Lumpers or splitters? Evaluating recovery and management plans for metapopulations of herring

The long-term management of a stock representing a metapopulation has been simulated in a case study loosely based upon herring to the west of the British isles, where stocks are currently assessed and managed by management area, although there is evidence of mixing between stocks (in terms of connectivity, migrations, and exploitation). The simulations evaluate scientific advice (based on virtual population analysis, VPA) and the sustainability of fishing under two population-structure scenarios, corresponding either to discrete stocks, which only mix on the feeding grounds, or where diffusion between stocks takes place. The ability of stock assessment to monitor stock status and exploitation levels was evaluated for defining stocks based on fishing areas and for stocks that combined fishing areas. The study showed that assessment based on VPA of the metapopulation could fail to detect overexploitation of stocks and fail to detect and distinguish between the effects of exploitation and regime shifts.