Browsing by Author "Johnston, Susan J"
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- ItemRestrictedThe 2004 age-structured production model assessments and projections for the South Coast rock lobster resource(2004) Johnston, Susan J; Butterworth, Doug SThe assessment conducted in 2003 has been 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. Results are generally more optimistic than those for the 2003 assessment; this is shown to be a result of both the new abundance data now available as well as the revision of the historic catch series, with the former having slightly the larger effect. The Reference Case (RC) scenario suggests that a TAC of 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 appropriate level for the TAC is increased to 390 MT. On the other hand, the scenario which assumes the 1995+ 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
- 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.
- ItemOpen AccessThe 2006 age-structured production model assessments and projections for the South Coast rock lobster resource-routine update(2006) Johnston, Susan J; Butterworth, Doug SThe assessment conducted in 2006 has been 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. The sustainable yield estimates are generally very similar to those for the 2005 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.
- 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 2007 age-structured production model assessments and projections for the South Coast rock lobster resource-routine update using Pope's approximation model fitting to catch-at-age data including scenarios for time-varying selectivity(2007) Johnston, Susan J; Butterworth, Doug SThe assessment conducted in 2006 (WG/06/06/WCRL3) has been routinely extended (except that the Baranov equation has been replaced by Pope’s approximation), 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 a little less than 330 MT or less would be appropriate to prevent biomass decline in the future. The other four scenarios reported suggest higher values than this, ranging from 350 MT to 405 MT. Spawning biomass trends over the last 10 years are downward for all the models considered.
- ItemOpen Access2009 updated stock assessments for West Coast rock lobster(2009) Johnston, Susan J; Butterworth, Doug SThe west coast rock lobster assessment of 20061 based on data to 2004 is updated to include data up to 2008. Over the last four years the exploitable biomass trend is upwards for Areas 7 and 8 and the resource as a whole, but downwards for Areas 5+6 and almost level for Areas 1+2 and 3+4. The overall increase since 2006 is significant at the 5% level. While better than median projections at the time the current OMP developed, the increase remains within the 95% probability intervals calculated at the time.
- ItemOpen Access2010 Estimates of sustainable rock lobster yield for the four islands of the Tristan da Cunha group(2010) Johnston, Susan J; Butterworth, Doug SA simple replacement yield model is fitted to CPUE and annual catch data from 1997 for the four islands of the Tristan da Cunha group. The results suggest that the current TAC be maintained, with a slight increase (perhaps 5 tons) to the allocation to Gough, and the same decrease to the allocation to Tristan.
- ItemOpen Access2012 South coast rock lobster operating models-some initial results(2012) Johnston, Susan J; Butterworth, Doug SInitial updated assessments are developed for the South Coast rock lobster resource incorporating the recently revised estimates of somatic growth rate by area developed by OLRAC, and fitting to area-specific CPUE and scientific catch-at-length data. The initial focus is on a model structure which splits the resource into areas A1E, A1W and A2+3. A problem arises because the slow growth rate estimate for A1E results in an unrealistically high estimate of preexploitation biomass, and some approaches to circumvent this are explored. The pack-category-based catch-at-length data are not consistent with these results and it is suggested that they be excluded from future model fits. Allowing selectivity at length to vary over time makes little difference to stock trajectory estimates, but scarcely improves non-random patterns in the catchat-length residuals, so that the investigation of alternative functional forms for this variation is suggested. An alternative model structure based on areas A1, A2 and A3 yields an estimate of the current depletion of the spawning biomass which is appreciably less than in previous assessments which were based on this structure; the reasons for this need to be identified.
- ItemOpen Access2015 updated South Coast Rock Lobster assessment results(2015) Johnston, Susan J; Butterworth, Doug SThe 2013 assessment of the resource is updated given three further years of data now available. Recruitment is estimated to have been poor over the further three seasons for which this can now be estimated. The spawning biomass trajectory has flattened after a preceding increase, and is now showing a slight downward trend.
- ItemOpen AccessThe 2016 Horse Mackerel Assessment Model(2016-09) Johnston, Susan J; Butterworth, Doug SFurman (2014) developed and described the horse mackerel assessment model for his MSc thesis. This document extracts pertinent sections from his thesis which describe the population assessment model, and provides a description of assumptions made for projecting the resource into the future. The data, as updated in 2016, are also provided.
- ItemOpen Access2016 Horse Mackerel projections(2016-10) Johnston, Susan J; Butterworth, Doug SThe projections conducted in 2015 are updated to include the further data now available. The results are somewhat more positive than previously as a result of the CPUE for 2015, although still low, being higher than that for 2014. Nevertheless, given the range of interpretations possible for these low CPUE data, the projections continue to point to the need for an effort restriction to be applied to the midwater trawl fishery.
