Impacts of Climate Change: Abundance and Catch of Pelagic Fish Predicted to Decline in North Pacific Temperate Biome and Increase in California Current Region During the 21st Century

PIFSC scientists Phoebe Woodworth and Jeff Polovina, along with colleagues Julia Blanchard (University of Sheffield) and John Dunne (NOAA's Geophysical Dynamics Laboratory (GFDL)), have coupled a size-based ecosystem model with a climate model to project the effects of climate change on fish abundance and catch for the North Pacific pelagic ecosystem over the 21st century.

The size-based food web model is too computationally intensive to run for the entire North Pacific, so seven regions were selected for investigation based on previous work conducted with output from the GFDL's Earth System Model ESM2.1. In a 2011 study employing ESM2.1, Polovina et al. defined three biomes: temperate, subtropical, and equatorial upwelling, and predicted that the temperate biome would expand over the 21st century while the other 2 biomes shrank. The study regions used in the current size-based modeling represent areas projected to remain within their respective biomes through the 21st century (biome interior regions), areas projected to become part of the expanding subtropical biome (biome boundary regions), and an area in the California Current ecosystem (California Current region).

Top: Percent change in density of large phytoplankton over the 21st century, as projected by ESM2.1.  The green boxes 
               represent the seven areas examined with the size-based food web model.  Bottom:  Percent change in ESM2.1 output variables 
               and size-based model catch for each of the seven areas examined.  In both figures, the grey lines represent the subtropical 
               biome boundary at the beginning of the 21st century (mean over 2001–2020) and the red lines represent the same boundary at 
               the end of the century (mean over 2080–2100).
Top: Percent change in density of large phytoplankton over the 21st century, as projected by ESM2.1. The green boxes represent the seven areas examined with the size-based food web model. Bottom: Percent change in ESM2.1 output variables and size-based model catch for each of the seven areas examined. In both figures, the grey lines represent the subtropical biome boundary at the beginning of the 21st century (mean over 2001–2020) and the red lines represent the same boundary at the end of the century (mean over 2080–2100).

In areas projected to become part of the subtropical biome, the abundance and catch of pelagic fish are projected to decline by 52-77% over the 21st century. In areas projected to remain within their respective biomes, declines of 0-38% are projected. However, in the California Current region, the catch and abundance of large pelagic fish are projected to increase by 43%.

The projected changes at the top of the pelagic ecosystem are a response to changes in the phytoplankton community. The density of large phytoplankton is projected to decline by 61-83% in biome boundary areas and by 15-38% in biome interior areas. A decline in large phytoplankton reduces the amount of energy available to larger size classes of consumers, thus limiting the number of large fish the ecosystem can sustain. Conversely, in the California Current region, a projected 32% increase in large phytoplankton density would increase the amount of energy available to larger size classes and, in turn, the number of large fish.

While this work only examined seven regions in the North Pacific, it showed a strong relationship between projected changes in the phytoplankton community and the abundance and catch of large fish. Based on this relationship, nearly all of the North Pacific, apart from the California Current region, may see a significant decline in the abundance and catch of large pelagic fish over the 21st century.

Woodworth and her colleagues have submitted a manuscript on the research for publication in the peer-reviewed journal Global Change Biology.