Browsing by Author "Johnston, S J"
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- ItemOpen AccessThe 2017 Horse Mackerel Updated Assessment(2017-09) Johnston, S J; Butterworth, Doug SFISHERIES/2016/SEP/SWG-DM/51 provided a full description of the current stock assessment model for the South African Horse Mackerel. FISHERIES/2016/SEP/SWG-DM/52 reported updated 2016 assessments for a number of model variants where the assessments were extended to allow for better fits to the low CPUE values (2014-2015). This document here provides updated 2017 assessment results for a number of model variants which follow on from the variants that were explored in 2016 and take the data from 2016 now available into account.
- ItemOpen Access2017 updated South Coast Rock Lobster assessment results(2017-08) Johnston, S J; Butterworth, Doug SThe 2017 assessment of the resource is updated given two further years of 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 is steady recent years. Current spawning biomass is estimated to be 32% of K.
- ItemOpen AccessInitial OMP candidates for the Nightingale rock lobster fishery(2017-07) Johnston, S J; Butterworth, Doug SThe performances of a number of OMP candidates are reported here. Simulation results do not vary much amongst the various candidates. For this and other reasons it is recommended that at this stage CMP3 be adopted as the new OMP. This CMP has a TAC Ceiling of 85 MT. Very little further resource protection (which is in any case already certainly adequate) is provided by a CMP with a lower TAC ceiling. All the CMPs have appropriate feedback, so that if the resource declines in the future, so will the future TAC. An added precautionary feature defines an Ilim catch rate below which TAC can decline faster than the baseline constraint under the occurrence of “Exceptional Circumstances” is also build into the CMPs.
- ItemOpen AccessInshore versus offshore split of available quota(2017-01) Johnston, S J; Butterworth, Doug SAlthough each of the five super-areas used in the management of the West Coast rock lobster fishery is treated as homogenous in assessments, it is clear that there are inshore areas where predominantly hoops and bakkies are deployed, and offshore areas where the larger trap boats operate in deep waters, and further that resource dynamics may differ in these two regions.
- ItemOpen AccessRECOMMENDATIONS ON ROCK LOBSTER TACs FOR THE TRISTAN GROUP OF ISLANDS FOR THE 2017/18 SEASON(2017-08) Butterworth, Doug S; Johnston, S JOMPs have recently been accepted as the basis to recommend TACs for Tristan, Inaccessible and Gough islands. Given that for Inaccessible and Gough the recent catch rates continue to be above the associated OMP’s target catch rate, the OMPs indicate increases to the TACs for both islands for the 2016 season: Inaccessible: 81 MT to 85 MT Gough: 110 MT to 116 MT The updated OMP for Tristan sets the TAC for 2017 at 120 MT. The newly developed OMP for Nightingale sets the TAC at either 75 MT or 78.75 MT depending on which final value for the TACceiling is selected for the OMP (either 75 MT or 85 MT).
- ItemOpen AccessUpdated (and rescaled) Tristan GLM-standardised lobster CPUE to take account of data for the 2016 season(2017-05) Johnston, S J; Butterworth, Doug SThe Powerboat CPUE series for Tristan was last updated in 2016 (MARAM/Tristan/2016/MAY/08) and took data for the 1994-20141 seasons into account. This GLM incorporated a method whereby the full GLM for Tristan can be rescaled to take into account the overall fishing efficiency changes informed by the data on fishermen’s names (available for only 2005-2007 and 2013-2016 seasons). This document updates this GLM analysis by incorporating data from the most recent 2016 season. A continued improvement in the 2016 CPUE is evident.
- ItemOpen AccessUpdated 2017 GLMM-standardised lobster CPUE from the Tristan da Cunha outer group of islands(2017-05) Johnston, S J; Brandão, A; Butterworth, Doug SThe longline CPUE series for Inaccessible and Gough islands are GLMM standardised through to 20161. For Nightingale, the fishery was closed for the 2011 season and catches were set at precautionary levels for the 2011 and 2012 seasons. The Nightingale GLMM model thus excludes 2011 and 2012, although it now includes 2013-2016 for this island. Year, month, area, trap-type, soak time, depth and year-area interactions are treated as fixed effects, and year-month interactions treated as a random effect. The standardised CPUEs for Inaccessible and Nightingale continue to be optimistic, and both show increases since the previous season. For Gough, the standardised CPUE value for the current season is lower than that of the previous season, but still well above the OMP target level Itar.
- ItemOpen AccessUpdated 2017 Nightingale island rock lobster assessment(2017-05) Johnston, S J; Butterworth, Doug SThis paper provides a further updated assessment of the rock lobster resource at Nightingale island. This assessment includes updated data from both the commercial fishery and the biomass surveys. The recent (2013-2016) high GLM standardised CPUE values (and biomass survey index values) at the island which were not initially anticipated, suggest that the negative impact of the OLIVA incident on adults may have been overestimated previously. The recent high CPUE indicates that the adult mortality in 2011 due to the OLIVA incident was much less than originally assumed. The 2017 RC assessment thus now assumes zero adult mortality in 2011 due to the OLIVA incident, but continues to assume an 80% juvenile mortality due to this incident in 2011. Projections suggest that the resource could readily sustain future catches kept constant at 75 MT or somewhat higher, though there may be a brief downturn in catch rates in the near future if the OLIVA incident led to a high mortality of juveniles at that time. An OMP is to be developed for this fishery to be used in setting the TAC for the next 2017/18 season.
- ItemOpen AccessUsing decision analysis to evaluate candidate OMPs for the South African west coast rock lobster fishery(Academy of Science of South Africa, 2002) Moloney, C L; Johnston, S JSimulation models frequently are used to provide information to assist in decision-making in natural resource management. The sheer volume of information to process can be overwhelming, however, and there is a danger that some management objectives are not adequately considered, because of the difficulties of making decisions when there are multiple, conflicting objectives. Formal decision-analysis methods have been used in conservation biology to choose among simulation model results. This paper suggests that they can also be used in South African fisheries management, where simulation models are used extensively in developing operational management procedures.
- ItemOpen AccessWest coast rock lobster projections to 2021 under the current TAC 2016 levels(2017-01) Johnston, S J; Butterworth, Doug SThe SWG TAC recommendations for the next five years corresponded to reducing the previous recovery target by 2021 compared to 2006 from a 35% increase down to about a 7% decrease in median terms. The final DAFF decision was to leave the TAC for 2016 unchanged. If this current DAFF TAC is maintained for the next five years, a drop to 85% of the 2006 abundance by 2021 in median terms is indicated, with this decrease being to 35% at the lower 5%-ile. Approximate projections concerning Exceptional Circumstances suggest that A56 and A7 will remain above their EC thresholds, but that A34 and A8+ will exceed them by marginal amounts only, and A12 will quickly drop below the EC threshold for that super-area.