A fishery managed at 50% of its assumed natural size is considered to be a sustainable fish stock. The 50% level is international best practice. The Rescue Fish policy is based on rebuilding depleted fisheries to 50% because when a fish stock is at that level it can provide for economical harvest whilst ensuring productivity and marine ecosystem services are maintained. Rescue Fish policy.

The default minimum management target for most New Zealand fish stocks is currently 40% of unfished biomass. According to Fisheries NZ, a fish stock below 20% is deemed to be ‘depleted’. A fish stock below 10% is deemed to be collapsed. In reality, many of our fish stocks are currently between 15% and 30% of unfished biomass. Biomass is the measure of the amount of fish present compared to pre-industrial harvesting numbers.