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

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 and 25 m 2 for video, and 2 m 2 for stills. Model curves exhibited initial steep positive responses reaching thresholds for the number of live coral heads at 0.11 m −2 (50 m 2 ), 0.14 m −2 (25 m 2 ), and 0.85 m −2 (2 m 2 ). Both live and dead coral cover were positively correlated with the number of associated taxa up to about 30% cover, for all spatial scales (24.5-28%). We discuss the results in the context of past and future efforts to develop criteria for identifying VMEs.

Benthic Assemblage Composition of South Atlantic Seamounts

“…although efforts are underway to propose both a classification and density thresholds (e.g., ICES, 2020;Rowden et al, 2020). It has however been confirmed that expert opinion can often determine VME status based on a single image, particularly when considering coral reef structures(Baco-Taylor et al, 2020).…”

A Data-Informed Approach for Identifying Move-on Encounter Thresholds for Vulnerable Marine Ecosystem Indicator Taxa

“…Additional work is required, to support the approach taken by SPRFMO in developing encounter protocols as a safeguard against the uncertainty associated with the effectiveness of spatial management measures in preventing significant adverse impacts on VMEs. We think this work should include the determination of taxa-specific catchability estimates (e.g., by comparing VME indicator taxa bycatch weights from trawls to those determined from seafloor imagery from the same area swept by the trawls), and the determination of the particular seafloor density/biomass of VME indicator taxa that represents an actual VME [e.g., by examining the relationship between the density/biomass of a VME indicator taxa and the diversity of associated species, see Rowden et al (2020) this issue]. Obtaining this information will allow for the identification of encounter thresholds that are more ecologically meaningful and at the same time prevent trawling impacts to VMEs while assessing the accuracy of the habitat suitability models on which the spatial management measures are based.…”

Vulnerable, but Still Poorly Known, Marine Ecosystems: How to Make Distribution Models More Relevant and Impactful for Conservation and Management of VMEs?

“…In these ecosystems, ecological processes are usually highly dependent on these structured systems. Further, such ecosystems often have high diversity, which is dependent on the structuring organisms policy-decisions remains relatively limited (Ardron et al, 2014;Vierod et al, 2014;Gianni et al, 2016;Thompson et al, 2016;Bell et al, 2019;Rowden et al, 2020). The main reasons for this are: (i) the overall disagreement about what constitutes a VME, (ii) the inherent uncertainty of model predictions, especially since most are fitted on observations from fishery bycatch data with limited reliability and accuracy, and (iii) model outputs are not always easily understandable or relevant for environmental managers and conservationists.…”

“…To model VMEs using assemblage data is however limited by the scarce availability of this data type, since it generally requires underwater imagery sampling (see section Connections Between the Type of VME Data and Model Use), and by the lack of consensus around the quantitative definition of VME assemblages, e.g. composition or density of taxa (Rowden et al, 2020).…”