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Most seafood production depends
ultimately on the creation of new organic matter (primary
production) by phytoplankton or seaweeds. Before this
primary production can become fisheries production, it has
to go through a number of steps, depending on how close the
animal being harvested is to the beginning of the food web.
Each type of organism has its
own production and consumption characteristics.
Phytoplankton produce more than 200 times their average
biomass in a year, whereas adult fish usually produce less
than one third. Squid eat more than 20 times their average
biomass in a year, compared with less than three times for
some adult fish. Therefore, biomass alone is not a good
guide to the productivity of these animals. We need to take
these differences into account when evaluating
sustainability of fisheries and aquaculture.
How do we know when a system
is very tightly constrained or when there is excess primary
production? One method creates a type of annual food budget
that takes into account the most important food interactions
in an ecosystem and the different production and consumption
characteristics of its key types of animals. There is now a
tool available that provides a framework within which to
construct such an annual food budget. This tool is known as
Ecopath with Ecosim. We have applied this framework
to an offshore region (Southern Plateau) based on fisheries
data and data collected during the Ocean Ecosystems
programme, funded by the Foundation for Research, Science
and Technology.
There is great potential for
this tool to be used in near-shore regions that are the
subject of aquaculture proposals. An Ecopath
analysis serves to determine not only the important food
interactions, but also if, on average, there is enough extra
primary production to support aquaculture of shellfish
without negatively affecting other parts of the ecosystem.
This technique has the added advantage of providing a
framework for pulling together the concerns of all
stakeholders.
Details about Ecopath
with Ecosim can be found at
www.ecopath.org.
Janet
Bradford-Grieve
j.grieve@niwa.co.nz
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