Changes between Version 7 and Version 8 of EwEugRepresentationOfMultiStanzaLifeHistoriesInEcopathEcoSimAndEcospace
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 20101122 01:56:16 (8 years ago)
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EwEugRepresentationOfMultiStanzaLifeHistoriesInEcopathEcoSimAndEcospace
v7 v8 11 11 (2) the species population as a whole has had relatively stable mortality and relative recruitment rate for at least a few years, and so has reached a stable agesize distribution. 12 12 13 Under the stable age distribution assumption, the relative number of age "''a''" animals is given by'' l,,a,,/'' ''S''''l,,a,,'' where the sum is over all ages, and ''l,,a,,'' is the population growth ratecorrected survivorship,13 Under the stable age distribution assumption, the relative number of age "''a''" animals is given by'' l,,a,,/''S l,,a,,'' where the sum is over all ages, and ''l,,a,,'' is the population growth ratecorrected survivorship, 14 14 15 15 [[Image(wiki:EwEugImages:0800001D.png)]] … … 27 27 then setting ''B,,s,,'' = ''b,,s,,B'' for the other stanzas. ''Q/B'' estimates for nonleading stanzas are calculated with a similar approach, assuming that feeding rates vary with age as the 2/3 power of body weight (a "hidden" assumption in the von Bertalanffy growth model). This method for 'extending' biomass and ''Q/B'' estimates over stanzas avoids a problem encountered in earlier 'splitgroup' EwE representations, where users could enter juvenile biomasses and feeding rates quite inconsistent with the adult biomasses and feeding rates that they had entered. The internal calculations of survivorship and biomass are actually done in monthly age steps, so as to allow finer resolution than one year in the stanza biomass and mortality structure (e.g., larval and juvenile stanzas that last only one or a few months). 28 28 29 On entry to Ecosim from Ecopath, the stanza agesize distribution information (''l,,a,,'', ''w,,a,,'') is passed along and is used to initialize a fully sizeage structured simulation for the multistanza populations. That is, for each monthly time step in Ecosim, numbers at monthly ages ''N,,a,t,,'' and body weights ''w,,a,t,,'' are updated for ages up to the 90% maximum body weight age (older, slow growing animals are accounted in an 'accumulator' age group). The body growth ''w,,a,t,,'' calculations are parameterized so as to follow von Bertalanffy growth curves, with growth rates dependent on body size and (size and timevarying) food consumption rates. Fecundity is assumed proportional to body weight above a weight at maturity, and sizenumbers dependent monthly egg production is used to predict changes in recruitment rates of age 0 fish. Compensatory juvenile mortality is represented through changes in ''Z'' for juvenile stanzas associated with changes in foraging time and predator abundances. Egg production is allowed to vary seasonally or over longterm through a userdefined forcing function (see [ [Egg production.htmEgg production]]). If an egg production curve is defined the egg production term is multiplied according to the userdefined function.29 On entry to Ecosim from Ecopath, the stanza agesize distribution information (''l,,a,,'', ''w,,a,,'') is passed along and is used to initialize a fully sizeage structured simulation for the multistanza populations. That is, for each monthly time step in Ecosim, numbers at monthly ages ''N,,a,t,,'' and body weights ''w,,a,t,,'' are updated for ages up to the 90% maximum body weight age (older, slow growing animals are accounted in an 'accumulator' age group). The body growth ''w,,a,t,,'' calculations are parameterized so as to follow von Bertalanffy growth curves, with growth rates dependent on body size and (size and timevarying) food consumption rates. Fecundity is assumed proportional to body weight above a weight at maturity, and sizenumbers dependent monthly egg production is used to predict changes in recruitment rates of age 0 fish. Compensatory juvenile mortality is represented through changes in ''Z'' for juvenile stanzas associated with changes in foraging time and predator abundances. Egg production is allowed to vary seasonally or over longterm through a userdefined forcing function (see [EwEugEggProduction Egg production]). If an egg production curve is defined the egg production term is multiplied according to the userdefined function. 30 30 31 31 In Ecospace, it is not practical to dynamically update the full multistanza age structures for every spatial cell (computer time and memory limits). The multistanza dynamics are retained, but the population numbers at age are assumed to remain close to equilibrium (changes in numbers at age associated with changes in mortality rates, foraging times, etc. are assumed to 'immediately' move the numbersatage composition to a new equilibrium). In practice, we have found that this movingequilibrium representation of population numbers generally gives results quite close to those obtained when full agesize accounting is done dynamically, provided feeding and mortality rates do not change too rapidly. This is similar to the general finding with Ecospace that time predictions of overall abundance change are quite similar to those obtained with Ecosim, despite how the "dynamic" calculation in Ecospace is really just a stepwise movement toward predicted spatial equilibrium values for all variables.