Evidence for Ecosystem-Level Trophic Cascade Effects Involving Gulf Menhaden (Brevoortia patronus) Triggered by the Deepwater Horizon Blowout

Charles Peterson,Christine Voss,Maria Vozzo,Vincent Guillory,Jeffrey Short,Harold Geiger,James Haney

doi:10.3390/jmse9020190

Abstract

Unprecedented recruitment of Gulf menhaden (Brevoortia patronus) followed the 2010 Deepwater Horizon blowout (DWH). The foregone consumption of Gulf menhaden, after their many predator species were killed by oiling, increased competition among menhaden for food, resulting in poor physiological conditions and low lipid content during 2011 and 2012. Menhaden sampled for length and weight measurements, beginning in 2011, exhibited the poorest condition around Barataria Bay, west of the Mississippi River, where recruitment of the 2010 year class was highest. Trophodynamic comparisons indicate that ~20% of net primary production flowed through Gulf menhaden prior to the DWH, increasing to ~38% in 2011 and ~27% in 2012, confirming the dominant role of Gulf menhaden in their food web. Hyperabundant Gulf menhaden likely suppressed populations of their zooplankton prey, suggesting a trophic cascade triggered by increased menhaden recruitment. Additionally, low-lipid menhaden likely became “junk food” for predators, further propagating adverse effects. We posit that food web analyses based on inappropriate spatial scales for dominant species, or solely on biomass, provide insufficient indication of the ecosystem consequences of oiling injury. Including such cascading and associated indirect effects in damage assessment models will enhance the ability to anticipate and estimate ecosystem damage from, and provide recovery guidance for, major oil spills.

Highlights

Despite widely held assumptions, demonstrating ecosystem-level damage from large marine oil spills remains challenging
These two separate year classes (2010 and 2011) of Gulf menhaden, which shared planktonic food resources in the same coastal habitats, were both characterized by exceptionally poor physiological condition in 2012
These include: (1) the exceptionally low weights-at-age of the 2010 year class at age 1 in and at age 2 in 2012, along with the 2011 year class at age 1 in 2012, based on the NMFS port sampling program (Figure 2); (2) the low condition indexes of Gulf menhaden sampled under the opportunistic DFI sampling program (Figure 3); and (3) the low oil yields of DFI catches in 2011 and 2012 (Figure 4)

Summary

Introduction

Despite widely held assumptions, demonstrating ecosystem-level damage from large marine oil spills remains challenging. Difficulties in clearly establishing ecosystem-level effects include the high natural variability of affected populations, inadequate or absent baseline data, and poorly understood ecological linkages. Mass mortalities of vulnerable species that are major components of marine ecosystems provide one of the more likely sources of ecosystem-level effects following large environmental perturbations such as oil spills Species such as seabirds and marine mammals that routinely occupy or traverse the air–water interface are vulnerable to direct contact with oil, as are species that inhabit the intertidal zone [2]. These species may be vulnerable to additional mortality from contact with dispersants, or the from effects of shoreline clean-up efforts [2].

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Results

Conclusion