Fishing Without a Trace? Assessing the Balanced Harvest Approach Using EcoTroph

Jennifer Rehren,Didier Gascuel

doi:10.3389/fmars.2020.00510

Abstract

Distributing fishing mortality across the widest possible range of species, stocks and sizes in proportion to their natural productivity (i.e. balanced harvest, BH) has been suggested as a new paradigm of fisheries management to minimize the effects of fishing on the ecosystem structure while maximizing overall yield. Models that have been used to test the effects of BH, however, usually concentrate on fish and assume full alignment of fishing mortality with the productivity of each species. Here, we used the trophic-level-based approach EcoTroph to investigate the effects of BH on the biomass and catch trophic spectra of a virtual ecosystem assuming 1) a full implementation, where all trophic levels can be fished according to their productivity and 2) a more realistic implementation, where low and intermediate trophic levels are only partially exploitable by fisheries mimicking current technological and practical limitations. EcoTroph simulations show that a BH fishing pattern does not fully maintain ecosystem structure but minimizes structural changes when maximizing total yield. The resulting catch, however, was dominated by low trophic levels (i.e. 2-2.5). Considering that fishing mortality cannot be fully aligned to all species, we observed an additional adverse impact of BH: the increase in unexploitable biomass. In contrast, protecting lower trophic levels appeared as an efficient way to limit the impact of fisheries on the highest trophic levels, which play a crucial role in ecosystem stability and biodiversity. We conclude that given our inability to align fishing mortality to the productivity of each species, BH could lead to strong adverse impacts on the ecosystem. Instead of expanding fishing pressure towards new species and trophic levels, we first should ensure the sustainable management of those that are currently harvested beyond their capacity to replenish.

Highlights

Balanced harvest (BH) has been suggested as a new paradigm of fisheries management, which minimizes the effects of fishing on the structure of marine ecosystems, while simultaneously maximizing overall yield (Garcia et al, 2012; Kolding et al, 2016c)
We address the effect of BH on the biomass trophic spectrum and the relative trophic composition of the catch when balancing fishing mortality with (1) productivity and (2) production rate and investigate which fishing pattern leads to full maintenance of the ecosystem structure
Under a BH scenario, where fishing mortality is set proportional to the productivity, the same proportion of production is harvested at each trophic level, and as a result fishing mortalities scale with trophic level (Figure 2)

Summary

Introduction

Balanced harvest (BH) has been suggested as a new paradigm of fisheries management, which minimizes the effects of fishing on the structure of marine ecosystems, while simultaneously maximizing overall yield (Garcia et al, 2012; Kolding et al, 2016c). With the intention to maintain ecosystem structure, while increasing fisheries yield a BH approach aims to meet international efforts toward an ecosystem-based approach to fisheries management and efforts toward global food security (UN Sustainable Development Goal 2, SDG) (Kolding et al, 2016b) Thereby, it partly reconciles ecological and socioeconomic goals (Zhou et al, 2019), making it a attractive concept. A transition of fisheries management to such an approach, reduces fishing pressure on higher trophic levels, while increasing it on the more productive lower trophic levels, and induces a general fisheries expansion (Law et al, 2013; Zhou et al, 2019)

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Conclusion