Harvesting impact on a commercially important red macroalga on a North-East Atlantic rocky shore

Red macroalgae in the genus Porphyra are widely collected for human consumption and increasingly harvested along the North Atlantic coasts but their ability to withstand repeated harvesting and maintain high cover and abundance on natural shorelines remains poorly explored, raising sustainability concerns. Therefore, we investigated the effect of frequent hand-harvesting on Porphyra dioica in a population in South Wales, UK, where it is commercially harvested. Twelve control and twelve treatment quadrats were installed in spring and six control and treatment plots in autumn. Monthly hand-harvesting of P. dioica individuals larger than 5 cm was performed in the treatment plots, while the control plots remained undisturbed. We monitored P. dioica percentage cover, yield, thallus length and recruit density, as well as benthic flora and fauna. Harvesting only impacted P. dioica thallus length, resulting in 41% smaller thalli compared to unharvested plots during summer, with possible long-term implications (e.g. reproductive output). Strong seasonality of P. dioica cover and yield was observed, peaking during spring and summer. The results suggest high resilience of the opportunistic life strategy of P. dioica under frequent harvesting, but to allow for full recovery, we urge for a recovery period of more than the tested 4 weeks.