(a) All business income of water transportation carriers shall be apportioned to this state in accordance with AS 43.19 (Multistate Tax Compact) as modified by the following:

(1) the numerator of the property factor is the sum of the value for property in a fixed location, including buildings and land used in the business, and intrastate equipment and personal property determined according to AS 43.19 (Multistate Tax Compact), and the value of interstate mobile property determined on a days-spent-in-ports basis as provided in (4) of this subsection; the denominator of the property factor is determined according to AS 43.19 (Multistate Tax Compact);

(2) the numerator of the payroll factor is the sum of the wages and salaries of employees assigned to fixed locations determined according to AS 43.19 (Multistate Tax Compact) and the wages and salaries of employees assigned to interstate mobile property determined on a days-spent-in-ports basis as provided in (4) of this subsection; the denominator of the payroll factor is determined in accordance with AS 43.19 (Multistate Tax Compact);

(3) the numerator of the sales factor is the sum of all revenues from intrastate activities and revenues from interstate activities determined on a days-spent-in-ports basis as provided in (4) of this subsection; the denominator is determined in accordance with AS 43.19 (Multistate Tax Compact);

(4) the portions of the numerator of the property, payroll, and sales factors which are directly related to interstate mobile property operations are determined by a ratio which the number of days spent in ports inside the state bears to the total number of days spent in ports inside and outside the state; the term “days spent in ports” does not include periods when ships are tied up because of strikes or withheld from Alaska service for repairs, or because of seasonal reduction of service; days in port are computed by dividing the total number of hours in all ports by 24.

(b) The department shall, by regulation, adopt formulas to ensure that the total income subject to apportionment under this chapter has been apportioned only to those states having jurisdiction to tax the income. Transportation carriers other than water carriers shall apportion their income to the state by means of the formulas adopted by the department.