Historical Arctic Logbooks Provide Insights into Past Diets and Climatic Responses of Cod

Abstract

Gadus morhua (Atlantic cod) stocks in the Barents Sea are currently at levels not seen since the 1950s. Causes for the population increase last century, and understanding of whether such large numbers will be maintained in the future, are unclear. To explore this, we digitised and interrogated historical cod catch and diet datasets from the Barents Sea. Seventeen years of catch data and 12 years of prey data spanning 1930-1959 cover unexplored spatial and temporal ranges, and importantly capture the end of a previous warm period, when temperatures were similar to those currently being experienced. This study aimed to evaluate cod catch per unit effort and prey frequency in relation to spatial, temporal and environmental variables. There was substantial spatio-temporal heterogeneity in catches through the time series. The highest catches were generally in the 1930s and 1940s, although at some localities more cod were recorded late in the 1950s. Generalized Additive Models showed that environmental, spatial and temporal variables are all valuable descriptors of cod catches, with the highest occurring from 15-45°E longitude and 73-77°N latitude, at bottom temperatures between 2 and 4°C and at depths between 150 and 250 m. Cod diets were highly variable during the study period, with frequent changes in the relative frequencies of different prey species, particularly Mallotus villosus (capelin). Environmental variables were particularly good at describing the importance of capelin and Clupea harengus (herring) in the diet. These new analyses support existing knowledge about how the ecology of the region is controlled by climatic variability. When viewed in combination with more recent data, these historical relationships will be valuable in forecasting the future of Barents Sea fisheries, and in understanding how environments and ecosystems may respond.

Fig 1. Total cod total stock biomass (blue) in the Barents/Norwegian Sea (ICES sub-areas I and II) from 1916–2013 (1916–1945 from Kjell Nedreaas, IMR, Bergen pers. comm., [40]; 1946–2013 from [13]), compared with a Climate Index (green) for the Barents Sea (1900–2009) [39].

Fig 2. Cod CPUE at each station from 1930–1959.

Crosses represent stations where no cod were caught. Size of the circles represent CPUE. Blue = winter, purple = spring, red = summer, orange = autumn.

Fig 3. Long-term change in cod CPUE (kg/hr) averaged by day (±SE) for all stations combined.

Note the large number of zero and near-zero CPUE values, but also very high CPUE values in some years.

Fig 4. Relationship between CPUE and spatial and environmental variables.

Fig 5. Annual mean percentage occurrence of the five major stomach content categories.