Harvesting strategies for Norwegian spring-spawning herring

The overfishing of an increasing number of fish populations has put focus on the need for development of robust sustainable harvest strategies that can be easily implemented. This requires estimates and modelling of the deterministic and stochastic components of the population dynamics as well as an evaluation of the contribution of different harvest strategies to future population fluctuations. Here we present an example of such an approach, using the collapse of Norwegian spring-spawning herring stock as a case. We demonstrate that the collapse probably was due to overfishing, and that the large influence of the environmental stochasticity could only influence the timing of the collapse. We suggest that a proportional threshold strategy with a threshold around 14 billion individuals (4 200 000 tons), combined with a harvest of 30-40% of the individuals above this threshold will give a sustainable yield with little annual variation. The choice of harvest strategy should also be strongly influenced by the uncertainty in the assessment of stock size. When the population stock is estimated with uncertainty, the proportional threshold strategy give a mean annual yield close to the optimum for known population size.