High-latitude climate variability and its effect on fisheries resources as revealed by fossil cod otoliths

Cod (Gadus morhua) otoliths from archaeological sites in northern Norway were analysed to reconstruct the temperature regime and determine the age structure, growth, and population identity of the fish harvested. Otoliths were selected from late- and post-medieval sites (700-300 years ago) to evaluate historical changes in the geographic region that matches the present-day stocks of Northeast Arctic cod (NEAC) and Norwegian coastal cod (NCC). Seasonal temperature cycles were reconstructed from stable isotope (delta O-18) measurements along transects representing fish ages 1.5-3 years old. Reconstructions of the size, age, and growth characteristics of individual fish were based on otolith growth increments. The geographical source and stock identity of the individuals were estimated based on otolith elemental composition and otolith growth features. Both NCC and NEAC fish were represented at Masoy and Vanna. The results indicate that fishing at Vanna exploited NEAC during their spawning migration, compared with fishing at Masoy, which was restricted to more coastal fish. Fish growth patterns appeared to be affected by changes in the temperature regimes as estimated from otolith delta O-18 and back-calculated fish length-at-age, with evident differences between pre- and post-1600 periods.