B50 describes a fish population at 50% of its estimated original stock size (biomass). Biomass is an estimate of the total quantity of fish in a given area. 

If a fish stock is estimated to be at B50 that means that there is around half the amount of fish in a given area compared to the initial biomass. 

‘Initial’ biomass is an estimate of a fish population before industrial fishing began, or the population size it would eventually return to if fishing stopped (NIWA).

In B50, ‘B’ represents initial Biomass (which is B0)’, and ‘50’ as in 50% of the initial biomass. 

B50 is an important baseline for fisheries management as it can be used to assess whether a fish stock is overfished.

Read more on our thoughts on B50 here.