Fishing Mortality and Spawning Stock Biomass
Regime shifts in juvenile cod abundance in Norwegian Skagerrak did not occur in an environment in which cod mortality was affected solely by natural causes. It is well established that fishing can dominate other sources of change in the mean and variance of population size (Shelton and Mangel 2011). In the present context, between 1963 and 2017, the mean instantaneous rate of fishing mortality (F ) on North Sea cod aged 2-4 yr, of which Skagerrak cod are a part, was 0.82 (ICES 2019). By contrast, the average annual natural mortality of cod comprising >90% of the catch ranged between 0.2 (cod older than 3 yr) and 0.74 (2-yr-old cod). Thus, since at least the early 1960s, fishing mortality experienced by North Sea cod has always exceeded natural mortality.
Changes in fishing mortality (F ) and the reproductive component of North Sea cod (the spawning stock biomass, or SSB) can be expressed relative to their limit reference points. When SSB falls below its limit reference point (Blim ), the population is considered to have increased risk of impaired reproductive capacity (ICES 2018). F should not exceed Flimbecause such a level of prolonged overfishing is thought to be associated with population dynamics that lead to stock collapse. For North Sea cod, Flim = 0.54. By comparison, the fishing mortality corresponding to the maximum sustainable yield (FMSY ) is 0.31 (ICES 2019).
The BOCPD algorithm was applied to the fishing mortality data for North Sea cod. Multiple regime shifts were detected and these were remarkably similar at different hazard rates (Fig. 5). Stock assessment modelling output indicates that fishing mortality on North Sea cod has rarely been less than Flim (Fig. 6), steadily rising from the early 1960s through the late 1990s from 0.9 Flimin 1963 to a maximum of 2.0 Flim in 1999 (the year that initiated the second Skagerrak cod regime shift). WhileF steadily increased, stock biomass experienced a decline from a peak SSB of 2.00 Blim in 1971. By 1999, SSB was less than its limit reference point (0.80 Blim ), part of a continuing decline that did not halt until 2006 when it had declined to 0.41 Blim .