OpenUCT :: Browsing by Author "Johnston, Susan"

Browsing by Author "Johnston, Susan"

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Open Access
2018 GLMM-standardised Tristan lobster CPUE compared with the current OMP target levels (Itar)
(2018-05) Johnston, Susan; Butterworth, Doug S
This document reports the 2018 GLMM-standardised Tristan lobster CPUE compared with the current OMP target levels that are in place for each island.
Open Access
The 2018 south coast rock lobster reference case operating model
(2019) Johnston, Susan; Butterworth, Doug S
This document details the current 2018 Reference Case Operating Model which is to be proposed to be used for simulation testing of a new OMP for South Coast rock lobster.
Open Access
2018 Updated horse mackerel assessments
(2018-10) Johnston, Susan; Butterworth, Doug S
The assessments for the two hypotheses (reduced catchability and a one-off additional mortality event) for the recent reduced Desert Diamond catch rate are updated to take one further year’s data into account. The 2017 CPUE somewhat increased above its 2016 value, and now almost returned to the values typical before the 2014 low. If the same basis as in the recent past is used to make management recommendations with an unchanged effort limitation, the total allowable midwater catch would be set at 23 851 MT. However the assessment results, given this data update, are suggestive of the catchability hypothesis now being more plausible than the additional mortality hypothesis. This in turn suggests that consideration should be given to some increase in the cap on effort, which would lead to a concomitant increase in the midwater TAC.
Open Access
2018 updated south coast rock lobster assessment results
(2019) Johnston, Susan; Butterworth, Doug S
The 2017 assessment of the resource for South Coast rock lobster is updated given the two further years of catch-at-length data and one further year of CPUE data now available. Recruitment is estimated to have decreased over the last two seasons for which this is now estimable (2007 and 2008). The spawning biomass trajectory decreases slightly over recent years. Current spawning biomass is estimated to be 29% of K. The Reference Case (RC) MSY estimate is 358 MT at a Bsp/K of 0.29. Fmsy is estimated to be 0.29, though MSY-related estimates are heavily dependent on assumptions made about the stock-recruitment relationship. The RC model estimates that the resource is currently at Bmsy.
Open Access
2018 updated south coast rock lobster assessment results
(2018) Johnston, Susan; Butterworth, Doug S
The 2017 assessment of the resource is updated given two further years of catch-at-length data now available. Recruitment is estimated to have increased over the last two seasons compared to the previous three seasons when recruitment was estimated to be poor. The spawning biomass trajectory decreases slightly over recent years. Current spawning biomass is estimated to be 27% of K. The RC MSY estimate is 259 MT at a Bsp/K of 0.27. Fmsy is estimated to be 0.112. The RC model thus estimates that the resource is currently slightly above Bmsy.
Open Access
2018 updated west coast rock lobster assessments
(2018-11) Johnston, Susan; Butterworth, Doug S
A new set of 2018 assessments for all five super-areas for the west coast rock lobster resource has recently been completed. These provide the bases for projecting the resource forwards (in each super-area) under alternative future catches to in turn provide a basis for management advice, as is reported in MARAM/IWS/2018/WCRL/P4. Limitations of the analysis are also discussed.
Open Access
Addendum to: Further 2018 west coast rock lobster assessment and associated biomass projections results
(2018-08) Johnston, Susan; Butterworth, Doug S
This Addendum provides further results for the two-step TAC phase down option, where the distribution of the TAC amongst the Super-areas is changed. The shifts from A8+ are to either or both of A34 and A7. The Results are shown in the Tables and Figures below, and correspond to the “7% recovery” option.
Open Access
Adjustments made to recent catches and CPUE for West Coast Rock Lobster in Areas 3+4 and 5+6
(2018-08) Butterworth, Doug S; Glazer, Jean; Johnston, Susan; van Zyl, Daniel
During the 2017/18 fishing season, some catches made in Area 3+4 by trapboats were landed in Area 5+6 as hoopnet catches. This document reports the methodology used and the results obtained from an attempt to correct the catches reported for these super-areas to adjust for this malpractice.
Open Access
Age-Structured Production Model (ASPM) assessments of the Alfonsino (Beryx splendens) resource in the SIOFA area of the Southern Indian Ocean
(2020) Brandão, Anabela; Butterworth, Doug; Johnston, Susan
An Age-Structured Production Model is applied to assess the Alfonsino (Beryx splendens) resource in the West and East SIOFA areas of the Southern Indian Ocean. Data limitations restrict these applications to deterministic variants, which assume no variation in annual recruitment about the predictions from a stock-recruitment relationship. The models are fitted to the CPUE series and single year of commercial catch length distribution information available. Both West and East stocks are estimated to be at about 60% of their pre-exploitation spawning stock biomass levels, and well above the levels corresponding to MSY (MSYL). These results are insensitive to all sensitivities explored, except for changes in the value assumed for natural mortality (M). For the Base case assumption of M=0.2, projections under constant annual catches up to 40% above the 2018 levels remain above MSYL for the next two decades. However, if a less productive situation is assumed (M=0.15), there are some cases of spawning biomass dropping below MSYL within 10 years for both areas, including even for continuation of the 2018 catch for the East area.
Open Access
Application of a minor variant of OMP-2014 to the updated base case operating model for the SCRL resource
(2019) Johnston, Susan; Butterworth, Doug S
Projected TAC outputs for OMP-2014 at that time are compared to those that subsequently eventuated. A minor variant of OMP-2014 is applied to the adjusted base case SCRL operating model for a number of values for the central parameter CPUE_targ.
