Benefits of threshold strategies and age-selective harvesting in a fluctuating fish stock of Norwegian spring spawning herring Clupea harengus

The current state of the world’s fisheries resources requires further investigation into the means and methods for sustainable use of fish stocks. In this study, I assess the performance of different harvesting strategies on a population model developed for Norwegian spring spawning herring Clupea harengus, a stock historically known as one of the largest and most valuable fish stocks in the world. This stock is further characterized by strong long-term fluctuations in stock size, which considerably complicate successful population management. The results support the use of threshold strategies, where harvesting is only allowed if the population size of the target population is above a predetermined threshold. Threshold strategies are beneficial for maintaining spawning biomass, yield, and prevention of population collapse. The downside of a threshold approach is the fisheries moratoria associated with low stock size. To overcome this, I introduce a precautionary strategy including 2 thresholds for reducing the likelihood and length of moratoria, such that if the population size decreases below an upper threshold, the harvest ratio is decreased in order to achieve fast recovery to the target population size. I also test the effect of changing the timing of fish entering the harvestable stock by means of age-selective harvesting. I show that allowing reproduction, preferably more than once, before the fish become vulnerable to harvesting increases the spawning biomass and yield, and decreases the variability in yield and the risk of stock collapse