Harvesting can cause substantial changes in wild populations aside from reducing population size, including skewing or truncating age/size structure toward younger ages/smaller sizes. In fisheries, size-selective harvesting often occurs, which can exacerbate age truncation. As in many other organisms, fish traits such as fecundity and timing of birth and migration often vary with age, and thus trait diversity may decline if age structure is truncated. Such effects may have negative consequences for population and community stability by reducing productivity, selecting for slower growth and earlier maturity, and perhaps increasing population sensitivity to environmental variation. Despite the severity of the potential influences of age truncation on population and community dynamics, the condition of the age structure is unknown for many species, and the effects of fishing on age structure have yet to be empirically synthesized. We addressed this knowledge gap by quantifying the extent of change in age structure induced by fisheries across five ocean regions. We derived theoretical expectations for how fishing reduces the relative number of old fish and compared this to our estimates of the change in age structure of 63 fish populations, the vast majority for which numbers at age data are available globally.
The proportion of individuals in the oldest age classes significantly decreased in 78-95% of assessed populations, with 43% decreasing by more than 90% compared to the unfished baseline. Although this age truncation is pervasive, it is less severe than our theoretical expectations. Somewhat to our surprise, the proportion of old fish actually increased in 1-18% of the populations, mostly caused by long-term favorable conditions in climate regimes, short time series of data starting after extensive fishery exploitation (shifting baselines), or environmental stochasticity. However, it is clear that fishing generally causes substantial changes in age structure, thus implementing measures such as no-take areas or slot limits (both minimum and maximum size regulations) may be useful if preservation of a broad age structure and associated trait diversity is desired. Given the growing body of research demonstrating the positive relationship between diversity, stability, and ecosystem function, our results imply that age truncation will generally reduce trait diversity and may in turn alter the ecological roles, productivity, distribution, stability, sensitivity to climate change, and resilience of fish populations.