I don't know for sure, but I had a few guesses. 1. Juvenile, so immature? Softer/more vulnerable scales? 2. The scales may be optimized for speeding along qui

I don't know for sure, but I had a few guesses. 1. Juvenile, so immature? Softer/more vulnerable scales? 2. The scales may be optimized for speeding along quickly through water and the net basically tore the scales apart? 3. The scales may not be durable since the tuna's primary mode of defense is probably speeding away. 4. The end of the article does say that "only one in tens of millions becomes an adult," so it could just be a weakness that was never changed in the course of evolution because it's not detrimental or common enough to force selection.