CCAMLR sets catch limits using a set of ‘decision rules’ to determine what proportion of the stock can be fished while still achieving the objective of the Convention.

To do this the population of the harvested species is projected forward in time using a population model to allow the effects of different catch levels to be simulated.

The distribution in blue shows the range of potential starting points for the simulations. For each projection a starting point is chosen at random; the time over which the projections are made and the key parameters used vary depending on the species.

The simulations are run thousands of times - varying key parameters to account for both the effects of the fishery and natural variability.

Two population levels have been agreed as the basis of the decision rules. These are the fractions of the population that can be taken by the fishery each year so that the target population old enough to spawn (the spawning biomass):

1. only drops below 20% of the pre-fishing median on 1 in 10 of the 20-year period of fishing (shown by the red dot)
2. after a 20-year period of fishing is 75% (for krill) or after a 35-year period of fishing is 50% (for toothfish) of the median before fishing started (the right hand blue distribution)

The actual catch limit is set at the lower of these two proportions multiplied by the estimate of the stock size. In the case of krill the size of the stock is estimated from acoustic surveys, for fish species a range of approaches, including surveys, tag-recaptures and catch statistics are used.

For icefish, a species that shows very highly variable population size, the ‘decision rule’ is for an annual yield which results in a 5% probability that the spawning stock biomass is reduced to below 75% of the level that would occur in the absence of fishing over a two-year projection period. This short-term projection is made using a biomass estimate provided by a survey of the population every one or two years.

For more information on the catch limits and the population parameters used please see the Fishery Reports for the relevant fishery.