Need help!

If it's packed tight, presumably 15 ml. (1 tablespoon = 15 ml.)

1.5 oz
An ounce-and-a-half.

As far as I can tell (a few different conversion tables) it's half an ounce (30 ml = 1 oz.)

Might be a fluid/ solid difference, though.

A ml is a measure of volume, not weight. A milliliter (ml) is, as the prefix would indicate, 1/1000 of a liter. The interrelation between weight and volume in the metric system is that 1 ml of water weighs precisely 1 gram. Therefore, 1 tablespoon of water (which is indeed very nearly 15 ml in volume, will weigh close to 15 grams, because the material is over ninety percent water. However, being slightly more dense than water, it will weigh a bit more. Your choice in determining the weight is to know the exact density of yellowfin tuna, or simply find an accurate scale and weigh it.