Causes of mortality in depleted populations of Atlantic cod estimated from multi-event modelling of mark-recapture and recovery data

Knowledge on mortality causes is key for an effective management of animal populations and can help to restore depleted fish stocks. Here we investigated the mortality dynamics of coastal Atlantic cod (Gadus morhua) in Skagerrak, southern Norway, by analyzing local mark-recapture and recovery data collected from 2005 to 2013 (N = 9360 fish, mean length = 41 cm, range = 16-93 cm). By applying multi-event models to the data, we could link field observations to multiple dead states and estimate the proportion of deaths associated with different fishing gears while controlling for unobserved mortality and detection errors. Deaths due to hand lines and fixed gear types were dominant compared with other causes, especially in legal-sized cod (>= 40 cm). Gear-specific mortality changed over time and between size classes, but annual survival remained low and stable (similar to 0.3). Assuming fully additive mortality, we predicted annual survival of cod to be above 0.5 if only one or both of the dominant gear types were removed, providing insights on the relative impact of diverse harvesting practices on local population dynamics.