Leveraging the Highly Data-Rich Northeast Arctic Cod (gadus Morhua, Gadidae) to Identify Key Biophysical Factors Behind Recruitment Success

Forecasting the number of individuals entering a fishery is critical for sustainable harvest and thereby management actions, but it remains challenging due to the considerable inter-annual variability and the complex nature of causal factors. This long-standing 'recruitment problem' has become even more urgent under ongoing climate change and the associated regime shifts overturning traditional predictive models. Leveraging the highly data-rich Northeast Arctic cod (Gadus morhua, Gadidae), this study proceeds beyond traditional data and method limitations. Such an initiative appears most timely because its recruitment (R), measured as number of 3-year-olds, is currently low and raising socio-economic concerns. We hypothesize a causal map and then apply Dynamic Structural Equation Model with sensitivity analyses to explore complex response patterns. The higher temperature exhibited a dual role: positive for spawning stock biomass (SSB) but negative for abundance of 1-year-olds (Age1) and R. Predation on postlarvae by northward-migrating Northeast Atlantic mackerel (Scomber scombrus, Scombridae), but not by herring (Clupea harengus, Clupeidae), appeared to influence Age1 negatively. As expected, fishing reduced SSB, whereas prey availability, represented by mesozooplankton biomass and stomach fullness, exerted a positive effect on Age1 and R. These suite of links remained robust, but shorter time series (< 30 years) may blur underlying patterns. Essentially, the low recruitment success (R/SSB) seen since 2010 (from the 2007 year class onwards) seems related to an overall poor situation for the 1-year-olds. Predictions give a less optimistic outlook and underline the high importance of maintaining today's advanced precautionary management strategies for this world's largest cod stock.