System Zoo Z409 Fishery dynamics from Hartmut Bossel (2007) System Zoo 2 Simulation Models. Climate, Ecosystems, Resources
Fisheries represent an interaction between ecological and economic systems. All else being equal, fish populations can sustain fishing indefinitely if extraction rates are below renewal rates, but above this, catch starts to fall with increasing fishing effort. Economic pressure makes it difficult to stay below those limits.
It is necessary for fishers to meet their costs to keep fishing over time, so a minimum profit must be met; as more people join the fishery, there is less available for each person fishing. Unmanaged fisheries are often over-exploited so that catch is much lower than it could be. Proper management often means putting limits on people - limiting the number of boats or number of fish that can be caught.