Open Access
Biomass Survey results for Nightingale which include the recent February 2016 Leg 2 survey results
(2016-04) Johnston, Susan
At the end of 2015, following indications in the recent biomass survey CAL data, provided by the September 2015 survey, that there may be a reduction in the 50-65mm size classes, concern was raised that this could be a “post-OLIVA” effect moving through the size classes and becoming evident only now due to the time lag between the OLIVA incident and possible resultant juvenile mortality. It was decided that although the stock assessments take only Leg 1 biomass survey data into account – and that the Leg 2 survey are no longer performed – that a once off Leg 2 biomass survey at Nightingale would be useful to examine this matter.
Open Access
Comparison between methods of estimating future recruitment for west coast rock lobster projections for super-area A8+
(2019) Johnston, Susan; Butterworth, Doug S
Initial applications of alternative approaches to forward projection of the rock lobster abundance in super-area A8+, based on suggestions made at the 2018 international workshop (IWS), are presented.
Open Access
Comparison between methods of estimating historical and future recruitment for the west coast rock lobster super-areas
(2019) Johnston, Susan; Butterworth, Doug S
This document introduces a new approach for estimating historical recruitment, and consequently projecting future recruitment, for west coast rock lobster, which takes account of the recommendations of International Panel at the 2018 stock assessment workshop (IWS). The results are compared to those from the previous approach for each super-area. Results of the new approach are sensitive to the weighting given to the residuals about an average over time in the new method, and a proposal for a common choice to address this is made.
Open Access
The consequences of different inshore/offshore splits of the WCRL TAC
(2018-11) Johnston, Susan; Butterworth, Doug S
Coarse estimates are provided of relative inshore (<30m depth) and offshore (30m+ depth) biomasses for each of the five super-areas using two methods – one being based on rough estimates provided by van Zyl of the hoop/trap catchability ratio, and the other on using the FIMS relative inshore/density results. Results indicate that the proportion of the biomass inshore in A34 is between 28-42% and between 5-25% for A8+. Constant catch projections for the inshore A34 and A8+ areas are also provided for a range (10%-40% or 10%-30% respectively) of assumptions regarding the inshore biomass as % of total biomass, and indicate that increases in the current inshore allocations in these two super-areas will lead to a rapid and substantial decrease in inshore abundance.
Restricted
CTRM Program Redevelopment: Interim Summary Report
(2010) Furman, Liam; Johnston, Susan
Open Access
The final OMP (2017) recommended for Nightingale.
(2018-01) Johnston, Susan; Butterworth, Doug S
This document provides the final specifications of the OMP recommended for Nightingale island.
Open Access
Final updated 2019 assessments and projections of west coast rock lobster for different poaching scenarios
(2019) Johnston, Susan; Butterworth, Doug S
The 2018 assessments and projections for the five super-areas for the west coast rock lobster resource are updated to 2019, using basically the same approach as in 2018. The update takes account of a further year’s catch and resource monitoring data, uses improved recruitment and projection methodologies, and incorporates major revisions of estimates of past poaching levels following extensive further examination of the associated data. There is clear evidence that quantity of lobsters poached has dropped from the 2017/18 to the 2018/19 season. Projections are developed on an identical basis to that used in 2018, with the results for three options shown for two different assumptions about the historical poaching trend. These are that future legal catches are set to zero, that they are set so that the abundance of the whole resource is equal (in median terms) to that in the 2006 benchmark year by the time the 2025 target year is reached, and hallway between these two options (in 2018 the WCRL SWG’s TAC recommendations were based on the third of these three options), The key projection results are listed in Table 1, with graphical representations provided in Figure 1. These reflect an improvement compared to the situation a year previously, with very little difference for the two historical poaching scenarios considered
Open Access
Further 2018 west coast rock lobster assessment and associated biomass projections results
(2018-08) Johnston, Susan; Butterworth, Doug S
This document provides results requested at the WCRL SWG meeting on Friday 24 August. First negative log-likelihoods are provided for the base case and two sensitivity poaching scenario assessment model fits. Then results are provided for a two-step TAC reduction programme for the BC poaching scenario, ranging from zero TAC (maximum recovery) projection to a scenario where the resource is at its 2006 target level by 2030 (sustainable but no recovery). Appendix 1 provides summary results for similar one- and three-step TAC reduction programmes.
Open Access
Further CMP results for the SCRL resource
(2019) Johnston, Susan; Butterworth, Doug S
Results are reported for three further CMPs for the south coast rock lobster. Simulations now take the 2017 catches and CPUE values into account.
Open Access
Hake Data: problems, solutions and GLM CPUE sensitivity to alternate scenarios
(2009) Fairweather, Tracey; Glazer, Jean; Leslie, Rob; de Decker, Michelle; Johnston, Susan; Butterworth, Doug S
The checks carried out on the hake offshore trawl catch and effort data, which provide the basis to split the catches by species and to obtain GLM-standardised CPUE indices of abundance by species, have focused on two separate steps. The first was a re-extract of the data from the original files to check for possible errors in the earlier extracts (this applies to the subset of the data for which size composition information can be unambiguously extracted). The second step was to increase the proportion of the data utilised (and at the same time to check for possible selection biases in the existing sample) by assigning size composition to drags for which this information was not directly available, on the basis of the average for other vessels for which this was available for the area and time at which the fishing concerned took place. The first step revealed a problem with extractions over the last few years which at times for certain vessels had linked the accumulated catch for the day with only the effort for the last trawl of the day, rather than for all trawls that day combined.