Set in stones: the influence of long-term environmental changes on Northeast Arctic cod viewed through the analysis of otoliths

Abstract

The living conditions of marine ecosystems are currently changing rapidly under the influence of human exploitation and contributions to climate change, especially in high latitude regions such as the Arctic. Predicting their future response and developing appropriate management strategies requires a good understanding of the factors influencing their biology and life history at different spatial and temporal scales, which may be enabled by long-term reconstructions and analyses of past populations. The Northeast Arctic cod is currently one of the largest and most commercially important Atlantic cod (Gadus morhua) populations in the world, although it experienced significant variability throughout the past century. Given the fast-changing nature of the Barents Sea region, there is therefore an increasing need to determine the response of cod to environmental changes. In this thesis, I used century-long biological records and multiple modelling approaches to investigate the influence of changes in climate, fish population trends and human exploitation on cod growth and life history throughout the last 100 years, using otoliths as a proxy of individual life history. Significant variations in cod growth and an earlier maturation trend throughout the past century could be related to changes in density-dependent competition and warming sea temperatures, providing evidence of synergistic influences of climate, exploitation and population dynamics. Contrasting effects of climate change at different temporal scales suggested that, while increasing temperatures benefit faster growth and earlier maturation, longer term warming may cause ecosystem-level changes that could be detrimental. Finally, although fishing mainly influenced cod biology through density-dependent release, the only partial reversal of maturity trends after exploitation pressure decreased may be indicative of fishing-induced evolution, which could have unforeseen consequences for the future response of NEA cod to climate change.