From research vessel surveys, for separate species of fish, each average catch per tow --determined from a series of tows--in each geographic stratum of a region is multiplied by that area (square nautical miles) of the stratum in which the tows were made. All of the individual products are added together and the total is divided by the sum of the entire area of the region. The final result is the stratified mean catch per tow; this is used as an index of relative abundance. For example, a scientist wishes to calculate the stratified mean catch per tow of cod in a region (perhaps Georges Bank) that measures 100 square nautical miles. The region has been divided into 5 strata on the basis of depth. In each stratm, 5 tows are made and the average catch of cod is calculated.