Dramatic changes in spawning stock age-structure of Barents Sea cod

Abstract

Atlantic Cod (Gadus morhua) is one of the commercially most important fish species in the North Atlantic and plays a central role in several ecosystems. Fishing pressure has been heavy over a prolonged period and the recent decades have shown dramatic decline in abundance of many stocks. The Arcto-norwegian (northeast Arctic) cod stock in the Barents Sea is now the largest. We show that the age composition of the spawning stock has changed distinctly during the last 60 years, the average age of spawners has decreased from between 10 and 11 in the late 1940s to 7-8 now. Although less clear, there is also a tendency towards reduced age diversity in the spawning stock, fewer age classes now contributing. The trend towards a younger, less diverse spawning stock is worrying, in particular since earlier work by other authors has thoroughly documented that older Barents Sea cod produce disproportionately more and higher quality eggs than first-time spawners. There is evidence for fluctuations in climate, particularly sea temperature, being a main cause for the pronounced recruitment variability of this stock, higher temperatures being favourable for survival throughout the critical early life stages. We document, through studies of time series, that the climate-cod recruitment link has strengthened during the last decades. Our results suggest that this is a result of cod now being less resilient to adverse climate conditions due to the reduction in spawning stock age and age diversity.