Data
Our measure of biological productivity reflects the abundance of juvenile Atlantic cod (ages 0+ to 2+ yr) in Skagerrak, a strait running between the southeast coast of Norway, the west coast of Sweden, and the Jutland peninsula of Denmark. Standardized beach-seine surveys have been conducted along coastal Norwegian Skagerrak annually since 1919 (Stenseth et al. 1999). The time series of data available for our analyses extended from 1919 to 2014.
In addition to cod, we examined time-series data for hypothesized drivers of cod productivity. Winter NAO data (December through March) were obtained for the years 1864 through 2018 (Hurrell et al. 2018). As a measure of juvenile cod food supply, we tested for the presence of regime shifts in the abundance of the energy-rich calanoid copepodCalanus finmarchicus, estimated by the Continuous Plankton Recorder (CPR) survey for the survey area closest to, and overlapping with, the Norwegian Skagerrak (area C1) (https://www.cprsurvey.org/data/data-charts/) (Edwards 2019). We pooled the CPR data for the months March through August, the period during which zooplankton are available to, and consumed by, cod larvae and juveniles in the North Sea (Nicolas et al. 2014). Sea-surface water temperatures in Skagerrak are available from the Flødevigen Research Station, Institute of Marine Research (http://www.imr.no/forskning/forskningsdata/temperatur_flodevigen/draw.map?boey=1). Temperatures recorded at Flødevigen are positively correlated with temperatures elsewhere in Skagerrak (e.g., Fromentin et al. 1998). These data were analysed on a monthly basis for the years 1925 to 2017. Estimates of fishing mortality (F ) and spawning stock biomass (SSB) are available for North Sea cod, the management unit of which Skagerrak cod is a part (ICES 2019).