Seaweed farms worldwide face the challenge of fouling from macrophytes and macrobenthos, which can cause significant damage to the farmed seaweed. Some macrophytes, such as Ulva (formerly Enteromorpha) and Hydroclathrus spp can cause mortality of the farmed seaweed through overgrowth of the thallus, especially when the swathe is >50% of the seaweed thalli. Seaweed farms in the Zanzibar archipelago in Tanzania, have experienced such challenges caused by macro-fouling. For example, at Bweleo village farmers have abandoned their farms in 2019-20, due to a serious loss of their seaweed crop through fouling, particularly by Gracilaria and Hydroclathrus. Information on the species diversity and extent of the fouling problem, however, is lacking. A study on the types of macro-organisms fouling Eucheuma and Kappaphycus seaweed farms in Zanzibar was, therefore, conducted during August – October 2020 and February-March 2021.
The study concentrated on a seaweed farm in Muungoni, SW Coast of Zanzibar, as part of a larger study on seaweed farm biosecurity. Three replicate areas on the farm were used to collect data on the fouling organisms during the harvesting of the seaweed, i.e., after a full 45-day growth cycle. Macro-fouling organisms were collected by hand, placed in sealed containers and identified to the lowest possible taxonomic group using field guides. The macro-fouling was observed to cover 40-60% of the cultivated seaweed lines. Twenty-four species of macroalgae were identified. The majority of species consisted of brown algae and some species were directly attached to the farmed seaweed, in addition to the cultivation lines. Seven species of seagrasses and 7 types of macrobenthos, some attached to the seaweed were also identified. Further investigations are now in progress to determine if the regular removal of this macro-fouling will increase the productivity of the seaweed farms in Tanzania.