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

James R. Watson,P. Daniël van Denderen,Ken H. Andersen,Charles A. Stock,Colleen M. Petrik

doi:10.3389/fmars.2020.588482

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.

Highlights

Anthropogenic greenhouse gas emissions are increasing global ocean temperatures, altering stratification and mixed layer depths, and decreasing pH and dissolved oxygen (Stocker et al, 2013)
We focus on patterns of trophic amplification, contrasts in the response of fish functional types, and the sensitivity of both these critical processes to uncertainties in widely applied metabolic allometries and temperature dependences in fisheries models
We present an overview of the fish production (FEISTY) model, details on the Earth system model (ESM2MCOBALT) that provides the physics, biogeochemistry, and lower trophic level production, a description of how parameter uncertainty was incorporated into projections, and specifications on the simulations that were run and their analysis

Summary

Introduction

Anthropogenic greenhouse gas emissions are increasing global ocean temperatures, altering stratification and mixed layer depths, and decreasing pH and dissolved oxygen (Stocker et al, 2013). Centennial-scale projections with coupled climateocean-biogeochemistry Earth system models (ESMs) show global increases in temperature, and most project decreases in primary productivity (Bopp et al, 2013; Stocker et al, 2013). Temperature indirectly affects ecosystem structure through the effects of stratification on primary production. The total amount of energy available at the base of the ecosystem and phytoplankton size affect the number of trophic steps between primary producers and fish, and the amount of energy available to upper trophic levels (Ryther, 1969; Pauly and Christensen, 1995)

Methods

Results

Discussion

Conclusion