Changes between Version 7 and Version 8 of EwEugDealingWithOpenSystemProblems


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Timestamp:
2010-01-28 14:27:51 (9 years ago)
Author:
varunr
Comment:

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  • EwEugDealingWithOpenSystemProblems

    v7 v8  
    99There are two broad options for dealing with directed migration to and from the Ecopath study area so as to avoid misleading predictions in Ecosim: 
    1010 
    11   The ‘diet import’ approach:  for species that migrate to/from the study area for part of each year, include all fisheries/catches that impact the species, independent of whether these are taken within the study area.  In the [wiki: EwEugDietComposition Diet Composition] form, set the diet import proportion to the proportion of time spent outside the system, and set remaining diet proportions to the diet proportions while in the system times the proportion of time spent in the system.  Using this convention, Ecosim then will allow policy exploration of all fisheries that may impact the migratory species, and will treat the food intake rate (per biomass) as constant over time for the time spent feeding outside the system.  Ecopath and Ecosim will ‘automatically’ account for reductions in prey impacts caused by the species for the proportion of time that the species spends feeding in outside areas.  Note that the list of fisheries impacting migratory species can involve splitting fleets into ‘inside’ and ‘outside’ fishing components (which can be varied or ‘managed’ separately in Ecosim), to represent possible policy changes in where/when the migratory fish are harvested.The ‘model expansion’ approach:  If it is considered unsafe to assume that food consumption rates obtained while outside the system (by migratory species) will remain constant in the future, then Ecosim must be provided information on possible changes in food organism populations in those outside areas.  That is, the outside areas must be ‘internalized’ as part of the modelled system, by adding functional groups representing the outside food web structure.  Often, adding such groups may simply mean replicating the initial Ecopath group structure, with the second set of groups labelled ‘outside species X’ and with diet matrix entries set so that the added groups feed on one another but not on the ‘inside’ groups. 
     11  The ‘diet import’ approach:  for species that migrate to/from the study area for part of each year, include all fisheries/catches that impact the species, independent of whether these are taken within the study area.  In the [wiki:EwEugDietComposition Diet Composition] form, set the diet import proportion to the proportion of time spent outside the system, and set remaining diet proportions to the diet proportions while in the system times the proportion of time spent in the system.  Using this convention, Ecosim then will allow policy exploration of all fisheries that may impact the migratory species, and will treat the food intake rate (per biomass) as constant over time for the time spent feeding outside the system.  Ecopath and Ecosim will ‘automatically’ account for reductions in prey impacts caused by the species for the proportion of time that the species spends feeding in outside areas.  Note that the list of fisheries impacting migratory species can involve splitting fleets into ‘inside’ and ‘outside’ fishing components (which can be varied or ‘managed’ separately in Ecosim), to represent possible policy changes in where/when the migratory fish are harvested.The ‘model expansion’ approach:  If it is considered unsafe to assume that food consumption rates obtained while outside the system (by migratory species) will remain constant in the future, then Ecosim must be provided information on possible changes in food organism populations in those outside areas.  That is, the outside areas must be ‘internalized’ as part of the modelled system, by adding functional groups representing the outside food web structure.  Often, adding such groups may simply mean replicating the initial Ecopath group structure, with the second set of groups labelled ‘outside species X’ and with diet matrix entries set so that the added groups feed on one another but not on the ‘inside’ groups. 
    1212 
    1313A good modelling tactic is to try both approaches and see whether they give different answers.  However, note that the first approach can lead to misleading answers upon entry to Ecospace, if the Ecospace mapped area includes the ‘outside’ system: in that case, the model will continue to ‘import’ part of the diet and food consumption of migratory species.  Thus when the model development plan includes use of Ecospace to represent a larger spatial system, the functional group organization for that larger system needs to be included in the initial Ecopath/Ecosim model definition (approach 2).