As fish move along the supply chain, from fishers to middlemen to exporters to consumers, the yellowfin tuna in our model are traded three times. At each trading point or market, prices are negotiated and fluctuate with supply and demand as the fish product changes hands.
We have arbitrarily named each market after the buyers: the middleman market, the exporter market, and the consumer market.
Each market has a current price, and tracks the volume of fish traded. Suppliers bring the fish to sell, and buyers bring a demand for fish. If demand outstrips supply, they negotiate a higher price. If there is a surplus of supply, prices drop and part of the product may get spoiled.