Experimental assessment of growing substrates for the early life stages of five large brown seaweeds

Seaweed cultivation, especially for large brown algae (LBA; e.g. Laminariales, Fucales) is in development in many regions worldwide. Identifying appropriate growing substrates is a key step for establishing successful seaweed cultivation. We investigated the effects of four substrates with aquaculture and restoration potential on the density and length of recruits of five LBA in a series of 42-day-long experiments. The substrates assessed were green-lipped mussel shells (<i>Perna canaliculus</i>), concrete gravel, fibreglass, and polyester twine. Reproductive material used to produce spores or gametes for experiments was sourced from the wild and collected in three seasons: early winter (June), late winter (August) and spring (October) in 2023. The LBA recruited and grew on all substrates. <i>Ecklonia radiata</i> had the greatest density and length on mussels and fibreglass. <i>Macrocystis pyrifera</i> had the lowest density and length on fibreglass. <i>Landsburgia quercifolia</i> had the greatest density and length on concrete and mussels. <i>Cystophora retroflexa</i> were most dense on fibreglass and twine but were longest on concrete and mussels. <i>Cystophora scalaris</i> were most dense on twine but grew the longest on mussels. Our study demonstrated that optimal growing substrates for LBA are species-specific and identified novel, recycled materials for growing LBA for aquaculture or restoration purposes.