The basic idea is that we should be managing fisheries to avoid breaching the precautionary reference points. Both of these have to be thought about when decisions are made about management of the stock. Reference points for spawning stock For the spawning stock the scientists make an estimate of spawing stock biomass. It is the weight of all the mature fish in the sea of that particular stock. It varies from year to year because of the varying numbers of recruits into the stock and the impact of fishing and other factors affecting mortality.
Recruits are the young fish which survive from each year's spawning to enter the fishery.
For North Sea cod the most recent estimate of spawning stock biomass was 136,000 tonnes. In order to apply the precautionary approach the scientists have established two reference points for the spawning stock biomass. For North Sea cod the red danger signal is if the biomass falls below 70,000 tonnes. This is the lowest level that the stock has ever been known to fall to and the scientists have proposed it as the limit below which the stock must never be allowed to decline. Recruit numbers fluctuate naturally from year to year but can collapse if the parent stock gets too low.