Changes between Version 3 and Version 4 of EwEugDensityDependentChangesInCatchability


Ignore:
Timestamp:
2010-11-22 01:33:53 (9 years ago)
Author:
shermanl
Comment:

--

Legend:

Unmodified
Added
Removed
Modified
  • EwEugDensityDependentChangesInCatchability

    v3 v4  
    99 1. almost always, the actual area ''A'' occupied by fish and fishers decreases with decreasing fish abundance, (i.e., the occupied range 'collapses' as stock size decreases) due to fish behaviours like shoaling and also cumulative effects of localized stock depletion events. The second problem is particularly important for fisheries analysis, since it implies that ''q'' increases, sometimes grossly, as A decreases (''q'' = ''a / A'' increases when ''A'' decreases) whether or not there is a change in fishing technology as represented in the area swept per unit effort. 
    1010 
    11 On entry to Ecosim, Ecopath has provided a base fishing rate ''F,,o,, = ''Catch / (Ecopath biomass)'.  You can specify time scenarios for ''F'' (''F,,t,,, t'' > 0) relative to this F,,o,, by using either the fishing rate 'sketch pad' interface (Ecosim [[Run ecosim.htm|Run Ecosim]] form) or time reference data files (see [EwEugTimeSeries Time series]).  The Ecosim default is to treat your time series values as relative or absolute fishing rates without reference to changes in ''q''.  But you can also treat the time input values as relative fishing efforts ''f'',,''t'',, where ''f'',,''o'',,'' = F'',,''o'',, (efforts scaled so base ''q'',,o,, = 1).  Then to create density-dependent catchability effects, set a value greater than 1.0 for the ''q'',,''max'',,'' / q'',,''o'',, ratio(s) in the Ecosim [EwEugGroupInfo Group info] form. 
     11On entry to Ecosim, Ecopath has provided a base fishing rate ''F,,o,, = ''Catch / (Ecopath biomass)'.  You can specify time scenarios for ''F'' (''F,,t,,, t'' > 0) relative to this F,,o,, by using either the fishing rate 'sketch pad' interface (Ecosim [EwEugRunEcosim Run Ecosim] form) or time reference data files (see [EwEugTimeSeries Time series]).  The Ecosim default is to treat your time series values as relative or absolute fishing rates without reference to changes in ''q''.  But you can also treat the time input values as relative fishing efforts ''f'',,''t'',, where ''f'',,''o'',,'' = F'',,''o'',, (efforts scaled so base ''q'',,o,, = 1).  Then to create density-dependent catchability effects, set a value greater than 1.0 for the ''q'',,''max'',,'' / q'',,''o'',, ratio(s) in the Ecosim [EwEugGroupInfo Group info] form. 
    1212 
    1313For example, setting a value of 5 represents assuming that ''q'' can be as much as 5 times higher than the ''q'' that led to the Ecopath base ''F,,o,,'', if stock size is very low. That is, suppose you set ''q,,max,, / q,,o,,'' = 5, then run a scenario where you shape the relative fishing rate over time (now treated as relative effort) so as to cause a gross stock depletion followed by a return to ''f,,t,, = f,,o,,''. In this scenario, the 'realized' fishing rate ''F,,t,,'' can be as much as 5 times ''F'',,''o'',, even for ''f,,t ,,= f,,o,,'', due to density-dependent decrease in the effective area ''A'' occupied by the depleted stock.