Browsing by Subject "Abalone"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Restricted
    Abalone spatial- and age-structured assessment model preliminary results for Zones A, B, C and D in 2008
    (2008) Plagányi, Éva E
    A summary is presented of the results obtained from the 2008 Reference-case model that was fit to Zones A, B, CNP, CP and D in combination (hereafter referred to as the “combined ABCD model”). The full details of the spatial- and age-structured production model (ASPM) are provided in previous documents (Plagányi and Butterworth 2007). The 2008 base-case model uses an updated CPUPE index, and differs from last year’s base-case in estimating one more parameter for Zone A (the historic catch multiplier). The results of an “Alternative Policing Efficiency index” model version are also presented, based on the “Pedro case” series as described in document SWG-AB17. Model base-case results estimate a pristine spawning biomass, B sp 0 (in tonnes), of 8185, 5735, 6765 and 9064 for Zones A, B, C and D respectively. The current (inshore+offshore) spawning biomasses of abalone in Zones A, B and D are estimated at ca. 33 %, 35 % and 13 % respectively of their pre-exploitation levels. The “nonpoached” CNP and “poached” CP areas of Zone C are estimated at ca. 5 % and 4 % respectively with the inshore region particularly depleted: the model predicts zero remaining abalone in the inshore CNP, CP and Zone D areas. Equivalent est8imates for Zones A and B are 16% and 32%. Natural mortality is reasonably estimated (e.g. 0.33 yr-1 for age 0 and 0.14 yr-1 for age 15+) and in Zones C and D, the additional mortality estimated for 0-yr old abalone (due to the ecosystem-change effect) corresponds to near zero current annual survival rates. Poaching is severely impacting the resource, with Zone A particularly impacted in recent years. The combined Zones A-D model-predicted 2008 poaching estimate is 580 MT and corresponds to the assumption that, on average, 23% of all poached abalone are confiscated.
  • Thumbnail Image
    Item
    Open Access
    Abalone spatial- and age-structured assessment model results for Zones A, B, C and D in 2005
    (2005) Plagányi, Éva E; Butterworth, Doug S
    A summary is presented of the results obtained from the 2005 Reference-case model that was fit to Zones A, B, CNP, CP and D in combination (hereafter referred to as the “combined ABCD model”). The full details of the spatial- and age-structured production model (ASPM) are provided in Appendix 1. Model results estimate a pristine spawning biomass, Bsp 0 (in tonnes), of 11930, 6190, 6890 and 7900 for Zones A, B, C and D respectively. The current spawning biomasses of abalone in Zones A, B and D are estimated at ca. 38 %, 40 % and 28 % respectively of their preexploitation levels. The “nonpoached” CNP and “poached” CP areas of Zone C are estimated at ca. 23 % and 10 % respectively with the inshore region particularly depleted: the model predicts zero remaining abalone in the inshore CP area. Natural mortality is reasonably estimated and in Zones C and D, the additional mortality estimated for 0-yr old abalone (due to the ecosystem-change effect) corresponds to near zero current annual survival rates. Poaching is severely impacting the resource, with Zone A particularly impacted in recent years. The combined Zones A-D model-predicted 2005 poaching estimate of 1150 MT (corresponding to the assumption that, on average, 19% of all poached abalone are confiscated) is more than six times the legal 2005 commercial TAC for these zones.
  • Thumbnail Image
    Item
    Open Access
    Effects of protection and environmental factors on rock lobster, abalone, and giant periwinkle abundances in the Table Mountain National Park Marine Protected Area
    (2022) Gardner, Kathryn; Attwood, Colin; Kock, Alison
    The Table Mountain National Park Marine Protected Area (TMNP-MPA) was established in 2004 to restore and protect the Cape Peninsula's biodiversity, including the populations of exploited macro-invertebrate species. Scientific publications on MPAs support the idea that biodiversity protection is well served by MPAs, particularly no-take MPAs. Focusing on west coast rock lobster, abalone and giant periwinkle, historically the three most important invertebrate resources of the region, this study assesses the effectiveness of spatial regulations in the TMNP-MPA. Three comprehensive peninsula-wide SCUBA surveys were conducted in 2000, 2003 and 2019. Patterns in the abundance of the three species were analyzed using generalized-linear models (GLM) to explore the effects of the TMNP-MPA and no-take zones within, whilst accounting for habitat, depth, species interactions, and the physical protection offered by False Bay. Overall, the rock lobster abundance around the Cape Peninsula decreased since the implementation of the TMNP-MPA, however, to a lesser degree than the dramatic overall decline of the rock lobster population in South Africa over this time. This decrease is possibly due to increased illegal harvesting of adults and juveniles and increased predation from recovering fish populations within TMNP-MPA. However, rock lobster abundance did respond positively to the length of protection in no-take zones, suggesting that the enforcement of no-take zones within the MPA was effective. The beneficial effects of the no-take zones are likely insufficient to compensate for increased harvesting in adjacent areas within the MPA but prevented a greater decline in rock lobster. No change in abalone abundance was detected over the years in the TMNP-MPA or within no-take protected zones. The regional abalone population has declined drastically over a similar period due to poaching. Effective policing has likely prevented the abalone within the MPA from following suit. Giant periwinkle abundance increased since the establishment of TMNP-MPA. However, it is unknown if this pattern reflects the regional giant periwinkle population, so it cannot be determined if the increase is a direct result of the MPA. However, the giant periwinkle abundance had not benefited from the no-take zones, suggesting general harvest restrictions in TMNP-MPA are affording them sufficient protection, or that the greater population is increasing. A combination of environmental factors, such as warmer water temperatures and reduced nutrients, could explain the decreased abundance of rock lobster in False Bay, compared to those along the Peninsula's west coast. Predation by rock lobster could explain the opposite pattern found in both the giant periwinkle and abalone that were more common in False Bay. Expected abundance patterns based on the habitat preferences of each species, associated with water depth, the presence of sand, and kelp and urchin density were found. The creation of the TMNP-MPA, with its many and varied no-take and harvest zones provided an excellent opportunity to test the effectiveness of no-take spatial protection in a complex marine community and in the vicinity of a heavily exploited coastal zone near a metropolitan setting. This is especially valuable for macro-invertebrates, which are not commonly the focus of MPA studies in South Africa. This study supports the use of MPAs as a protection measure for overexploited macro-invertebrates. The extent of the success of this protection is evident by way of comparison with the magnitude of declines elsewhere. As both rock lobster and abalone fisheries are heavily plagued by poaching, the impact of no-take zones and the enhanced policing brought by the TMNP-MPA as a whole, likely helped to buffer the declines of rock lobster and abalone, respectively.
