Why were ancient fish expensive and fishermen poor?

It is a problem for middlemen to earn the difference. Generally speaking, they grow in rivers and valleys. In addition to farming, ordinary people probably go fishing. So there are many people fishing and there are many sources of goods. Those businesses will naturally make more profits by lowering their purchase prices and raising their selling prices. Moreover, since there is no shortage of fishermen and goods, and fishermen compete with each other, then merchants will see who gives them low prices and good quality, so fishermen are poor.