- Title
- Abalone stock enhancement at Cape Recife, Eastern Cape Province of South Africa
- Creator
- Witte, Warren
- Date Issued
- 2017
- Date
- 2017
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10962/45695
- Identifier
- vital:25532
- Description
- The potential for restocking depleted natural reefs with hatchery abalone, Haliotis midae, to support a commercial ranching venture in the Eastern Cape Province of South Africa was investigated. A baseline stock assessment and analysis of historical fishery data provided compelling evidence of significant declines in the abalone stock, attributed to uncontrolled illegal fishing over the last 20 years at the study site. A ranching pilot study was undertaken where small-scale replicated seeding experiments were set up which quantified the effect of seed size, density and habitat on survival and growth of hatchery-reared abalone. Thirty-six experimental plots were seeded with 8400 abalone spread evenly over two broad habitats. Mean retrievals of seeded abalone were 17% ranging from 4 - 42% after a minimum of eight months at large. Release density and large-scale habitat had no significant effect on seed survival, but smaller seed size (40.0 ±3.7mm shell length) achieved higher (F1, 22 = 4.64, p = 0.042) retrievals than larger seed size (53 ±3.7mm). Growth rates were an average of 30% higher in the shallow sites compared deeper sites (t1, 1091 = -2.79, p = 0.005). There was seasonal component in growth with a 0.55 ± 0.04 mm per month higher rate in summer than during winter (t1, 1091 = -12.45, p < 0.001). Abalone seed releases were also conducted on a larger scale, which focused on the economic potential for commercial-scale ranching. Over 160 sites were seeded with 1.4 million hatchery reared abalone or 30 metric tonnes. Seventeen sites underwent long-term monitoring of survival with repeated sampling events on sites over a period of up to 666 days. Nine of these sites were seeded with 140800 small abalone (29 – 40 mm), three sites with 25100 medium abalone (30 – 46 mm) and five sites with 14700 large abalone (60 – 84 mm). There was a significant difference in survival between the seed sizes (F4,6329 = 304.3, P < 0.001). The Generalized Linear Mixed Model (GLMM) calculation estimated the instantaneous apparent mortality rate for small, medium and large abalone at 1.19 yr-1 , -0.9 yr-1 and -0.67 yr-1 , which equated to a survival rate of 21%, 28% and 36% per year respectively. The weighted mortality rate calculated according to the seed size differential mortality and the number of seed released for those sizes was 1 yr-1 or 25% survival. In conjunction with a conservative somatic growth rate of 1.2mm.m-1 and plausible emigration rates, a range of biomass harvesting estimates were obtained. Movement and mortality were investigated using video analysis which showed that hatchery seed had the ability to move considerably once released onto natural reefs and confirmed that mortality immediately following releases was due to the heavy predation pressure as confirmed by shell collections. This project has demonstrated that potential exists for ranching in South Africa and highlights critical factors that need to be addressed if ranching is to be an economically viable operation.
- Format
- 168 leaves
- Format
- Publisher
- Rhodes University
- Publisher
- Faculty of Science, Ichthyology and Fisheries Science
- Language
- English
- Rights
- Witte, Warren
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