An alabarch was a traditionally Jewish official in Alexandria during the Hellenistic and Roman periods, seemingly responsible for taxation and especially customs at the harbor.
[1] Professor Samuel Krauss in the 1906 Jewish Encyclopedia suggests that the alabarch was the leader of the Jews in Alexandria, and would have been called "ethnarch" by gentiles including Strabo.
Such a derivation would imply that the Alabarch was a farmer of taxes, certainly from the time of the Ptolemies; and, judging by inscriptions which give a similar title to an office of the Thebaid in Egypt, he must also have collected the toll on animals passing through the country.Krauss also mentions an older etymology using ἄλς (hals = sea).
He is in all probability identical with the ἀραβάρχης, whose office was that of chief superintendent of customs on the Arabian frontier, i.e. on the east side of the Nile.
Perhaps it is the office of the alabarch that is in view when Josephus says that the Romans 'continued (to the Jews of Alexandria) the position of trust given them by the kings, namely, the watching of the river' (c. Apion.
: 'maximam vero eis fidem olim a regibus datam conservaverunt, id est fluminis custodiam totiusque custodiæ' [the last word is certainly corrupt]).
[3]Philo's brother Alexander was alabarch (customs official) in the 30s A.D., and another Jew, Demetrius (otherwise unknown) held the same post late in Claudius' principate; neither case excites comment from Josephus as unusual.