Theory 07/Marine Biology & Fisheries /Fisheries science/3 min
Stocks Collapse When the Age Structure Goes
A stock can have plenty of fish and no future. The missing year classes are the warning.
Fish stocks collapse when their age structure is truncated rather than when biomass falls below a threshold, so collapse should be predictable from the loss of older age classes years before the biomass signal, and management should regulate age diversity rather than tonnage.
01The evidence
- Size-selective fishing has been shown to alter the demography of marine fish populations through changes in size and age structure, which is the mechanism a portfolio account requires.Gear selectivity overview
02The tension
Management is built on biomass reference points: fish until the spawning stock falls to a limit. The limit is chosen from historical data and applied across stocks, and collapses still occur in stocks that were above the limit until shortly before they were not.
03The theory
A stock's resilience is a portfolio: different year classes recruit under different conditions, so a stock with many year classes can absorb a run of bad recruitment. A truncated stock has one or two cohorts carrying reproduction, and a bad year is then a demographic catastrophe.
If resilience is portfolio-based, then the warning signal is the loss of age diversity, which happens before biomass falls because the old fish are removed first and the young ones have not yet accumulated.
The prediction is that collapse should be predicted by an age-diversity index better than by biomass, and that stocks with truncated structure should collapse at higher biomass than stocks with intact structure. That is testable on existing assessment time series.
The management consequence is that a minimum age diversity could be a reference point in its own right, which would protect the portfolio before the biomass limit is approached.
04What this is not
Biomass limits treat the stock as a mass. This treats it as a portfolio of cohorts, which predicts collapse at higher biomass when diversity is low.
05Predictions
- P1Collapse should be predicted by age diversity better than by biomass.
- P2Truncated stocks should collapse at higher biomass than intact ones.
- P3Protecting age diversity should delay collapse at equal harvest.
06What would refute it
Collapse predicted by biomass alone, with age diversity adding nothing, would leave the reference-point system adequate.
07What it would change
It would add an age-diversity reference point and change how quotas are set.
08Evidence base
- Gear selectivity and the demography of marine fish populationshttps://www.sciencedirect.com/topics/earth-and-planetary-sciences/gear-selectivity