Sustainable management of Fucus beds – testing of UAV-assisted biomass mapping and evaluation of re-growth after harvest at individual and population level

Experimental harvests of Fucus beds were conducted at three sites in central Denmark applying four harvest treatments: apical tip harvest, full harvest, thinning harvest (full harvest of one third), and control (no harvest) to 1) determine correlations between Fucus areal cover and biomass harvest potential, and 2) examine effects of harvest treatment on biomass yield and recovery capacity. The study examined recovery capacity at individual algae scale through the capacity for generating new shoots and/or biomass regrowth, and at population scale using unoccupied aerial vehicles (UAVs) to estimate areal cover recovery. The results indicated significant variability in harvest yields and recovery responses, where full harvest provided the highest but variable yields, ranging from 0.33 to 5.85 kg FW m^-2. The full harvest also had the highest impact on individual algae biomass and population scale areal cover with observable slower recovery capacity compared to other harvest treatments. For the full harvest treatment, UAV-based assessments showed areal cover recovery six months post-harvest, whereas individual algae had not fully recovered one year after full harvest. The use of UAVs proved useful for monitoring the areal cover of Fucus beds, but less reliable in estimating the standing stock biomass. Future studies should focus on refining UAV-based methodologies to enhance accuracy and reliability in estimating areal cover and standing stock, and on large scale, long-term harvest impacts.