Age-Structured Biomass Dynamics of North Atlantic Shortfin Mako with Implications for the Interpretation of Surplus Production Models

The 2017 North Atlantic shortfin mako (Isurus oxyrinchus) assessment has resulted in a substantial negative shift in perception of the stock status and future projections, which we summarize here as the “The Good, the Bad and the Ugly”. The existence of large mature sharks not caught in ICCAT fisheries has probably retarded the stock collapse (“The Good”). This biomass is as cryptic to the fishery as it is unobservable in the available abundance data (“The Bad”). By ignoring the strong lag effect between exploitable and reproductive biomass, earlier surplus production model assessments have probably contributed to a false perception about the long-term sustainably of the fishery. The inability to predict the long-term impact of unsustainable fishing over the last 30 years has likely created a “time bomb” scenario towards a collapse of the mature biomass (“The Ugly”). While it is probably too late to halt the collapse, rebuilding chances will depend on the time it takes to implement effective management interventions.