Dr Carl O'Brien:
Before I joined fisheries in the mid-90s, virtual population analysis was used, which is an age structure-based model. You actually use age data. As long as you can age fish, you can model the development of fish as they grow, the same as you would with human populations—one-year-olds become two-year-olds, who then become three-year-olds. You can take into account natural mortality through natural deaths and also exploitation rates—death through fishing.
The typical data-rich models are those that have the age-based data. The data-limited ones are those where, for various reasons, we either cannot age the fish or it is too expensive to age the fish, so we have simpler methods, such as the size of the fish or maturity ogives, which are simpler types of metrics. However, we can still come up with so-called proxies. Back in 2015, within ICES, I was developing methods with our Portuguese and French colleagues to come up with MSY proxies, which, as the Minister knows, the Commission will now accept as MSY values. They are not treated as second-class MSY values. They are appropriate for the data-limited stocks.