The biological collapse of fisheries is a global problem. In this tutorial, we explain the apparent paradox of the fishing industry: they seem to act against their own interests by overexploiting a renewable resource to the point of exhaustion, even though their livelihood depends on sustainable catch.
Ocean fish populations have traditionally been organized as common property resources, meaning that anyone with nets and a boat can harvest the resources. In this context, there are financial incentives to expand the fishing fleet as long as there are profits. Often, by the time this point is reached, fisheries are on the brink of collapse, resulting in the so-called "tragedy of the commons."
Throughout the tutorial, we study the phenomenon with the help of the free online educational simulator "Fisheries Management v1.0". We keep all the parameters at their default values (see the "Documentation" tab of the simulator for more information), except for "Simulation time", which is set to 33 years.
In Figure 2, we can clearly see that from the beginning, as the fishing fleet grows, the biomass of the fishery decreases. In Figure 3, at t = 26.83 [year], we observe how the net regeneration capacity of the biomass begins to decrease significantly.
Biomass and regeneration capacity are data that fishermen cannot infer based on their catches.
However, from the fishermen's perspective, the situation is completely different. The fleet's annual catch rate does not start to decrease slightly until t = 29.83 [year] and then falls sharply until t = 30.83 [year] ("Fleet catch rate" in Figure 3), coinciding with the beginning of an abrupt disinvestment policy ("Ships" in Figure 2) due to the significant decline in fishery biomass. By the time fishermen see clear signs of what is happening, the economic disaster is inevitable.
Additionally, the annual catch rate per ship remained constant at a value of 0.025 [kton/(ship*year)] until t = 26.83 [year] ("Ship catch rate" in Figure 4). At t = 29.83 [year] (when the fleet’s annual catch begins to slightly decline), the catch per ship is only 4% lower than it had been for nearly three decades (0.024 [kton/(ship*year)]).
It is important to highlight that the catch rate per ship starts to decrease (t = 26.83 [year]) before the fleet's total catch rate (t = 29.83 [year]) because, between these two events, the fleet continues to grow, thereby compensating for the decline in catches per ship.
Let’s remember that, in the real world, during the first 26.83 years, the annual catch rate per ship fluctuates slightly around 0.025 [kton/(ship*year)] due to environmental factors unrelated to fishing activity. These fluctuations contribute to masking the 4% decline over the 3 years mentioned earlier. For this reason, the early detection of the ecological, financial, and social disaster is even more challenging than what we observe in the simulation results.
The net biomass regeneration for fish densities above 60% decreases sharply (Figure 5), while catches per ship remain at their maximum level for the same density range (Figure 6). This difference in sensitivities to fish density keeps the real situation of the fishery "hidden" if we only monitor catches or economic profit.
The collapse of fisheries represents a serious ecological, financial, and social problem on a global scale. Neither catches nor economic profit are adequate indicators to prevent the issue, as by the time they alert us, there is no time left to react.
It is necessary to monitor the internal state of fisheries to define an exploitation policy that treats them as a renewable resource. This approach will help prevent the "tragedy of the commons" and the collapse of fisheries.
