Commons: Fishery

Game Description

Each round, each student decides how many fish to catch from an open-access lake (a common-pool resource). The tragedy of the commons occurs when overfishing prevents the sustainability of the fish stock for future use.

In this game, the primary societal cost of overfishing is borne in future periods as the fish stock dwindles. For a game where over-exploitation of a common-pool resource reduces current payoffs, see our Tragedy of the Commons game.

Learning Objective 1: Tragedy of the Commons

Since fish in a public lake are a common resource, each individual has an incentive to overfish.

Learning Objective 2: Resolving the Tragedy of the Commons

Communication may help prevent the tragedy of the commons. Additionally, increased future benefits should lead to more sustainable levels of fishing.

Brief Instructions

The game starts with the fish stock at the lake's capacity: 40 fish for each student. At the end of each round, the fish double, up to a maximum of lake capacity. Catching half of the stock can be sustained indefinitely. At the end of the game, the remaining fish stock is doubled (at most to lake capacity) and evenly divided amongst the group. As a result, the group catching half of the stock in every round maximizes total fish caught. A student earns one point per fish caught, meaning she has an incentive to catch an additional fish rather than let it double and split it with her group.

A student fishes by clicking on his boat, and stops by either doing nothing for ten seconds or clicking on his anchor. A round ends when all students in a group stop fishing or when the time you specify in Round Duration (seconds) expires.

Key Treatment Variations

Allowing and encouraging in-game chat facilitates coordinate and may lead to more efficient behavior. Alternatively, increasing the number of rounds (akin to having players care more about the future) increases the cost of current over-fishing.


In this game, tragedy of the commons (overfishing) can be seen when fish are caught faster than they can reproduce. This can be seen in either of two graphs, the first depicting the change in the stock of fish, the second depicting the catch rate.

Figure 1: Summary Table

To generate summary graphs, you select desired group(s) in the summary table (Figure 1) and click Generate. The final column of this table lists for each group the average catch per boat across all rounds. This average will be low for groups that overfish.

Figure 2: Fish Stock Across Rounds

The first graph (Figure 2) shows the fish population at the start of each round. The population starts at capacity, a smaller population at the start of any subsequent round represents previous overfishing. In the worst case, a group will catch all the fish, meaning no fish in future rounds.

Figure 3: Percentage of Stock Caught Across Rounds

The second graph (Figure 3) shows each round's catch as a percentage of the beginning-of-round-population. Once again, 50% is the maximum sustainable catch rate, and values greater than 50% mean a declining population.

Robot Play

Robot players are conditional cooperators with probabilistic defection. There exist two different scenarios based on beginning-of-round fish stock versus lake capacity. As long as the beginning-of-round fish stock is at least 90% of lake capacity, the robot fishes sustainably with probability 0.9, catching 20 fish, and fishes aggressively with probability 0.1. If the the beginning-of-round stock is less than 90%, the probabilites are inverted, e.g. the robot player fishes aggressively with probability 0.9, with a cap of beginning-of-round fish stock divided by the number of players, and fishes sustainably with probability 0.1.

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