In the past as well as the present, changes in distribution of the largest and most economically important pelagic fish stocks such as mackerel have caused disagreement over the proper management of such resources.
To understand the present and predict the future, we need to learn from history. One of the largest observed changes in North Atlantic mackerel happened when the so-called "North Sea mackerel" – a component of the stock which spawned in the North Sea – collapsed due to overfishing. Despite protection, it has remained in a depleted state. Central to this interpretation was that the "North Sea mackerel" was considered to be a distinct spawning component; however, a recent study has shown that this may not be the case.
In light of this new understanding, Jansen reviewed the history of mackerel spawning in the North Sea and found and that the traditional explanation of the collapse did not account for a range of unfavourable environmental changes. High fishing pressure was followed by decreasing temperatures that reduced the spawning migration into the North Sea. This was further supplemented by unfavourable changes in food and wind-induced turbulence. On a population level, this was, therefore, not a local stock collapse, but a southwest shift in spawning distribution combined with a reduction in that portion of the population cline with an affinity for spawning in the northeastern part of the spawning area, including the North Sea.
No indication of irreversible genetic or behavioral losses caused by the events was found. The previously unexplained lack of rebuilding of spawning in the North Sea consequently seems related to two environmental factors that have remained unfavourable: (i) zooplankton concentration and (ii) wind-induced turbulence. Furthermore, the large commercial autumn–winter fishery in the North Sea continues to land unknown quantities of mackerel that have an affinity for spawning in the northeastern part of the spawning area, including the North Sea.
This paper concludes a series of six about the spatial dynamic nature of mackerel. The full paper can be read via the link on the right.
