1.1 Depletion of Benthis métiers

Depletion rates d by sediment class and Benthis métiers

Global depletion rates were first estimated by Hiddink et al. 2017 for four generic gear types (otter trawls, beam trawls, toothed and hydraulic dredges). These estimates were split by substrate type (mud, sand or gravel) by Pitcher et al. 2022 and by Benthis-metier type by Rijnsdorp et al. 2020. The proportional grain size distribution from Mitchell et al (2019) was categorized in three classes (mud, sand and gravel) following Pitcher et al (2022) to estimate sediment-specific depletion rates for each grid cell in the study region.

Gear	dr_Hiddink	Sediment class	dr_Pitcher	Benthis métier	dr_Rijnsdorp	dr_SEAwise
BT	0.14	gravel	0.174	TBB_CRU	0.060	0.108
BT	0.14	mud	0.181	TBB_CRU	0.060	0.115
BT	0.14	sand	0.113	TBB_CRU	0.060	0.047
BT	0.14	gravel	0.174	TBB_DMF	0.140	0.174
BT	0.14	mud	0.181	TBB_DMF	0.140	0.181
BT	0.14	sand	0.113	TBB_DMF	0.140	0.113
BT	0.14	gravel	0.174	TBB_MOL	0.140	0.174
BT	0.14	mud	0.181	TBB_MOL	0.140	0.181
BT	0.14	sand	0.113	TBB_MOL	0.140	0.113
OT	0.06	gravel	0.108	OT_CRU	0.107	0.193
OT	0.06	mud	0.115	OT_CRU	0.107	0.205
OT	0.06	sand	0.047	OT_CRU	0.107	0.084
OT	0.06	gravel	0.108	OT_DMF	0.026	0.047
OT	0.06	mud	0.115	OT_DMF	0.026	0.050
OT	0.06	sand	0.047	OT_DMF	0.026	0.020
OT	0.06	gravel	0.108	OT_MIX	0.049	0.088
OT	0.06	mud	0.115	OT_MIX	0.049	0.094
OT	0.06	sand	0.047	OT_MIX	0.049	0.038
OT	0.06	gravel	0.108	OT_MIX_CRU	0.097	0.175
OT	0.06	mud	0.115	OT_MIX_CRU	0.097	0.187
OT	0.06	sand	0.047	OT_MIX_CRU	0.097	0.076
OT	0.06	gravel	0.108	OT_MIX_CRU_DMF	0.076	0.137
OT	0.06	mud	0.115	OT_MIX_CRU_DMF	0.076	0.146
OT	0.06	sand	0.047	OT_MIX_CRU_DMF	0.076	0.060
OT	0.06	gravel	0.108	OT_MIX_DMF_BEN	0.029	0.052
OT	0.06	mud	0.115	OT_MIX_DMF_BEN	0.029	0.055
OT	0.06	sand	0.047	OT_MIX_DMF_BEN	0.029	0.022
OT	0.06	gravel	0.108	OT_MIX_DMF_PEL	0.073	0.132
OT	0.06	mud	0.115	OT_MIX_DMF_PEL	0.073	0.141
OT	0.06	sand	0.047	OT_MIX_DMF_PEL	0.073	0.057
OT	0.06	gravel	0.108	OT_SPF	0.009	0.017
OT	0.06	mud	0.115	OT_SPF	0.009	0.018
OT	0.06	sand	0.047	OT_SPF	0.009	0.007
OT	0.06	gravel	0.108	SDN_DMF	0.009	0.017
OT	0.06	mud	0.115	SDN_DMF	0.009	0.018
OT	0.06	sand	0.047	SDN_DMF	0.009	0.007
OT	0.06	gravel	0.108	SSC_DMF	0.017	0.030
OT	0.06	mud	0.115	SSC_DMF	0.017	0.032
OT	0.06	sand	0.047	SSC_DMF	0.017	0.013
TD	0.20	gravel	0.254	DRB_MOL	0.200	0.254
TD	0.20	mud	0.261	DRB_MOL	0.200	0.261
TD	0.20	sand	0.193	DRB_MOL	0.200	0.193

Swept Area Ratio SAR by Benthis métiers

The system’s study area was defined by ICES Divisions 4abc excluding depths below 400 m, as these constitute fisheries regulated by the deep-sea access regulations Van Denderen et al. 2021, and delineated by the availability of longevity data (Rijnsdorp et al. 2018). SAR data were available for 2021 at 0.05 degree resolution for all Benthis métiers formatted in a similar way as the freely available 2009-2020 data from ICES (2021). SAR estimates were pooled into broad categories that could be matched to the fleets and métiers as used in the FLBEIA model. These were otter trawls targeting fish (combining Benthis métiers: OT_DMF,OT_MIX,OT_MIX_DMF_BEN,OT_MIX_DMF_PEL), Nephrops-directed otter trawls (OT_CRU,OT_MIX_CRU_DMF), flatfish beam trawls (TBB_DMF), flyshoot seines (SSC_DMF), Danish seines (SDN_DMF) and other mobile bottom-contacting gears such as shrimp beam trawls (DRB_MOL,OT_SPF,TBB_CRU,TBB_MOL).

Swept Area Ratio for pooled Benthis métiers in 2021

Depletion of pooled Benthis metiers

Swept Area Ratio (SAR) estimates were multiplied with their sediment-specific depletion rates in each grid cell to estimate the depletion caused by the Benthis métiers in 2021.

Code

depl_r <- sar_r_2021[[selmets]] * drate_r[[selmets]] # SAR and depletion rate are multiplied on a grid cell basis using spatRasters

Depletion for pooled Benthis métiers in 2021