Latitudinal differences in the timing of otolith growth: A comparison between the Barents Sea and southern North Sea

Seasonality of otolith zone formation can vary between fish species, and between populations within the same species. Although information on otolith zone formation is important for fish age estimation, there is incomplete knowledge of factors influencing variation in otolith opacity. In this study, marginal increment types (opaque or translucent) of two spatially distinct Atlantic cod stocks were assessed. Cod otoliths from the southern North Sea, where the fish experience high temperatures were compared with cod otoliths from the Barents Sea, where the fish experience much lower temperatures. There was approximately a shift of 5 months for deposition of translucent otolith material between the two areas. Cod from the southern North Sea deposited translucent otolith material in the period July–November, while it occurred in the period December–April in the Barents Sea. Inter-annual differences within areas were minor compared to differences between the two areas.