The multi-stanza representation of juvenile and adult biomasses was originally included in Ecosim to allow representation of trophic ontogeny (big differences in diet between juveniles and adults). To implement this representation, we found that it was necessary to include population numbers and age structure, at least for juveniles, so as to prevent ?impossible? dynamics such as elimination of juvenile biomass by competition/predation or fishing without attendant impact on adult abundance (graduation from juvenile to adult pools cannot be well represented just as a biomass ?flow?).
When we elected to include age-structured dynamics, we in effect created a requirement for model users to think carefully about the dynamics of compensatory processes that have traditionally been studied in terms of the ?stock-recruitment? concept and relationships. To credibly describe the dynamics of multi-stanza populations, Ecosim parameters for split pools usually need to be set so as to produce an ?emergent? stock-recruitment relationship that is at least qualitatively similar to the many, many relationships for which we now have empirical data (see data summary in www.mscs.dal.ca/~myers/data.html). In most cases, these relationships are ?flat? over a wide range of spawning stock size, implying there must generally be strong compensatory increase in juvenile survival rate as spawning stock declines (otherwise less eggs would mean less recruits on average, no matter how variable the survival rate might be).
When Ecosim users create multi-stanza dynamics, they need to be careful in setting model parameters that define/create compensatory effects. This begins with the Ecopath input parameters; in order for the juvenile dynamics to Figure compensatory mortality changes, at least two conditions are needed or helpful:
Compensatory effects can be increased (the recruitment relationship is flat over a wider range of adult stock sizes, with a steeper slope of recruitment curve near the origin) by:
It is especially important to test alternative values for the vulnerability of prey to juveniles (point i). If vulnerability is too high, the Ecosim emergent stock-recruitment relationship is likely to look almost like a straight line out of the origin, i.e. without compensatory effect. If vulnerability is too low, the relationship may develop a ?spurious? dome-shape.
Note that in Ecosim multi-stanza groups, the ?Adult? group is always the oldest stanza. The stock-recruitment relationship between this adult stage and each of the younger stages separately is calculated using the Stock recruitment? (S/R plot) form.
A stock-recruitment exercise in Ecosim
Always check the stock-recruitment curve shape, and play with Group info and Stage parameters that may affect it, before proceeding to other policy analysis. The simplest way to check this shape while minimizing complicating and confounding effects of trophic interactions is to set up Ecosim for a fairly long time scenario (40+ yrs):