How may a catastrophic event killing eggs and larvae in a single year affect the cod in the Lofoten-Barents Sea are?

Opening the sea areas around Lofoten and Vesterålen islands for the search and extraction of petroleum resources is a highly debated topic. One of the main concerns has been to what extent an accidental oil-spill may affect the ecologically and economically important species of fish (including cod, haddock and herring) that spawn in the area.  Since an oil-spill in these areas is expected mainly affect the eggs and larvae, the population impact will depend on how a reduction in a single year in the numbers at these stages is further translated to the population level. Quantifying the magnitude of potential future impacts at the egg and larval stages is highly uncertain due to unknown ecological conditions in the future. To circumvent this problem, we took an alternative and more general approach and asked what would the population level impact be if 50 % or 99 % of eggs and larvae were killed by such a catastrophic event. Such events could be caused by e.g. disease outbreak, biological invasions, extreme climatic events or possibly toxic spills. We used an advanced population model for cod that accounts not only for the observational error in the data but also for stochastic events affecting the biological processes involved. The model included the egg, larval, juvenile and adult stages (Figure 1). Importantly, the model accounted for both inter- and intra-cohort density dependence. These are important since they can dampen the impact of cohort loss at the early stages over time. We analysed what would have happened if such a catastrophic event had happened in any single year between 1960 and 2006. 

    Our results demonstrate that catastrophic events during the egg and larval stages in cod may have quite variable impacts at the population level depending on which year is affected (Figure 2). This is mostly due to differences in the relative size of the affected cohort, and the general age-structure of the population. Overall, our analysis reveals a remarkable resilience in the cod population to a severe loss in a single cohort.

Nevertheless, our findings also suggest that there may be significant economic loss due to reduced catch (Figure 3) as a result of the cohort loss. In conclusion, we show that severe die-offs during early life stages can be buffered by density dependence in vital rates and a broad age structure in long-lived marine species. Yet, even when population recovery is relatively fast, losses in harvest and economic value can be substantial.
 

Ohlberger, J., & Langangen, Ø. (2015). Population resilience to catastrophic mortality events during early life stages Ecological Applications, 25 (5), 1348-1356 DOI: 10.1890/14-1534.1