Catastrophic dynamics limit Atlantic cod recovery
How can two drivers, fishing pressure and climate change, interact in inducing discontinuous dynamics in 20 Atlantic cod stocks? And how can these dynamics affect stocks´ recovery? We are trying to solve this mystery in our new paper1 published in Proceeding of the Royal Society B!
Atlantic cod, picture by Heike Schwermer
In one of my last blogs, “the Atlantic cod´s tale”, I left you with a huge question: “Will cod stocks recover and still be our Christmas food?”… I know, you probably spent sleepless night trying to solve this dilemma and you have wondered why I have disappeared for so long (sorry, looong revision process). Finally, after a while you will get a first preliminary answer (I know, you wanted the definitive one, but I cannot serve you all at once right? Where would be the fun in that?)! So let´s look at what we have done!
We used data of 19 Atlantic cod stocks distributed all over the North Atlantic. The stocks show strong and in some cases abrupt declines of biomass over time, thus suggesting the possible presence of abrupt changes, so called “regime shifts” in ecology. Regime shifts are abrupt discontinuous transition of a state variable (in our case biomass) depending on multiple stressors (in our case we investigated temperature and fishing pressure) and are characterized by the presence of multiple states and hysteresis (a delayed return to previous conditions after the pressure is released)2. In order to understand whether cod stocks dynamics were really non-linear and discontinuous we applied a method from catastrophe theory, the stochastic cusp model. The stochastic cusp model describes abrupt changes of a state variable depending on two control variables and allows us to understand whether the system is stable or not. This method has been rarely applied to ecology, but has high potential since it can represent the interaction between two drivers (Fig.1).
The results clearly show that Atlantic cod stocks present discontinuous dynamics. While fishing pressure is found to be the driver regulating the dimension of the stocks, temperature increase is the driver that breaks the relationship between fishing and biomass, so that to a decline of fishing pressure does not correspond an increase of biomass. Moreover, our results highlight that depending on longitude and latitude the stocks present different vulnerabilities to fishing and climate. The presence of discontinuous dynamics suggests that recovery might be hindered especially under climate change. These results are very important for management purposes because they highlight that alternative and adaptive management measures might need to be adopted considering stocks discontinuous behaviors, and highlight the urgency to consider these pervasive but difficult-to-detect dynamics in management.
So, to come back to the torturing question: “Will cod stocks recover and still be our Christmas food?”…Even though our paper suggests recovery might be difficult, we do not have a definitive answer. This is part of the scientific process, where more proofs are always needed and there is never an absolute, final answer. Therefore, more analyses on other population traits and on management are needed combined with food-web and ecosystem studies to have the full picture and get a bit closer to this dilemma solution…You just need to be patient a bit longer, something new will come soon...stay tuned!!!
1Sguotti C, Otto SA, Frelat R, Langbehn TJ, Plambech Ryberg M, Lindegren M, Durant JM, Stenseth NC, Möllmann C (2019) Catastrophic dynamics limit Atlantic cod recovery. Proceeding of the Royal Society B, 286: 20182877. http://dx.doi.org/10.1098/rspb.2018.2877
2Sguotti C, Cormon X (2018) Regime Shift-A global challenge for the sustainable use of our marine resources. In: Jungblut S, Liebich V, Bode M (eds) YOUMARES 8 - Oceans across boundaries: learning from each other. Springer, Cham.