Recently, a paper published in Current Biology sought to measure age truncation in commercial fisheries. “Human extraction of living resources can reduce intraspecific trait variation by, for example, causing truncation of age and size structure of populations, where numbers of older individuals decline far more with exploitation than younger individuals.” Such considerations were largely overlooked, and the authors felt the extent of age truncation could compromise the stability of marine communities. Age truncation has long been assumed to be a product of fishing effort, but this paper represented the most complete documentation of that assumption.
The authors “quantified the extent of age truncation in 63 fished populations across five ocean regions, as measured by how much the proportions of fish in the oldest age groups declined over time. The proportion of individuals in the oldest age classes decreased significantly in 79% to 97% of populations (compared to historical or unfished values, respectively), and the magnitude of decline was greater than 90% in 32% to 41% of populations.” The US Southeast and the West Coast showed the most pronounced age truncation.
The authors cited three main tools for reducing age truncation in fisheries: marine reserves, rotational harvests, and more age-specific slot limits. The call for more marine reserves in this paper was heard and supported by National Geographic. “No-take marine reserves can restore a broad age structure within protected areas and can buffer against destabilizing effects of fishing and environmental variability through overall increases in multiple forms of resilience.” National Geographic echoed, “By protecting more areas as no-fishing zones, you assure some portion of the senior age class is never in danger of being caught.”
“Given the growing body of research demonstrating the specific relationship between age structure and population dynamics, and the general relationship between biodiversity at multiple scales, community stability and ecosystem function our results imply that fishing has decreased the productivity and stability of many fish populations and aquatic communities”. Though it should also be noted that the majority of the world’s fish stocks are at sustainable levels.
National Geographic cites the Pacific Whiting fishery as a primary concern because, “fishing had taken more than 90 percent of the oldest age group.” But at the same time, it is acknowledged to be “well-managed” with numbers that are “currently high”. NOAA confirmed the, “coastal [Pacific Whiting] stock remains large and is the most abundant commercial fish stock on the Pacific Coast.”
Lastly, as overfishing has been reduced by lowering fishing pressure, the length structure has been rebuilt in many fisheries. A good example would be Norwegian Cod which had record landings of large, older Cod landings in recent years.
These landings were partly due to climate change, bringing warmer waters into the Norwegian Sea, but, “Norway also began to place limits on the commercial harvest in the 1980s, and this is bearing fruit,” said Knut Korsbrekke, a specialist at the Institute of Marine Research in Bergen, Norway.
This paper was successful in documenting widespread age truncation in key global fisheries on a new scale. However, skeptics of a correlation between biodiversity and fishery health will find holes in the authors’ conclusions.