Spatial and temporal dynamics of movements in eastern Atlantic cod

Movement in fish species can be incredibly complex. While some species can remain resident throughout most of their life cycle (e.g., the blacktail seabream Diplodus sargus capensis), other species exhibit more migratory behaviour with occasional long-distance migrations (e.g., in several species of tuna). Even more interestingly, some species are characterized by a mix of resident and migratory individuals.
The Atlantic cod (Gadus morhua) is one such species with complex movement patterns. Both resident, migratory, and stocks and populations with a mix of these two ecotypes have been documented which can cause issues for management. This is exemplified in MPAs (marine protected areas) where seasonal movement outside the MPA can compromise the efficacy of the MPA, such as in the resident Gilbert Bay cod. In addition, larval transport from the North Sea to the Kattegat and the subsequent return migrations of the adult North Sea cod similarly exemplifies this issue. This highlights the need to document cod movement behaviour in detail in order to understand underlying causes and connectivity to surrounding areas in order to ensure efficient management.
The aim of this PhD study was thus to analyse the movement behaviour of Baltic and North Sea cod in detail using tagging data spanning ~70 years. The goal was to clarify the movement through time and, in the case of the Baltic Cod, try to understand the potential effect of the environment due to the significant past and projected changes in the Baltic Sea caused by climate change.
This dissertation consists of an introduction and three papers (Paper I-III) followed by a discussion. The introduction serves to give an extensive background on the Atlantic cod, including movement, ecotypes, and the effect of the environment on the cod physiologically and behaviourally. This also includes a review of the historical tagging projects in the Baltic Sea and surrounding waters to set the stage for the following papers. The first paper (Paper I) outlines general movement in Baltic and North Sea cod through a period of ~70 years, while the second paper dives into a more detailed analysis of connectivity in the Sound cod stock component (Paper II). Finally, the third paper (Paper III) showcases the results of extensive simulations of movement where the resulting distribution patterns are based on Baltic cod following optimal values of oxygen, temperature, and salinity as summarized by a habitat index. Finally, the results of the three papers are discussed and put into perspective.