Langangen, O. | bioRxiv Science

bioRxiv2025

On the status of Atlantic cod in the Oslofjord

There are serious concerns about the status of the Atlantic cod (Gadus morhua) in the Oslofjord region. However, quantitative estimates of the cod biomass are scarce. Typically, analysis of juvenile fish has been used to assess the state of the cod, and datasets that contain recruited individuals tend to be limited. We here quantify the biomass, with an estimate of uncertainty, over the last 25 years based on a large dataset of commercial landings. By accounting for the number of boat-landing events, we can account for the variation in effort over this period. Our analysis indicates that cod biomass has been reduced by more than 80% from a peak in 2004-2005 to 2018. After 2018, the biomass was further reduced below 10% of the previous levels, but these estimates are more uncertain. Given large public interest and a recent decision to close large areas in the Oslofjord for fishing, our results are highly topical and can serve as a baseline for further investigations into why the cod has collapsed in this area, as well as a contrast to potential future recovery.

ecology↗

bioRxiv2019

Estimating cause- and size- specific mortality hazard rates using mark-recapture-recovery data

O_LIEvidence-based management of natural populations under strong human influence frequently requires not only estimates of survival but also knowledge about how much mortality is due to anthropogenic versus natural causes. This is the case particularly when individuals vary in their vulnerability to different causes of mortality due to traits, life-history stages, or locations. C_LIO_LIHere, we estimated harvest and background (other cause) mortality of a landlocked migratory salmonid over half a century. In doing so, we quantified among-individual variation in vulnerability to cause-specific mortality resulting from differences in body size and spawning location relative to a hydropower dam. C_LIO_LIWe constructed a multistate mark-recapture model to estimate harvest and background mortality hazard rates as functions of a discrete state (spawning location) and an individual time-varying covariate (body size). We further accounted for among-year variation in mortality and migratory behavior and fit the model to a unique 50-year time-series of mark-recapture-recovery data on brown trout (Salmo trutta) in Norway. C_LIO_LIHarvest mortality was highest for intermediate-sized trout, and outweighed background mortality for most of the observed size range. Background mortality decreased with body size for trout spawning below the dam and increased for those spawning above. All vital rates varied substantially over time, but a trend was evident only in estimates of fishers reporting rate, which decreased from over 50% to less than 10% throughout the study period. C_LIO_LIWe highlight the importance of body size for cause-specific mortality and demonstrate how this can be estimated using a novel hazard rate parameterisation for mark-recapture models. Our approach allows estimating effects of individual traits and environment on cause-specific mortality without confounding, and provides an intuitive way to estimate temporal patterns within and correlation among different mortality sources. C_LI

ecology↗