ABSTRACT: We examined otolith opacity of Baltic cod in relation to environmental conditions in order to evaluate the formation mechanisms of seasonal patterns used in age determination. Adult fish were tagged with data storage tags (DSTs) and a permanent mark was induced in the otoliths by injection of a strontium chloride solution. Based on environmental conditions experienced, fish were classified into different behavioural types: non-reproducing ‘non-spawner’, and ‘spawner’ undertaking spawning migrations. Otolith opacity, an indicator of otolith and fish somatic growth and condition, was examined in relation to these environmental drivers. Temperature was the only environmental variable with a significant effect, overlaying a strong size-related effect. The temperature effect was not uniform across behavioural types and spawning periods. Opacity showed a negative correlation with temperature as expected—but in non-spawning fish only. In spawners, the general trend was a decrease in opacity from pre- to post spawning. A significant—but positive—temperature effect was only found in the pre-spawning period. The negative effects during and following spawning were not significant. In spawners, this decoupling leads to an otolith structure with stronger contrasts and more abrupt changes, while in non-spawners, opacity changes more smoothly. The trigger for this decoupling seems to be an interaction between temperature exposure and seasonal variations in food availability and may serve as a tool to identify the occurrence and repetitiveness of spawning in Baltic cod.