Stock and recruitment in North Sea herring (Clupea harengus); compensation and depensation in the population dynamics

The recovery of a stock after severe exploitation is of major interest to fish ecologists and managers alike. Understanding the dynamics of recruitment at low stock sizes is crucial to the simulation of stocks as they recover. Compensation in recruitment has occurred in North Sea herring, and it was stronger after the collapse of the stock. The compensation appears to be a product of both increased production of larvae per spawner and increased survival to the juvenile stage. There is only slight evidence for depensation and the point at which North Sea herring has zero recruitment appears close to the origin. There is more variability in recruits per unit spawning stock size when the stock is smaller, this is probably as a result of the potential larger diversity in contributions from spawning components in an unexploited stock compared to an overexploited stock. Mimicking this dynamic in population models will increase the uncertainties in a projection of stock recovery. The lack of observations at higher stock sizes hinders our ability to compare the dynamics of recruit to SSB across the full range of stock sizes, however, recent apparent over-compensation at higher spawning biomass has resulted in less than one recruit per mature adult being produced per year.