Determining Coral Density Thresholds for Identifying Structurally Complex Vulnerable Marine Ecosystems in the Deep Sea

Ashley A Rowden,Owen F Anderson,David Anthony Bowden,Tabitha R R Pearman,Malcolm Ross Clark

doi:10.3389/fmars.2020.00095

Abstract

Vulnerable marine ecosystems (VMEs) are at risk from the impacts of deep-sea trawling. Identifying the presence of VMEs in high seas fisheries management areas has to date relied mainly on presence records, or on habitat suitability models of VME indicator taxa (e.g., the stony coral species Solenosmilia variabilis Duncan, 1873) as proxies for the occurrence of VMEs (e.g., cold-water coral reefs). However, the presence or predicted presence of indicator taxa does not necessarily equate to the occurrence of a VME. There have been very few attempts to determine density thresholds of VME indicator taxa that relate to a “significant concentration” which supports a “high diversity” of associated taxa, as per the current criterion for identifying structurally complex VMEs (FAO 2009). Without knowing such thresholds, identifications of VMEs will continue to be subjective, impeding efforts to design effective spatial management measures for VMEs. To address this issue, we used seafloor video and still image data from the Louisville Seamount Chain off New Zealand to model relationships between the densities of live Solenosmilia variabilis coral heads, as well as percent cover of live and dead coral matrix, and the number of other epifauna taxa present. Analyses were conducted at three spatial scales; 50 m2 and 25m2 for video, and 2 m2 for stills. Model curves exhibited initial steep positive responses reaching thresholds for the number of live coral heads at 0.11 m-2 (50 m2), 0.14 m-2 (25 m2), and 0.85 m-2 (2 m2). Both live and dead coral cover were positively correlated with the number of associated taxa up to about 30% cover, for all spatial scales (24.5 to 28%). We discuss the results in the context of past and future efforts to develop criteria for identifying VMEs.

Highlights

Deep-sea trawling impacts the structure and function of seafloor communities and habitats
Vulnerable marine ecosystems (VMEs) indicator taxa may be recorded from trawling bycatch and used for encounter protocols [e.g., Parker et al, 2009 for the South Pacific Regional Fisheries Management Organisation (SPRFMO)] or their distribution may be predicted by habitat suitability modeling (e.g., Georgian et al, 2019 for SPRFMO)
The density thresholds we identified for live coral heads using video data, 0.11 m−2 and 0.14 m−2, were the same or similar to the threshold of 0.11 m−2 used previously to identify coral reef VME on the Lousiville Seamount Chain

Summary

Introduction

Deep-sea trawling impacts the structure and function of seafloor communities and habitats (see review by Clark et al, 2016). VME indicator taxa may be recorded from trawling bycatch and used for encounter protocols [e.g., Parker et al, 2009 for the South Pacific Regional Fisheries Management Organisation (SPRFMO)] or their distribution may be predicted by habitat suitability modeling ( called species distribution modeling) (e.g., Georgian et al, 2019 for SPRFMO). Howell et al (2011) noted that the observed distribution of a coral reef in the North Atlantic is a subset of the wider predicted distribution the indicator taxon Desmophyllum pertusum (Linnaeus, 1758) [formerly Lophelia pertusa] that can form such a VME when it occurs in sufficient density. Rowden et al (2017) used abundance-based habitat suitability models of a coral species and a subjective density-based definition of a coral reef to predict VME distribution. The definition used was not related directly to any of the criteria that the FAO guidelines specify for what constitutes a VME (FAO, 2009)

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Discussion

Conclusion