The spawning distribution of NA cod along the Norwegian coast has been highly variable during the period 1866–1969, and through time the proportion of total catches in the spawning ground fisheries have been increasing in the north and decreasing in the south.
Observation of the fluctuating spawning distribution is not new. Sætersdal & Hylen (1964)
noticed the rapid decline in catches on the southern spawning grounds in the 1920s and stated that the southern NA cod fishery had been insignificant since the 1930s. They argued that the decline was not caused by reduced effort, but rather a natural change in the spawning ground distribution. More recently, Sundby & Nakken (2008)
studied the shifts in spawning ground distribution between Finnmark and Møre during the time period 1905–1969. They correlated smoothed sea temperature and the NA cod's roe indices, and suggested that spawning ground distribution is climate-driven, and that warm periods favour more northbound spawning compared with cold periods.
However, the present study demonstrates that there is also a strong effect of fishery on the spawning ground distribution, and that after the onset of the BSF the influence of the climatic periods is undetectable. The severe truncation of spawning ground distribution after the mid-1920s appears to take place just after the onset of the trawling fishery in the Barents Sea. The trawl fishery in the Barents Sea has a sigmoid selection curve causing higher fishing mortality for the larger individuals, contrary to the traditional line and gill net fishery taking place at the spawning grounds (Huse et al. 2000
). Over some time this may have truncated the size distribution of NA cod, and influenced the relative abundance of spawners along the large latitudinal gradient of alternative spawning grounds. This hypothesis is supported by a recent state-variable modelling experiment, predicting that larger and older NA cod in better condition should perform longer migrations than individuals in relatively poorer condition (Jørgensen et al. 2008
). Also, we have released and tracked particles in a general circulation model simulating the fate of larvae spawned at different known spawning areas for NA cod (Opdal et al. 2008
). This model demonstrated that spawning grounds further south provide better offspring conditions in terms of favourable temperatures and retention schemes.
The present study suggests that early trawl fishery in the Barents Sea not only caused a sixfold increase in total fishing mortality within a few years, but also truncated the size distribution of the stock. This in turn reduced the abundance of large individuals in the stock, again causing the decline in, and successive disappearance from, the southern spawning grounds. The hypothesis is supported by our earlier modelling studies, and by the time-series data presented here.
Truncation and northward shift in spawning grounds has heavily reduced the latitudinal range of offspring nursery areas, possibly making recruitment processes less resilient towards climatic changes, and could provide further support for the climate-cod recruitment link that has appeared during the last decades (Ottersen et al. 2006
). In the long run, high fishing mortalities will also increase allocation to reproduction, and reduce allocation to migration (Jørgensen et al. 2008