Seaweed farming is often depicted as a sustainable form of aquaculture,
contributing to poverty reduction and financial revenues in producer
countries. However, farms may negatively affect seagrasses and associated
organisms (e.g. invertebrate macrofauna) with possible effects on the flow
of ecosystem goods and services to coastal societies. The present study
investigates the influence of a seaweed farm, and the farmed seaweed
Eucheuma denticulatum in particular, on fishery catches using a traditional fishing method
(“madema” basket traps) in Chwaka bay (Zanzibar, Tanzania). The results
suggest that a seaweed farm, compared to a seagrass bed, had no influence on
catch per unit effort (no. of individuals per catch, or catch weight) or no.
of species per catch, but significantly affected catch composition (i.e.
how much that was caught of which species). The two species contributing most to differences
between the sites were two economically important species; the herbivorous
seagrass rabbit fish Siganus sutor, which was more common in the seaweed site and is
known to graze on the farmed algae; and the benthic invertebrate feeder
chloral wrasse Cheilinus chlorourus, more common in the seagrass site. Compared to
vegetation-free bottoms, however, the catches were 3−7 times higher, and
consisted of a different set of species (ANOSIM global R > 0.4). As traps
placed close to the seaweeds fished three times more fish than traps placed
on sand patches within the seaweed farm, the overall pattern is attributed
to the presence of submerged vegetation, whether seagrass or seaweed,
probably as shelter and/or food for fish. However, qualitative differences
in terms of spatial and temporal dynamics between seagrass beds with and
without seaweed farms, in combination with other factors such as
institutional arrangements, indicate that seaweed farms cannot substitute
seagrass beds as fishing grounds.