Roxanne Maranger an ecologist at the University of Montreal and other have a neat paper in Nature Geoscience Nitrogen transfer from sea to land via commercial fisheries that shows that commercial fishing removed substantial amounts of nitrogen from coastal oceans. They show that while fertilizer run-off into the ocean and fishery removal of nitrogen have increased over the past forty years, the increase in nitrogen inputs has been faster. Consequently the proportion of nitrogen removed from coastal zone has dropped from a global average of about 60% in 1960 to about 20% in 2000. This trend as well as the spatial pattern of nitrogen withdrawal are shown in figure 1 of their paper:

Figure 1. a, Total amount of N in fertilizer run-off (Tg N yr^-1=10¹² g N yr^-1) delivered to the global ocean (left axis, blue line) and N returned as fish biomass (left axis, red line) per year over time. The orange line (right axis) is the proportion of fish N removed relative to fertilizer N exported (ratio fish N:fertilizer N) reported as a percentage. b, The ratio of fish N removed to fertilizer N entering 58 different large marine ecosystems (LMEs) for the year 1995.

The paper shows that fishing can help reduce the impacts of nitrogen pollution. But that nitrogen pollution that destroys fisheries, through the creation of anoxic “dead zones”, can make nitrogen pollution even worse by removing a major source of nitrogen withdrawals. Similarly, overfishing the reduces the amount of fish biomass that can be removed from a system will make the system more vulnerable to eutrophication.