Removal of deep-sea sponges by bottom trawling in the Flemish Cap area: conservation, ecology and economic assessment

E. Kenchington,M. Maldonado,C. K. Pham,A. Colaço,C. Lirette,D. Ottaviani,F. J. Murillo

doi:10.1038/s41598-019-52250-1

Abstract

Deep-sea sponge grounds are vulnerable marine ecosystems, which through their benthic-pelagic coupling of nutrients, are of functional relevance to the deep-sea realm. The impact of fishing bycatch is here evaluated for the first time at a bathyal, sponge-dominated ecosystem in the high seas managed by the Northwest Atlantic Fisheries Organization. Sponge biomass surface created from research survey data using random forest modeling revealed 231,136 t of sponges in the area. About 42% of that biomass was protected by current fisheries closures. However, projections of trawling tracks estimated that the sponge biomass within them would be wiped out in just 1 year by the current level of fishing activity if directed on the sponges. Because these sponges filter 56,143 ± 15,047 million litres of seawater daily, consume 63.11 ± 11.83 t of organic carbon through respiration, and affect the turnover of several nitrogen nutrients, their removal would likely affect the delicate ecological equilibrium of the deep-sea benthic ecosystem. We estimated that, on Flemish Cap, the economic value associated with seawater filtration by the sponges is nearly double the market value of the fish catch. Hence, fishery closures are essential to reach sponge conservation goals as economic drivers cannot be relied upon.

Highlights

Deep-sea sponge grounds are vulnerable marine ecosystems, which through their benthic-pelagic coupling of nutrients, are of functional relevance to the deep-sea realm
We found that the fishers are operating mainly in areas located outside of dense sponge grounds, as previously suggested by Murillo et al.[2], and that the largest and densest sponge aggregations within the Northwest Atlantic Fisheries Organization (NAFO) fishing footprint are currently protected by fisheries closures
The results of this study highlight that the magnitude of impacts by bottom trawling on the deep-sea floor should be assessed at the scale of entire fishing fleets within ecosystems, because fishing pressure is not homogeneously distributed, making extrapolations based on small-scale studies highly uncertain

Summary

Introduction

Deep-sea sponge grounds are vulnerable marine ecosystems, which through their benthic-pelagic coupling of nutrients, are of functional relevance to the deep-sea realm. Projections of trawling tracks estimated that the sponge biomass within them would be wiped out in just 1 year by the current level of fishing activity if directed on the sponges Because these sponges filter 56,143 ± 15,047 million litres of seawater daily, consume 63.11 ± 11.83 t of organic carbon through respiration, and affect the turnover of several nitrogen nutrients, their removal would likely affect the delicate ecological equilibrium of the deep-sea benthic ecosystem. There has as of yet been no quantitative assessment of SAI of fishing on VMEs, despite the long history of research on trawling impacts on benthic ecosystems[5] This is in a large part due to a lack of knowledge on the spatial distribution of both VMEs and fisheries in the high seas, and on the basic functional connections within and between deep-sea benthic communities. We further undertook an economic evaluation of those impacts in terms of reduction of ecosystem function and we compared, in economic terms, the disruption of sponge water filtering activity due to trawling with the economic value of the commercial catch

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Conclusion