Large Pelagic Fish Are Most Sensitive to Climate Change Despite Pelagification of Ocean Food Webs

Abstract

Global climate change is expected to impact ocean ecosystems through increases in temperature, decreases in pH and oxygen, increased stratification, with subsequent declines in primary productivity. These impacts propagate through the food chain leading to amplified effects on secondary producers and higher trophic levels. Similarly, climate change may disproportionately affect different species, with impacts depending on their ecological niche. To investigate how global environmental change will alter fish assemblages and productivity, we used a spatially explicit mechanistic model of the three main fish functional types reflected in fisheries catches (FEISTY) coupled to an Earth system model (GFDL-ESM2M) to make projections out to 2100. We additionally explored the sensitivity of projections to uncertainties in widely used metabolic allometries and their temperature dependence. When integrated globally, the biomass and production of all types of fish decreased under a high emissions scenario (RCP 8.5) compared to mean contemporary conditions. Projections also revealed strong increases in the ratio of pelagic zooplankton production to benthic production, a dominant driver of the abundance of large pelagic fish vs. demersal fish under historical conditions. Increases in this ratio led to a “pelagification” of ecosystems exemplified by shifts from benthic-based food webs toward pelagic-based ones. The resulting pelagic systems, however, were dominated by forage fish, as large pelagic fish suffered from increasing metabolic demands in a warming ocean and from declines in zooplankton productivity that were amplified at higher trophic levels. Patterns of relative change between functional types were robust to uncertainty in metabolic allometries and temperature dependence, though projections of the large pelagic fish had the greatest uncertainty. The same accumulation of trophic impacts that underlies the amplification of productivity trends at higher trophic levels propagates to the projection spread, creating an acutely uncertain future for the ocean’s largest predatory fish.

Climate‐induced decrease in biomass flow in marine food webs may severely affect predators and ecosystem production

“…Considering predator at TLs higher than 3.5, the projected change in potential catch (by 2100 under RCP8.5) would be closer to 16.3%, based on production, than to 21.3% as expected from biomass. Trophic amplification in production (and not in biomass) is consistent with the projections based on a mechanistic model resolving trophic interactions and basic life cycle processes (Petrik et al, 2020). Interestingly, while we projected a decrease of 12.0% in total fish production, Petrik et al, (2020) projected total fisheries yield declines by 11.8% using a simple representation of fishing (constant over space, time, and TL).…”

“…Trophic amplification in production (and not in biomass) is consistent with the projections based on a mechanistic model resolving trophic interactions and basic life cycle processes (Petrik et al, 2020). Interestingly, while we projected a decrease of 12.0% in total fish production, Petrik et al, (2020) projected total fisheries yield declines by 11.8% using a simple representation of fishing (constant over space, time, and TL). However, they projected larger differences in fisheries yield between the low and the high TLs.…”

“…Trophic amplification has been previously shown for phytoplankton and zooplankton using different planktonic food web models and different ESMs (Chust et al, 2014; Kwiatkowski et al, 2019; Stock et al, 2014b). At the upper TLs, Petrik et al, (2020), based on a spatially explicit mechanistic model of three functional types of fish, showed the amplification of the projected changes in productivity by grouping functional types by TL. In a complementary way and using a trophic‐level‐based model, our projections highlighted a continuous and progressive amplification of changes in biomass and production when moving up the food web.…”

“…At the upper TLs, Petrik et al, (2020), based on a spatially explicit mechanistic model of three functional types of fish, showed the amplification of the projected changes in productivity by grouping functional types by TL. In a complementary way and using a trophic-level-based model, our projections highlighted a continuous and progressive amplification of changes in biomass and production when moving up the food web.…”

“…Only a few studies (e.g., Petrik et al, 2020) have explored both ecosystem biomass and production. For each ecosystem compartment, the latter is issued from animal growth and reproduction, implicitly referring to a gross production of living biomass (Gascuel et al, 2008; Gascuel et al, 2011; see Figure S1.1), which can be used in the system to feed the food web, detritus compartment, and fisheries if any, or to constitute a net production changing the current biomass of the considered compartment.…”