  • Thumbnail Image
    Item
    Restricted
    Illegal and unreported fishing on abalone—Quantifying the extent using a fully integrated assessment model
    (Elsevier, 2011) Plagányi, Éva E; Butterworth, Doug S; Burgener, Markus
    Illegal, unregulated and unreported (IUU) fishing is a major problem in many of the world's fisheries. The stocks most severely impacted centre on those characterised by high economic value, such as abalone, as well as long lived and slow growing species such as Patagonian toothfish (Dissostichus eleginoides). Effective management of these stocks as well as assessment of the impacts of IUU fishing on the resources is impeded by the technical difficulties associated with determining the magnitude of the IUU catches. The South African abalone Haliotis midae fishery rates as an extreme example of extraordinarily high levels of illegal and unreported (IU) catch. To assess the level and trends in IU catches, we used a combination of approaches that included collation of confiscation records from law enforcement, development of a novel index (the confiscations per unit policing effort—CPUPE), estimation of illegal catches using a spatial and age-structured assessment model, and cross-checking of model outputs through comparison with trade data on abalone imports in destination countries. The model-predicted 2008 IU estimate was 860 tonnes, more than 10 times the total allowable catch (TAC) for that year, and implied that, on average, 14% of all IU catches are confiscated. Associated management responses included the listing of H. midae on Appendix III of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), and a temporary closure of the commercial fishery. We summarise both technical and management lessons to be learnt from this integrated approach to assess and verify the magnitude of IU fishing.
  • Thumbnail Image
    Item
    Open Access
    Reference-case 2008 assessment model for abalone in Zones A, B, C and D
    (2008) Plagányi, Éva E
    A summary is presented of the results obtained from the 2008 Reference-case model and two variants that was fit to Zones A, B, CNP, CP and D in combination (hereafter referred to as the “combined ABCD model”). The full details of the spatial- and age-structured production model (ASPM) are provided in Appendices 1 and 2. The 2008 base-case model uses an updated CPUPE index, and differs from last year’s base-case in estimating one more parameter for Zone A (the historic catch multiplier). The three model versions presented in Table 1 are as follows: Model a) Concern was expressed by the AWG that the CPUPE trend for Zone B declines too steeply in recent years. This may be attributable to an incorrect partitioning of confiscated abalone between Zones A and B. Rather than estimating the amount poached in Zone B in recent years, this model combines the estimates of the amount poached from Zones A and B and then estimates a parameter that describes the proportion of this total that is taken from Zone A versus Zone B from 2000 onwards. The model estimated proportion poached from Zone A is 0.65 [90% Hessian-based confidence interval 0.55 – 0.75. This model version has been refined slightly from a preliminary version presented to the AWG in that parameter estimates for Zone A now result in an improved fit. Model b) The old Reference case model is similar but estimates the Zone B poaching amount (in numbers), that is assume to apply from 2005-2008. Model c) This model version used a single compartment per zone, rather than assuming inshore and offshore model regions. This model had the worst AIC value. The new Reference Case Model estimates a pristine spawning biomass, (in tonnes) with 90% Hessian-based confidence intervals shown in square brackets, of 9760 [6060; 13460], 5840 [5400; 6280], 7290 [7050; 7530] and 9650 [6440; 12850] for Zones A, B, C and D respectively. The current (inshore+offshore) spawning biomasses (and associated 90% confidence intervals) of abalone in Zones A, B, C and D are estimated at ca. 33 % [26%; 39%], 25 % [19%; 30%], 4% [1%; 6%] and 12 % [8%; 16%] respectively of their preexploitation levels. The “nonpoached” CNP and “poached” CP areas of Zone C are estimated at ca. 6 % and 3 % respectively with the inshore region particularly depleted: the model predicts zero remaining abalone in the inshore CNP, CP and Zone D areas. Equivalent estimates for Zones A and B are 21% and 22%. Natural mortality is reasonably estimated (e.g. 0.33 yr-1 for age 0 and 0.14 yr-1 for age 15+) and in Zones C and D, the additional Bsp 0 mortality estimated for 0-yr old abalone (due to the ecosystem-change effect) corresponds to near zero current annual survival rates. Poaching is severely impacting the resource, with Zone A particularly impacted in recent years. The combined Zones A-D model-predicted 2008 poaching estimate is 860 MT and corresponds to the assumption that, on average, 14% of all poached abalone are confiscated.