§ 91-a. Licensing of Hudson river pilots. The commissioners, or a majority of them, shall license, for such terms as they may think proper, as many pilots as they may deem necessary to pilot ships through that portion of the Hudson river specified in this act, the continuation of the Hudson river, and the main ship channel of the Upper bay, New York harbor, east of a line from Robbins reef light to the tower at Staten Island borough hall and down to the Verrazzano-Narrows Bridge, and such commissioners may specify in such licenses different degrees of qualifications appropriate to different parts or branches of pilot duty, according to the competency of the applicant.