method to determine longfin eels sex is through internal examination and only becomes easy to distinguish when the eels mature and  migrate.