Line configuration and farming depth markedly affect survival and growth in the kelp Ecklonia radiata

The native kelp Ecklonia radiata is a target species for the emerging seaweed aquaculture industry in New Zealand, however, the effect of line configuration on survival and growth of farmed seaweed is unknown. In this study, we outplanted nine seaweed growth lines (with 20 individuals each) in three line configurations (vertical, horizontal 2 m depth, and horizontal 4 m depth) in northern New Zealand for grow-out between August 2021 and April 2022. Initial survival rates were similar between line configurations, but only individuals growing on vertical lines survived after four months (38% ± 12 SD). Similarly, on vertical lines, average juvenile growth by elongation was 85% higher (0.11 cm day−1 ± 0.03 SD) and maximum average weight of individuals was 77% higher (106 g WW ± 73 SD), compared to horizontal lines. Unusually warm water temperatures were likely a central driver for the overall high mortality of E. radiata observed from January to April, while a combination of temperature and light stress was likely a central driver for higher mortality observed on vertical lines at shallow depths compared to deeper depths. In conclusion, this study supports farming E. radiata on vertical dropper lines rather than horizontal lines.