2 posts tagged with "Ocean Productivity"

(book chapter (chapter 10) in The Changing Wealth of Nations 2018: Building a Sustainable Future)

• After steadily increasing over decades, annual global production of capture fisheries has plateaued just above 80 million metric tons. From a peak of 86 million tons in 1996, global marine catches have shown a small downward trend of about 0.2 million ton per year.
• Globally, the proportion of fully fished stocks and overfished, depleted, or recovering fish stocks has increased from slightly more than 50 percent of all assessed fish stocks in the mid-1970s to about 75 percent in 2005, and to almost 90 percent in 2013.
• As global marine catches have stagnated and even declined, fishing effort has greatly expanded over the past 70 years. Over the same period the level of global marine catches has not even doubled, suggesting a steep decline in the catch per unit effort, often considered a measure of fishing productivity.
• At the global level, the data show that, overall, global fisheries have foregone US$83 billion of rent in 2012. Fisheries are heavily subsidized and in many countries resource rents from fisheries are negative—meaning that revenues do not fully cover the costs of fishing.

Photosynthesis fuels marine food webs, yet differences in fish catch across globally distributed marine ecosystems far exceed differences in net primary production (NPP). We consider the hypothesis that ecosystem-level variations in pelagic and benthic energy flows from phytoplankton to fish, trophic transfer efficiencies, and fishing effort can quantitatively reconcile this contrast in an energetically consistent manner. To test this hypothesis, we enlist global fish catch data that include previously neglected contributions from small-scale fisheries, a synthesis of global fishing effort, and plankton food web energy flux estimates from a prototype high-resolution global earth system model (ESM). After removing a small number of lightly fished ecosystems, stark interregional differences in fish catch per unit area can be explained (r = 0.79) with an energy-based model that (i) considers dynamic interregional differences in benthic and pelagic energy pathways connecting phytoplankton and fish, (ii) depresses trophic transfer efficiencies in the tropics and, less critically, (iii) associates elevated trophic transfer efficiencies with benthic-predominant systems. Model catch estimates are generally within a factor of 2 of values spanning two orders of magnitude. Climate change projections show that the same macroecological patterns explaining dramatic regional catch differences in the contemporary ocean amplify catch trends, producing changes that may exceed 50% in some regions by the end of the 21st century under high-emissions scenarios. Models failing to resolve these trophodynamic patterns may significantly underestimate regional fisheries catch trends and hinder adaptation to climate change.