Capelin or not for Barents Sea cod population dynamics

The Barents Sea capelin (Mallotus villosus) and the Northeast Arctic cod (Gadus morhua) have an overlapping distribution in the Barents Sea (Figure 1) and are known to interact. The cod is a known predator of the capelin, controlling the capelin abundance, while at the same time the irregular abundance of the capelin is thought to affect the cod dynamics.

Using a threshold state-space model (Durant et al. 2020 and post) and taking advantage of 37 years long survey data, we demonstrate that capelin is beneficial for the cod population only at high capelin abundance (Durant et al. 2022). Below a threshold amount of capelin, the cod is not affected by the amount of capelin. In other words, there was a nonlinearity in the relationship between cod and capelin (Figure 2).

Through the use of a state-space model that combined long-term population time series with environmental variables, we illustrated how historically established species interactions may be drastically modified if explored for nonlinearity (see post). This approach helps us to understand what  nonlinear dynamics may mean for the population and the trophic interactions in the system.

Non-additive population dynamics has been previously described for many species and notably for cod due to the long time-series available for this species (since the middle of the 20^th century for the Barents Sea cod). In our study, we show that nonlinearity in species interactions has an impact on population dynamics. This finding affects our understanding of the functioning of the food chain. In the Barents Sea, it is classically said that the abundance of capelin is driving the dynamic of its predators. This is only partially true. Our results show that the low abundance of capelin cannot explain the low abundance of cod which in case of low capelin abundance readily shift to other prey. While the Barents Sea cod diet is composed of about one-third capelin (Holt et al., 2019) since 1984, in the 1930s the herring Clupea harengus was much more preyed upon than capelin (Townhill et al., 2021). Note that the absence of capelin effect is not delimited to a particular period of time (for instance during the recent years) as illustrated in Figure 2.  

Stock assessment is often conducted on a single species basis. However it increasingly incorporates some known interaction between the species of interest and climate and/or other species. For instance, capelin in the Barents Sea is managed by taking into account the cod predation. Given the implication our results can have on the understanding of Barents Sea cod population dynamics, our threshold approach seems timely and necessary to improve the management these stocks.

References:

Bakketeig I.E., Hauge M., Kvamme C., Sunnset B.H., Toft K.Ø. 2016. Havforskningsrapporten 2016. Fisken og havet, 1, 99.

Durant J.M., Ono K., Langangen Ø. 2022. Empirical evidence of nonlinearity in bottom up effect in a marine predator–prey system. Biology Letters, 18, doi:10.1098/rsbl.2022.0309

Durant J.M., Ono K., Stenseth N.C., Langangen Ø. 2020. Non-linearity in interspecific interactions in response to climate change: cod and haddock as an example. Global Change Biology, 26: 5554– 5563, doi:10.1111/gcb.15264.

Holt, R.E., Bogstad, B., Durant, J.M., Dolgov A.V., Ottersen, G. 2019. Barents Sea cod (Gadus morhua) diet composition: long-term interannual, seasonal, and ontogenetic patterns. ICES Journal of Marine Science  doi:10.1093/icesjms/fsz082

Townhill, B.L., Holt, R.E., Bogstad, B., Durant, J.M., Pinnegar, J.K., Dolgov, A.V., Yaragina, N.A., et al. 2021. Diets of the Barents Sea cod (Gadus morhua) from the 1930s to 2018. Earth System Science Data, 13: 1361-1370. doi:10.5194/essd-13-1361-2021