Integrated Multitrophic Aquaculture: Filter Feeders Bivalves as Efficient Reducers of Wastes Derived from Coastal Aquaculture Assessed with Stable Isotope Analyses - Aquaculture and the Environment: A Shared Destiny - page 104

Weston, 1991), however, knowledge of fish farming impacts in the Mediterranean Sea is
scarce (Sarà et al., 2004, 2006; Vizzini & Mazzola, 2004), which have different hydrographic
characteristics, such as shallow waters and low current velocity areas, and are characterized
by oligotrophic waters.
1.1 Integrated multitrophic aquaculture
The growing concern on aquaculture's environmental impact has led to an increasing
research into feed formulations and digestibility, better conversion efficiency and improved
management (Skalli et al., 2004; Troell et al., 2003). Since the last decade, the emphasis has
been placed on the practice of integrated multi-trophic aquaculture (IMTA), with a potential
to mitigate some of the environmental problems associated with mono-specific aquaculture
(Soto et al., 2008). Originally used in freshwater practices, it involves the culture of two or
more species from different trophic level; generally finfish being simultaneously cultured
with both organic and inorganic extractive species, such as shellfish and seaweeds,
respectively; in which by-products from one species are recycled to become inputs for
another. Thus, the organic matter released in aquaculture systems might represent a source
of available food for filter-feeding organisms, such as bivalves, reducing its impact on the
environment (Shpigel et al., 1991) and can represent a potential economic income.
Filter feeders bivalves are essentially generalist consumers, and it has been demonstrated
that they can exploit organic matter from several sources (autochthonous, allochthonous or
anthropogenic), as a function of its availability (Stirling & Okumus, 1995). In a conceptual
open water integrated aquaculture, filter feeder bivalves are cultured adjacent to fish
floating cages, reducing nutrient loadings by filtering and assimilating particulate wastes
(uneaten food and faeces) as well as phytoplankton. In this way, bivalves would perform as
biological filters. Previous studies have determined that bivalves can be successfully
incorporated into integrated multitrophic aquaculture systems, based on the increased
growth displayed and the feeding efficiency on pellet feed and fecal products (Mazzola &
Sarà, 2001; Reid et al., 2010).
1.2 Stable isotopes as tracers of matter fluxes
The use of stable isotopes in biogeochemistry and marine ecosystem analysis is increasing
rapidly, especially in assessing trophic relationships and pathways of energy flow in food
webs (Bergamino et al., 2011). Traditionally, the origin and fate of organic matter in the
marine environment have been investigated through different approaches, such as lipid
biomarkers or chlorophyll pigments. In filter feeder bivalves, it has been evaluated with gut
content or fatty acid analyses, representing an instant snapshot of food ingested by bivalves.
This limitation could be solved using stable isotope analyses, as the evaluation of food
sources is based on assimilated instead of ingested food, which represents a time-integrated
food utilization.
Stable isotope analysis is based on the assimilation of 13 C and 15 N from sources, with a
slight enrichment of heavier isotopes ( 13 C and 15 N) as lighter isotopes ( 12 C and 14 N) are
used in metabolism. The enrichment between prey and consumer tissues has been
considered to be consistent across species, however, it has been demonstrated that it
depends on the assimilation process and it has been set at 1‰ for 13 C and 3-4‰ for 15 N