About Tuna Stock Growth
A fish stock consists of all of the fish of a given type that live in a fishery. Our model studies the yellowfin tuna stock in the Mindoro area of the Philippines, a biomass of roughly 20,000 tonnes of fish.1
Key Parameters and Observables
- Carrying Capacity The carrying capacity is the maximum tonnage of fish the fishery can sustain. In the absence of fishing by humans, the biomass of a fishery is always at or near the carrying capacity.
- Biomass The total amount of fish currently living in the fishery waters, measured as a total tonnage of fish rather than a head count.
- Growth Rate When fishing takes place, fish are removed from the stock. The growth rate is a measure of how quickly the fish population will repopulate when its biomass is below the carrying capacity. Higher growth rates lead to faster growth in the fish population.
- Maximum Sustainable Yield (MSY) The theoretical maximum rate of fishing, or number of tonnes caught per year, that can be compensated by the fish's own growth rate.
A fishery is said to be fully exploited when fished at Maximum Sustainable Yield. A fishery is said to be overfished when more fish are removed then are able to repopulate, leading to the collapse of the stock. Today, roughly 90% of worldwide fisheries are either fully exploited or overfished. 2
Click here to learn more about the outcomes of a fully exploited fishery
Click here to learn more about the outcomes of an overfished stock
Modeling Notes
We based our model of fish stocks off of the Gordon-Schaefer model.3 Gordon-Schaefer is a basic approach to modeling fish stock change which does not take into account, for example, changing ecological conditions that might lead to the carrying capacity changing over time, or the ages of the fish being caught and removed from the reproductive cycle. The goal of this fishery model is to investigate the interactions in the value chain, not the complexities of fish stock changes, and Gordon-Schaefer provides a reasonable model of the fish stock component of the fishery value chain.
