"We need to account for the potential for significant (i.e. 25–50%) changes in sustainable fish catch in the way we manage these resources," Charles Stock of NOAA, US, told environmentalresearchweb. "Where fisheries catch shows signs of decline, we need harvest controls that adjust for this so that the benefits of these resources can be maintained. In systems where fish yields may increase, we should also account for the potential to sustainably catch more fish."
The team made the finding by analysing why there are such big differences in fisheries catch between ocean ecosystems that have similar phytoplankton productivity. They examined the predominantly coastal large marine ecosystems that cover 22% of ocean area but account for 95% of global fish catch. Fisheries catch per unit area across these ecosystems varies by more than five orders of magnitude, with most producing catches of 0.01–1 g of carbon per square metre per year.
Catches in the Arctic and Antarctic were lowest, whilst most upwelling systems and many temperate and subpolar systems had high catches. The researchers incorporated data on fishing effort assembled by team member Reg Watson of the University of Tasmania, whose database includes details of the number and power of fishing vessels and how long they spend at sea. They also used estimates of catch from discards and small-scale fisheries reconstructed by Daniel Pauly of the University of British Columbia and his colleagues under the Sea Around Us project, as well as data on industrial fisheries from the UN FAO.
Finally, the team employed a prototype high-resolution global Earth system simulation developed at NOAA’s Geophysical Fluid Dynamics Laboratory to model plankton dynamics. "While its computational costs are quite high, we can use a short run to explore patterns in today’s ocean," said Stock, "though we must fall back on coarser simulations to explore climate-change implications".
Not only is phytoplankton production in upwelling regions higher but it is also delivered more efficiently to fish by the marine food web, according to the researchers. Food chains in more productive and/or temperate bloom regions tend to shorten as there’s more herbivory of larger phytoplankton. The food web is also more efficient as ingestion rates become large relative to basal metabolic costs.
"The work elucidates how phytoplankton production at the base of the marine food web and subsequent differences in energy transfers within marine food webs can combine to create very large differences in fisheries catch across ecosystems in today's ocean," added Stock. "It then shows how these same processes can generate steep fish catch trends under climate change."
As temperatures rise, the projected changes in fish catch are generally in the same direction as the changes in phytoplankton productivity, but larger. "Warming and a decline in phytoplankton production also leads to longer, less efficient food webs connecting phytoplankton and fish," said Stock.
While current ocean projections under high greenhouse-gas emissions scenarios suggest modest declines in phytoplankton production at low and mid-latitudes, the combination of declines in phytoplankton production and food-web dynamics may cause 25–50% declines in some regions, the study showed. "Our results further underline the importance of understanding shifting environmental baselines under climate change," added Stock.
"The idea that phytoplankton production and food-web dynamics must combine to create the very large differences in sustained fish catch we see in today's ocean goes back to a classic paper in the journal Science written by Dr John Ryther in 1969," said Stock. "Even with the sparse observations available at the time, Ryther noted that phytoplankton alone could not account for the very large differences in fish catch between systems."
Now, rather than finding broad tendencies across different latitudes, the team would like to constrain the effects of climate change on ocean productivity to a regional scale, for example projecting what exactly will happen in New England waters, the Gulf of Mexico, or the Bering Sea.
"While our results provide additional impetus to ongoing efforts to fully include climate and environmental information in marine-resource management, improved constraints on regional changes would make our projections more useful," said Stock. "We are pursuing this through several paths, including further development of high-resolution climate models like the prototype described in this paper, and partnerships with regional modeling groups and other partners to ‘downscale’ our global results to regional and local scales."
Stock and colleagues from NOAA, the University of South Carolina, Princeton University, The University of British Columbia, US National Marine Fisheries Service and University of Tasmania reported their findings in PNAS.