Effects of environmental change, fisheries and trophodynamics on the ecosystem of the western Scotian Shelf, Canada
Marine Ecology Progress Series
Supplement 1. Ecopath with Ecosim main features. In Ecopath, for functional groups that are simple aggregate biomass pools, i.e. with no size/age structure representation, the flow to and from each functional group is described by the first Ecopath basic equation: where P i is the total production; Y i is the total fishery catch; B i is the biomass; M2 i is the predation mortality rate; E i is the net migration rate (emigration -immigration); BA i is the biomass accumulation rate; and EE i is
... e 'ecotrophic efficiency' of i, with P i (1 -EE i ) being the other (non-predation) natural mortality term. Ecotrophic efficiency is the proportion of the production that is accounted for by fishing, predation, immigration and population growth. The Ecopath master equation is usually expressed as: where P/B i is the production/biomass ratio of i; B j is the biomass of consumer or predator j; (Q/B) j is the consumption per unit of biomass of j; and DC ji is the fraction of i in the diet of j. In the second basic equation of Ecopath, the energy balance within each species or group is ensured using the equation: where R i and GS i are the respiration and the proportion of food that is not assimilated, and the other parameters are as defined above. In most cases, at least 3 of the 4 basic parameters (B, P/B, Q/B and EE) are required for the model parameterization. The fourth is then estimated by Ecopath. If all 4 basic parameters are entered, Ecopath then estimates either the BA or E. Fully age/size-structured functional groups, representing ontogenetic changes in diet and changes in vulnerability to predation and fishing can be represented in Ecopath with Ecosim (EwE) as 'multistanzas'. In these cases, the user must enter the estimates of B and Q/B for the leading stanza, and total mortality, Z, for all stanzas. Note that for the functional groups represented by 2 or more stanzas, Z is used instead of P/B, since the equivalence between the 2 parameters is only valid for the population as a whole, i.e. the population averages of these parameters. In addition, estimates for BA, for the growth parameter K of the Von Bertalanffy growth function, the starting age in months of each stage and the ratio between the average weight at maturity and the asymptotic weight must be entered. The B and Q/B of the other stage(s) are then estimated by Ecopath.