Gross tonnage

The entire internal cubic capacity of the ship expressed in tons of 100 cubic feet to the ton, except certain spaces which are exempted, such as:

1. peak and other tanks for water ballast;
2. spaces above the uppermost continuous deck, such as: open forecastle, bridge and poop, certain light and air spaces, domes of skylights, condenser, anchor gear, steering gear, wheel house, galley and cabins for passengers.

Source: Military Sealift Command Definitions, Tonnages and Equivalents 

Fisheries