2. RBS estimation

Relative Benthic State (RBS) is a quantitative risk indicator of the benthic biomass under mobile bottom-contact fishing, expressed as the fraction of biomass remaining relative to the unfished carrying capacity of the community (1 = no depletion; 0 = fully depleted) using the balance between fishing‐induced mortality and natural recovery. RBS is derived by solving a Schaefer(1954)-type of logistic population model at equilibrium, with an additional term to estimate recurring depletion induced by mobile fishing gears (Ellis et al. 2014, Pitcher et al. 2017, Hiddink et al. 2017). Depletion is assumed to occur with an average intensity estimated by the Swept-Area Ratio SAR or the number of trawl passes per unit area per year and the depletion rate d or the fraction of biomass that each pass removes (see Depletion scenarios). Let B be the benthic biomass, K the unfished carrying capacity and r the intrinsic recovery rate (see Benthic sensitivity), then the equilibrium is found at:

\[ \text{RBS} = \frac{B}{K} = 1- \frac{d \cdot \text{SAR}}{r} \]

RBS is a linear index that falls between 0 and 1 to explicitly indicate that benthic state declines with fishing frequency and intensity (d), and improves with faster recovery (r).