Quantifying depletion rates of benthic macrofauna from the Danish Seine – A ‘low impact’ mobile bottom-contacting fishing gear

The Danish ‘anchor’ seine is a widely used commercial fishing gear, assumed to have relatively low seabed impact due to shallow sediment penetration. However, this assumption has lacked empirical testing. Here, we quantify benthic macrofaunal depletion from Danish seines for the first time via a Before-After-Control-Impact (BACI) experiment involving single and multiple hauls. By attaching GPS loggers to the seine gear, we analysed separate benthic effects of the two main seine components (rope and ground gear) using targeted sampling within the respective footprints. From 120 Van Veen grab samples, we observed little or no depletion in macrofaunal density, biomass, or species richness for either gear component or haul treatment. However, multiple hauls led to marked depletion of erect tube-building Phoronis spp., which declined by 90 % in the rope and 64 % in ground gear footprints. Overall, macrofauna depletion was greater when subjected to multiple hauls, with clearer negative effects in rope impacted areas. Conversely, metrics such as density and biomass unexpectedly increased in ground-gear impacted areas, possibly due to scavenging species or natural variability in macrofauna distributions. We further estimated a whole-gear single-haul biomass depletion rate of 0.068, though this estimate is based on large-bodied fauna (>4 mm) and was associated with high uncertainty. Despite considerable sampling effort, the contrasting results and chiefly species-specific effects indicate an overall low benthic impact from the Danish seine. This outcome supports the general assumption of its relatively low impact, and as a potential alternative to more damaging beam trawls that target the same species in similar habitats. • First empirical estimate of benthic fauna depletion for the Danish seine. • Low overall impact but significant depletion of Phoronis spp. after repeated hauls. • Higher benthic impact from seine ropes than ground gear components. • Danish seines may represent a potential lower-impact alternative to beam trawls.