Stock structure of Patagonian toothfish Dissostichus eleginoides (Smitt 1898, family Nototheniidae) in the Southwest Atlantic
- Authors: Lee, Brendon
- Date: 2022-10-14
- Subjects: Otoliths , Patagonian toothfish Geographical distribution , Fish tagging , Biogeography , Microstructure , Microchemistry
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/365599 , vital:65763 , DOI https://doi.org/10.21504/10962/365599
- Description: The identification of discrete self-sustaining productive units in marine populations is essential for achieving sustainable fisheries objectives. Marine fish populations frequently exhibit dynamic characteristics across their life-histories, displaying variability in spatial structure and mixing patterns, both within and among populations. The incoherent application of management boundaries on biological populations can bias stock assessment results and have important implications on sustainable fisheries management. Patagonian toothfish (Dissostichus eleginoides) is a long-lived, slow-growing, late-maturing, deep-sea, benthopelagic species. It forms the basis of important and highly lucrative industrial and artisanal fisheries across its distribution. Patagonian toothfish have complex life-histories characterised by high dispersal potential during the egg and larval phase, a wide depth range because of their ontogenetic migratory behaviour, and large adult size that is capable of undertaking long-distance active movements (>200 km). These characteristics provide opportunities for high levels of connectivity, and as such, the stock structure is not well understood. We applied an integrated, multidisciplinary approach to provide an improved understanding of the complex stock structure dynamics for Patagonian toothfish on the Patagonian Shelf, specifically in relation to the shelf, slope, and deep-sea plains around the Falkland Islands. Research results were focused on aspects pertaining to (1) geographic variation in phenotypic characters (otolith shape); (2) a description of the spatial-temporal distribution patterns; (3) the active movements of deep-sea adults (tag-recapture study); and (4) the identification of early life-history dispersal through otolith microstructure and microchemical chronologies. Results from the study indicate high regional connectivity during the early life-history stages derived from at least two spawning contingents into spatially discrete nursery areas (cohort groups) on the Falklands Shelf. Fish followed distinct ontogenetic pathways into deeper waters adjacent to the areas wherein juvenile settlement into a demersal habitat occurred. There is little to no evidence of mixing among cohort groups during their ontogenetic migration into deep-sea adult habitats, reflecting a mixed population based on oceanographically defined egg and larval dispersal. The majority of the adult component of the population continue to display high site fidelity. However, between 9 and 25% of the population, consisting predominantly of larger reproductively capable adults undertake long-distance dispersal behaviour, identified as home-range relocations from the adult deep-sea habitats towards three of the known southern spawning grounds in the region. Results are suggestive of a requirement for improved collaborative efforts for regionally-based management approaches with careful consideration of local stock contingents. Future monitoring and research priorities should focus on the identification of reproductive potential, dispersal pathways and settlement patterns of stock contingents to inform the dynamics of mixed stock origins across the Patagonian region. While many aspects regarding the stock structure remain unresolved, results derived from the current studies can be used to inform the development of management measures to ensure the continued recovery and sustainable management of Patagonian toothfish within the region. , Thesis (PhD) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
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- Date Issued: 2022-10-14
The biology of Oreochromis mossambicus and vulnerability to the invasion of Oreochromis niloticus
- Authors: Mpanza, Nobuhle Phumzile
- Date: 2022-10-14
- Subjects: Ecomorphology , Mozambique tilapia , Nile tilapia , Introduced fishes , Predatory aquatic animals , Predation (Biology) , Otoliths , Von Bertalanffy function
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362983 , vital:65380
- Description: Mozambique tilapia, Oreochromis mossambicus, a native southern African species now co-occurs with invasive Nile tilapia, Oreochromis niloticus throughout much of the distribution of the former. The spread of O. niloticus in South Africa has been attributed to escapees from aquaculture facilities, placing O. mossambicus at risk through competition for habitat and food resources, as well as through hybridisation. To better manage invasions, a comprehensive understanding of the biology, ecology and behaviour of both native and invasive species is required. The aim of this research was to comparatively assess the biology of O. mossambicus and O. niloticus, their food resource use characteristics and potential competitive interactions to infer impact risks associated with O. niloticus invasion dynamics. In addition to lack of sufficient autecological knowledge on O. niloticus in general, the challenge in the Eastern Cape is that relatively little regional knowledge is available on the biology and ecology of the native O. mossambicus. To address this, a total of 101 O. mossambicus individuals (32 - 297 mm LT) were sampled from the Sunday River catchment and their age and growth determined using sectioned sagittal otoliths. The largest female was 288.8 mm with a parameter estimate of LT (mm) = 272 (1-e-0.331(t=0.772)) and the largest male was 297 mm described as LT (mm) = 331.9 (1-e-0.167(t=1.192)). The growth parameter estimate age for combined sexes was best described as LT (mm) = 322.5 (1-e-0.201(t=1.027)). The growth rate was initially rapid for O. mossambicus and the asymptotic length reached after four years. The length-at-50% maturity was reached at 106.45 mm LT (R2 = 0.57) for the entire population. There was a significant difference (ᵡ2 = 8,047, df = 1, p-value = 0.0045) in the sex ratio between males and females which was skewed towards males 1:1.89 (F:M). Comparisons with O. niloticus were based on literature and these showed that O. niloticus had faster growth rates than O. mossambicus. These results serve as a baseline study in predicting the potential impacts of O. niloticus if it was to be introduced in the Eastern Cape region. Furthermore, although these two species are known to share habitat and food resources, feeding dynamics within the context of relative impact on prey resources, and competition potential between the species, are largely lacking. I used experimental functional response procedures to contrast the food consumption dynamics of each species and to assess for any multiple predator effects (MPEs) between these two closely related fishes. This was done by contrasting functional responses between individual species under single predator scenarios, predicted multiple predator functional response dynamics based on the individual species outputs, and actually observed functional responses under multiple predator conditions. Results showed that both Nile tilapia and Mozambique tilapia depicted a destabilizing Type II functional response. In both single and conspecific pairing Nile tilapia had significantly greater functional responses than Mozambique tilapia, hence greater overall predatory potential than its native congeneric Mozambique tilapia. Attack rates were also greater for Nile tilapia than Mozambique tilapia with both species showing similar handling times in single trials. However, no evidence for MPEs were detected, given lack of differences between predicted and observed functional responses under heterospecific conditions. These results suggest that Nile tilapia do not adjust their food intake in the presence of heterospecific competitors, but do consume more than Mozambique tilapia and are better at finding food when it is present at low densities. Feeding-related morphological characteristics may influence predatory performance of a species and can further provide information on the species’ capacity to locate, attack and consume different prey items. The feeding capacities between O. mossambicus and O. niloticus were compared based on morphological traits in order to determine whether differences existed, and if these differences place the invasive O. niloticus at an advantageous position in terms of resource acquisition and consumption over its native congener. Principal component analysis for functional morphology traits showed overlap between O. niloticus and O. mossambicus. Oreochromis niloticus had distinctively larger lower jaw closing force, gill resistance and gill raker length which facilitated greater feeding capacities for the invasive species over the native O. mossambicus. Trophic profiles depicted high dietary overlap between the two species. Although O. niloticus had a greater feeding capacity towards phytoplankton, plants, fish (ambush), fish (pursuit) and larvae, while O. mossambicus only showed greater feeding capacity towards zooplankton. While dietary overlap and similarities in morphological traits between native and invasive species may result in exploitative competition between the species, O. niloticus seems to be more versatile in its feeding and capable of consuming food web components that O. mossambicus may not be able to handle. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
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- Date Issued: 2022-10-14