Browsing by Subject "Age-Structured Production Model"
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- ItemRestrictedThe 2005 Age-Structured Production Model assessments and constant catch projections for the south coast rock lobster resource(2005) Johnston, Susan J; Butterworth, Doug SThe 2005 assessment was routinely extended, taking account of a further year’s catch, CPUE and catch-at-age data. CPUE shows a continuation of the increase that commenced in 1998. However, sustainable yield estimates are generally less than those for the 2004 assessment, although estimates of current biomass levels relative to K increase. The Reference Case (RC) scenario suggests that a TAC of about 360 MT or less would be appropriate to prevent biomass decline in the future. Other scenarios suggest either higher or lower values than this. If the catch-at-age data are down-weighted, then this 360 MT level for the TAC is increased to 390 MT. On the other hand, the scenario which assumes the 1996+ recruitment to be equal to the average of the previous 10 years is more pessimistic and suggests an appropriate TAC level of only some 300 MT or less to prevent biomass decline. A model (Model 2) which allows for time-varying selectivity is presented and shows promising results. Model 2 is better able to reproduce the recent CPUE trend. Preliminary results for a model that fits to catch-at-length rather than catch-at-age data, using a selectivity-at-length rather than selectivity-at-age function, are presented, but these do not as yet reflect satisfactory fits to the data so that they should not be considered reliable in the context of stock status estimates. These different scenarios reflect very different interpretations of the recent increase in CPUE for the resource. If the catch-at-age data are down-weighted, the model fit essentially ignores them and suggests a recent increase in abundance. However, under either the effort saturation or the time-varying selectivity approaches, spawning biomass is estimated to have decreased further over recent years.
- ItemRestrictedThe 2007 Age-Structured Production Model assessments and projections for the South Coast rock lobster resource-routine update using model fitting to catch-at-age data(2007) Johnston, Susan J; Butterworth, Doug SThe assessment conducted in 2006 (WG/06/06/WCRL3) has been routinely extended, taking account of a further year’s catch, CPUE and catch-at-age data. The observed CPUE shows a slight decrease for 2005 (2005/06 season). The sustainable yield estimates are generally very similar to those for the 2006 assessment, although estimates of current biomass levels relative to K increase. The Reference Case (RC) scenario suggests that a TAC of about 330 MT or less would be appropriate to prevent biomass decline in the future. The other two scenarios reported suggest higher values than this. If the catch-at-age data are down-weighted, then this 360 MT level for the TAC is increased to 390 MT.
- ItemRestrictedThe Age-Structured Production Model for the south coast rock lobster population(2005) Johnston, Susan J; Butterworth, Doug SThe south coast rock lobster resource is modelled using an age-structured-productionmodel (ASPM).
- ItemRestrictedThe Age-Structured Production Model for the South Coast rock lobster population extended to be sex- and area-specific and to fit to catch-at-length data(2007) Johnston, Susan J; Butterworth, Doug SThe south coast rock lobster resource is modelled using an age-structured-productionmodel (ASPM) which fits to catch-at-length data directly. The model is sexdisaggregated (m/f) and area-disaggregated (A=1,2,3).
- ItemRestrictedThe Age-Structured Production Model for the South Coast rock lobster population extended to be sex- and area-specific, to fit to catch-at-length data, and to use Pope's approximation(2008) Johnston, Susan J; Butterworth, Doug SThe south coast rock lobster resource is modelled using an age-structured-productionmodel (ASPM) which fits to catch-at-length data directly. The model is sexdisaggregated (m/f) and area-disaggregated (A=1,2,3). Population equations have been modified from Baranov equations to Pope’s approximation. This has reduced the number of estimable parameters, and speeded up runtime of the program. Note that the model now includes an option for annual variability in the proportion of recruitment (age 0 lobsters) to each area each year. Though formally there is not interArea movement after this recruitment, in effect this means that there is allowance for such movement, but only for ages less than those which the fishing exploits.
- ItemRestrictedPreliminary results of the Sex- and Area-specific Age-Structured Production Model for the South Coast rock lobster population(2008) Johnston, Susan J; Butterworth, Doug SPreliminary results of the sex- and area-specific age-structured production model for the South Coast rock lobster, as described in Johnston and Butterworth (2008) are presented here. The model explored to date incorporating time-varying selectivity with estimation in terms of the “MARAM” method (as reported in Johnston and Butterworth 2007). Future results will explore alternate methods.