Look for pectoral fins
A saltwater Tuna fish has relatively large pectoral fins and its tail is bordered by a thick white outline. If you or your friends have got lucky in catching a Tuna fish, it is strongly recommended that you look for the pectoral fins to identify whether it is a saltwater Tuna or not. If it matches the abovementioned description, then there is a strong possibility that you have caught a saltwater Tuna.
Check back and upper side
After you have confirmed the pectoral fins and tail of the Tuna fish, you must now check its back and upper side as there is not just one way of identifying the saltwater Tuna fish. To ensure that you have a saltwater Tuna, you must confirm that whether it has a strong steel blue colouring or not? Note that the steel blue colouring will be catered by almost 6-9 stripes that will be coming downward and forward. The saltwater Tuna is often mistaken for a Skipjack, thus you must look for the other clues. While checking the back and upper side, remember that the lower sides of these stripes will be of silver colour.
Compare the eyes
In order to confirm that your catch is a Tuna fish, you must compare its eyes with the Yellow fin Tuna fish. Both fishes have resembling appearance, thus you must check their eyes. Keep in mind that both fishes have same design and pattern on their body which often makes it difficult for people to identify a saltwater Tuna. The saltwater Tuna will have bigger eyes while its dorsal and anal fins will be relatively smaller than as compared to Yellow fin Tuna.
The saltwater Tuna will have a relatively dark colour while its fins are likely to have white borders.