Comparative assessment of the performance and economic feasibility of seaweed cultivation at different depths along the Kenyan coast

In recent decades, seaweed farmers in East Africa have faced declines in production and income, largely attributed to varying environmental conditions. Subtidal farming, promoted for its relatively more stable environmental conditions, has been proposed as a potential mitigation strategy, although empirical evidence remains limited. This pilot study assessed the technical feasibility and economic viability of cultivating three red seaweed species ( Kappaphycus alvarezii , Eucheuma denticulatum , and Gracilaria salicornia ) at three depths (0.5 m, 2 m, 4 m) in coastal Kenya, using different cultivation techniques: off-bottom, long-line, and bamboo raft. Seaweed was grown at three sites at the south coast of Kenya (Kibuyuni, Kijiweni, and Mwazaro) in one single cultivation cycle during the SEM season over a 45-day period, after which survival, growth and biomass were evaluated. Regardless of treatment and location, K. alvarezii exhibited the highest RGR (i.e. 2.5 ± 1.5% day −1 ) and biomass production among the three species examined, followed by E. denticulatum and G. salicornia (i.e. 1.3 ± 0.5% day −1 and 0.75 ± 1.3% day −1 , respectively). Survival decreased significantly at 4 m compared to 0.5 m for all three species. Site-specific variations were evident, with seaweed growth in Kibuyuni consistently exhibiting lower growth rates and biomass than Kijiweni and Mwazaro, indicating underlying environmental differences, despite the absence of strong differences in measured environmental parameters. Off-bottom farming at 0.5 m depth yielded the highest eucheumatoid biomass per rope (0.75 kg DW). Subsequent economic assessments revealed that, under current market conditions (USD 0.50 kg⁻¹ DW), profitability is attained solely by off-bottom farming in its present form, with break-even occurring after 29 cultivation cycles (approximately 3.7 years). In contrast, deeper water cultivation systems require substantial optimization before they can be recommended as viable options. Together, these results provide a practical evidence base for future scientists, aquaculture practitioners and policymakers seeking to design sustainable seaweed farming strategies in East Africa. • We assessed the technical feasibility of deepwater seaweed cultivation in Kenya. • Site- and species specific variations in growth and biomass yield were evident. • Across species, survival dropped significantly at 4 m depth. • Off-bottom farming at 0.5 m was the only method to reach profitability.