Changes between Version 7 and Version 8 of EwEugRepresentationOfMultiStanzaLifeHistoriesInEcopathEcoSimAndEcospace


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Timestamp:
2010-11-22 01:56:16 (8 years ago)
Author:
shermanl
Comment:

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  • EwEugRepresentationOfMultiStanzaLifeHistoriesInEcopathEcoSimAndEcospace

    v7 v8  
    1111(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 age-size distribution. 
    1212 
    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 rate-corrected survivorship, 
     13Under 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 rate-corrected survivorship, 
    1414 
    1515[[Image(wiki:EwEugImages:0800001D.png)]] 
     
    2727then setting ''B,,s,,'' = ''b,,s,,B'' for the other stanzas. ''Q/B'' estimates for non-leading 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 'split-group' 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). 
    2828 
    29 On entry to Ecosim from Ecopath, the stanza age-size distribution information (''l,,a,,'', ''w,,a,,'') is passed along and is used to initialize a fully size-age structured simulation for the multi-stanza 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 time-varying) food consumption rates. Fecundity is assumed proportional to body weight above a weight at maturity, and size-numbers 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 long-term through a user-defined forcing function (see [[Egg production.htm|Egg production]]). If an egg production curve is defined the egg production term is multiplied according to the user-defined function. 
     29On entry to Ecosim from Ecopath, the stanza age-size distribution information (''l,,a,,'', ''w,,a,,'') is passed along and is used to initialize a fully size-age structured simulation for the multi-stanza 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 time-varying) food consumption rates. Fecundity is assumed proportional to body weight above a weight at maturity, and size-numbers 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 long-term through a user-defined forcing function (see [EwEugEggProduction Egg production]). If an egg production curve is defined the egg production term is multiplied according to the user-defined function. 
    3030 
    3131In Ecospace, it is not practical to dynamically update the full multi-stanza age structures for every spatial cell (computer time and memory limits).  The multi-stanza 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 numbers-at-age composition to a new equilibrium).  In practice, we have found that this moving-equilibrium representation of population numbers generally gives results quite close to those obtained when full age-size 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.