Résumé
Marine organisms are key to food security and to combat malnutrition,
especially in the developing tropics. With the increase in human population,
its appetite for seafood also grows, while production from capture fisheries
is at its limit. There are conflicting views on whether aquaculture can
really fill this gap. For large scale industrial aquaculture, most authors
agree that it will not be able to feed the world, much less in a sustainable
way. With regard to small-scale, family or village-based systems, the future
looks bright.
Faced with resource limitations and environmental regulations, aquaculture
operations have to augment their output while decreasing the relative use of
resources, coastal space and effluent output. Integrated aquaculture is seen
as a strategy to meet demands for more efficient and sustainable aquaculture
production. Especially, integrated multi-trophic aquaculture (IMTA) is often
described as a solution, but proliferation of the industry has been slow,
uneven and research is still lacking in regards to its application to
tropical marine species.
This chapter presents case examples from Tanzania, based on IMTA experiments
with edible tropical species at three different trophic levels. A
proof-of-concept presents the viability of a low-technology recirculating
aquaculture system (RAS) of milkfish Chanos chanos,
integrating sea cucumber Holothuria scabra and halophyte
Sesuvium portulacastrum as extractive species. The species showed good
growth and the system maintained good water quality and low concentrations
of dissolved inorganic nitrogen even without an additional biofilter.
Removal, uptake and assimilation of feed-derived nutrients through the
cultivation of the extractive species were examined to determine strategies
to optimize bioremediation. The high-value sea cucumber H.
scabra was raised on solid waste from milkfish aquaculture. In
order to improve bioremediation efficiency, animals were supplied with
additional carbon based on cellulose or bagasse, demonstrating good
assimilation of nutrients from aquaculture waste. Increasing the C/N ratio
of the waste led to a higher biomass production and nutrient recovery and
subsequent turnover into sea cucumber biomass and bagasse, an agricultural
waste product, proved to be effective for carbon supplementation. Removal
and uptake of dissolved inorganic nutrients from milkfish effluents by
systems planted with S. portulacastrum showed that nitrogen
recovery in edible plant biomass was significantly higher in hydroponic
cultivation.
This chapter contributes to the knowledge needed to move towards sustainably
intensifying multi-trophic aquaculture in tropical countries. The results
hint at the intricate interplay between extractive species, their biotic
environment and nutrient cycles and highlight the complexity, but also the
opportunities of marine IMTA. Community-based production systems (e.g.
Indonesia, Vietnam, Tanzania, Madagascar) should be multiplied so that a
significant amount of seafood can be produced in a more environmentally and
socially friendly manner along the long coastlines of tropical
countries.