- ItemOpen AccessThe 2016 Horse Mackerel Updated Assessment(2016-09) Johnston, Susan J; Butterworth, Doug SFISHERIES/2016/SEP/SWG-DM/51 provides a full description of the current stock assessment model for the South African Horse Mackerel. This document provides the updated 2016 assessment results for a number of model variants.
- 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.
- ItemOpen AccessThe age-structured production modelling approach for assessment of the rock lobster resources at the Tristan da Cunha group of islands(2011) Johnston, Susan J; Butterworth, Doug SThe stock assessment approach for all four islands of the Tristan da Cunha group is to use an age-structured production model (ASPM) to fit to catch, longline standardised CPUE and catchat-length (CAL) data. The models consider only catches from 1990, i.e. models are initiated in 1990. The method for setting up the initial population age structure in 1990 is given below.
- ItemRestrictedAllowing for time-varying selectivity in South Coast rock lobster assessments(2007) Butterworth, Doug S; Johnston, Susan JThe Reference Case (RC) age-structured production model that has been used for past South Coast rock lobster assessments (ASWS/JUL07/SCRL/ASS/1) has assumed a time-invariant age-specific selectivity function. Previous work allowing the possibility of deviations from this assumption has used a model identical to the RC model, except that the age-specific selectivity function is allowed to vary over the time period for which catch-at-age data are available (1994 to yrecent).
- ItemOpen AccessAssessment of the west and east Australian breeding populations of Southern Hemisphere humpback whales using a model that allows for mixing on the feeding grounds and taking account of the most recent abundance estimates from JARPA(2005) Johnston, Susan J; Butterworth, Doug SThe most recent JARPA survey estimates of abundance are used to update the dynamic production model analyses of West and East Australian humpback breeding populations (Johnston and Butterworth 2002). These analyses take account of the results of both these surveys on the feeding grounds and of breeding area surveys, by applying a model that incorporates mixing of the two breeding populations on the feeding grounds of Areas IV and V. Results are quite precisely determined, with the available data providing a self-consistent picture of population recoveries well above their minima of the 1960’s. Best estimates are projected under continuing zero harvest, and show approaches to pristine levels in some 10 years for the western and 15-20 years for the more depleted eastern breeding population.
- ItemRestrictedAssessment results for humpback breeding stocks D, E1 and Oceania following recommendations from SC 65a(University of Cape Town, 2014) Ross-Gillespie, Andrea; Butterworth, Doug S; Johnston, Susan JFollowing recommendations made at IWC 65a, 2013, a single-stock BSD (Breeding Stock D, West Australia) model has been run for a range of Antarctic catch boundaries, and some two-stock BSE1 (Breeding Stock E1, East Australia)+BSO (Breeding Stock Oceania) models have been explored. The single-stock BSD model excluded the Hedley et al. (2011) absolute abundance estimate from the model fits, and instead utilised an uninformative uniform prior on the log of the target abundance estimate. The minimum value for this prior was based on calculations by Hedley of a minimum absolute abundance indicated by the 2005-2008 survey (Hedley et al. 2011). These changes markedly improve the fit to the BSD relative abundance series. The two-stock models considered consist of one model with fixed Antarctic boundaries that allowed for a proportion of each of the BSE1 and BSO stocks to feed in a common feeding ground between 170°E and 170°W, and a second model in which there was no overlap between the two stocks, but a range of different Antarctic catch boundaries have been explored. Results of these models showed that (a) the BSE1 growth rate remained virtually at 0.106 yr-1 (the demographic boundary imposed by the model), (b) fits to the BSE1 mark-recapture data were relatively poor and (c) the Nmin constraint remained problematic for BSO. Further two-stock runs, as well as a three-stock run, have not been included in this paper, but the authors aim to provide the results as an addendum to this paper at the meeting.
- ItemOpen AccessA Bayesian assessment of breeding stocks B, C and G of Southern Hemisphere humpback whales using a prior for growth rate from analyses for stocks D and E(2005) Johnston, Susan J; Butterworth, Doug SBayesian assessment of breeding stocks B, C and G of the Southern Hemisphere humpback whales are presented. These assessments incorporate a prior for the maximum growth rate r derived from similar assessments for breeding stocks D and E. This is the first time an assessment for breeding stock B has been attempted. Results show current abundance estimates for breeding stocks B and G are low (less than 0.15K), whilst the current abundance estimate for breeding stock C much further recovered at 0.79K. Projections under a zero continued harvesting strategy estimate breeding stock C to be fully recovered by 2020, whilst breeding stocks B and G will be fully recovered only by about around 2030.