> He will stop fishing as soon as everyone has plenty of fish to eat because there is no way to liquidate his assets.

Sounds like his problem is lack of demand, not currency liquidity.

> But if the island became connected to a trading route he could trade excess fish for gold or another non perishable asset, he could store up those assets. But he would only do that if he believed that those assets had liquidity.

Ok, but that kind of breaks the island economy experiment, given that he was already trading fish in a closed system for some other good or currency.