Observation error appears to explain the perception of earlier maturation in Atlantic cod following the industrialised fishing

Maturity data are required to estimate spawning stock biomass (SSB) which is the most common basis for setting quota. Such data are also important when studying stock responses to, for instance, biophysical variability and fisheries-induced evolution. The official maturity time series of the most data-rich Atlantic cod (Gadus morhua) stocks (≥40-50 years) display windows of sudden reductions in age-at-50 %-maturity (A₅₀). Here, we addressed whether these shifts in Northeast Arctic (NEAC) and North Sea (NSC) cod time series are artefacts caused by revisions to sampling. In the complementary ecophysiological approach, the analysis was extended to other cod stocks, including Northern cod (NC). The quality-assessment of the open-access NEAC and NSC databases clarified that the abrupt shifts in A₅₀ coincide with altered sampling rather than being rooted in biology. The exceptionally long NEAC commercial catch series did not show evidence of selective removal as a causal mechanism. A high stock age diversity in synergy with age-specific spawning time and inadequate sampling coverage heighten the risk for biased high A₅₀. We found no significant change in the 75-year NEAC A₅₀ series (p-value = 0.576), but so in the 44-year NSC A₅₀ series (p-value = 0.041). Histology strengthened that NEAC A₅₀ is less variable than frequently stated. The revised NEAC A₅₀ led to significantly higher historic SSB, with consequences for recruitment dynamics patterns and fisheries (biological) reference points. Essentially, the various cod stocks fitted into the same maturation plasticity scheme suggested that the decision to sexually mature is steered by temperature-dependent body growth rate.