Environmental Engineering Reference
In-Depth Information
consumption or environmental forcing. Thus, a tool was created that allows, by using a
balanced ECOPATHmodel as a reference, to explore ecosystem response of different
fishing policies and to follow ecosystem changes over time by fitting time series of
observed biomasses and catches to fishing effort and/or environmental parameters.
Application Range
ECOPATH requires the identification of main functional groups within a system,
and allows to quantify and visualize biomass flows, to identify key compartments of
high biomasses and productivities and to understand the relative importance of the
biomass export to the fishery for the energy cycling of the system.
Relevant research questions are related to (a) the relative impact of the fishery
and natural predation on the key resources of the system; (b) the overall functional
structure (i.e. the ratio of benthic to pelagic compartments; relevance of benthic
versus pelagic primary productivity etc.); (c) the trophic efficiency between trophic
levels; (d) network flow characteristics (such as degree of connectivity, amount of
internal cycling, role of detritus, etc.); (e) mean trophic level of the catch, Primary
Production Required (PPR) to sustain the catches and many more summary statis-
tics that can be derived from this holistic approach. It is frequently used for
comparing network structure, overall degree of development/disturbance and the
relative role of certain functional groups of aquatic ecosystems. Hundreds of
aquatic ecosystems have been modelled with this approach and the database for
comparative research has grown substantially over the past two decades. Following
the Large Marine Ecosystem (LMEs) initiative of Sherman et al. (2003) ECOPATH
models were and are being constructed for each of the LMEs. A substantial number
is already accessible on the web (see:
http://www.seaaroundus.org
).
5.2 Conceptual and Mathematical Basis of Approach
5.2.1 Mass-Balance Modelling: Ecopath
The core routine is based on the assumption of mass balance over a given time
period (usually 1 year, but any other interval may be chosen). In its present form
Ecopath is based on two master equations, one to describe the production term (5.1)
and one for the energy balance for each group (5.4).
Master equation one:
P
i
¼
Y
i
þ
M
2
i
B
i
þ
E
i
þ
BA
i
þ
M
0
i
B
i
;
(5.1)
where
Y
i
is the total fishery catch rate of
i
,
M
2
i
is the instantaneous predation rate for
group
i
,
E
i
the net migration rate (emigration
immigration),
BA
i
is the biomass
