A sardine is a very small, oily fish that is a member of the herring family. They are commonly eaten by humans.
Your answer that sardine is a sea fish was wrong becaus there are natural sardins growing naturally in fressh water lakes. The fresh water sardins are soled exactly like all other Sardins but the taste and the smell of that kind of sardins is much more milled from the sea water Sardins. Therefore: Fresh water Sardins do exist.
Having very small teeth or no teeth at all, sardines eat plankton, which they filter from the water through their gills.
The war between the Greeks and Persians was 499-494 BCE. Marathon was in a Persian amphibious raid against Eretria and Athens 490 BCE. The wars with Persia ent on sporadically for fifty years. It is hard to divide them up into First, Second ... etc.