Determining the impact of initial age structure on the recovery of a healthy over-harvested population

• We study the effect of the initial age structure on the recovery of a heavily exploited population, using the Baltic cod as an example species.
• Advantageous age structure is one of the main drivers in the early recovery of the cod stock.
• Meaning of initial age structure emphasizes under heavy harvesting pressure and environmental variance.
• The data on which the fisheries management is based should also include reliable information on the age structure of the stock.

Effective fishing has created a need for precautionary fishery management. However, both profitable and sustainable harvesting policies have been challenging to accomplish. One factor determining population resilience is the recovery capacity of the stock under environmental and harvesting pressure. Achieving sustainable fishing strategies requires enough knowledge of population recovery dynamics. We study the effect of the initial age structure on the recovery of a heavily exploited population. As an example species, we use Baltic cod (Gadus morhua callarias), the population dynamics of which are strongly influenced by environmental factors and fishing. The recovery process was scored during a time window of five years in the absence of harvesting and was also subjected to proportional and threshold harvesting. Our results show that initial age structure has a strong impact on population recovery capacity in the short run, especially in over-harvested populations and under strongly fluctuating environmental conditions. Thus, advantageous age structure is one of the main drivers in the early recovery of the stock. The data on which the fisheries management is based should include reliable information on the age structure of the stock.