An eco-physiological investigation of fisheries-induced evolution: comparing the resilience of larvae from exploited and unexploited commercial reef fish populations to projected ocean acidification
- Authors: Muller, Cuen
- Date: 2022-04-08
- Subjects: Ocean acidification , Fishes Climatic factors , Fishes Physiology , Fishes Metabolism , Fishes Respiration , Fishes Larvae , Fishery management , Chrysoblephus laticeps (Red roman)
- Language: English
- Type: Doctoral thesis , text
- Identifier: http://hdl.handle.net/10962/232579 , vital:50004 , DOI 10.21504/10962/232579
- Description: It is now accepted that anthropogenic-induced climate change is resulting in unprecedented rates of change to marine environments. Marine organisms are being challenged by rapidly increasing temperatures, acidification, expansion of oxygen dead zones, and higher frequencies and magnitudes of extreme weather events. Exploited fish populations are also undergoing selective harvesting. Certain traits, such as large size, fast growth, and/or bold/active behaviours, are being actively targeted and removed from the population gene pool. This selective removal of individuals may compromise the capacity of fish populations to resist or recover from environmental disturbances and reduce their ability to adapt to a changing environment as many of these traits are heritable. As most marine fishes' embryonic and larval stages represent the period when individuals are most sensitive to environmental disturbances, they are a critical bottleneck to population persistence in the face of exploitation and climate change. This thesis aimed to quantify and compare the metabolic physiology, growth, and development of an exploited and endemic sparid, the roman seabream Chrysoblephus laticeps, during the early larval stages under 1) ocean acidification conditions expected by the year 2100 and 2) from populations experiencing dissimilar rates of exploitation. To quantify and compare the physiology of larvae, adult C. laticeps from an exploited population were captured and field-spawned. Fertilised eggs were placed into either control/present-day conditions (pH = 8.03, pCO2 ≈ 420 μatm) or high-pCO2/hypercapnic treatment conditions (pH = 7.63, pCO2 ≈ 1400 μatm). The metabolic physiology of individual larvae was determined using a novel rolling-regression technique on static respirometry data. Here, estimates of the minimum and maximum oxygen consumption rates (VO2) could be determined with high test-retest reliability. The very early developmental stages (yolk-sac stage) appeared resilient to high pCO2 conditions despite being exposed to treatment conditions throughout the embryonic stage. Preflexion larvae showed sensitivity to treatment conditions by exhibiting reduced metabolic and growth rates, consistent with metabolic depression, associated with environmental stress. However, by the onset of flexion, which coincides with gill development, acid-base regulation, and muscle differentiation, metabolic and growth rates of treatment larvae were significantly greater than that of controls. This suggests that acid-base regulation imposes a high cost to maintain internal pH homeostasis. Importantly, these elevated metabolic costs were likely mediated through increased feeding rates in experimental conditions where food was ad libitum. In natural conditions, where food availability may be varied, high pCO2 conditions could be associated with higher mortality rates. Based on evidence that protected/unexploited populations are more genetically diverse and are composed of individuals representing a greater range of metabolic phenotypes, offspring were collected from a protected population experiencing otherwise similar environmental conditions to the exploited population. Metabolic rates of control larvae were generally similar to those of the exploited population. However, minimum rates of VO2 were typically higher for larvae from the protected population at comparable life stages. Preflexion treatment larvae from the protected population did not appear to undergo a period of reduced metabolism or growth compared to their control counterparts. While metabolic rates at the onset of flexion were significantly higher for treatment larvae, this was not associated with growth differences. Growth over-compensation following periods of growth depression is often associated with deleterious effects, such as organ damage and body or developmental malformations. This suggests somewhat improved resilience to ocean acidification conditions. This thesis found evidence that larval C. laticeps are sensitive to ocean acidification conditions expected by 2100. When this stressor is combined with increasing thermal variability, changing current coastal regimes, and heterogeneous food availability, also expected to occur by 2100, ocean acidification may compromise the population persistence of this species. However, an energetics approach to stress-tolerance suggests that larvae from the protected population may inherently show greater resilience to climate change-related environmental stressors. Evidence that exploitation affects the resilience of fish larvae to climate change highlights the need for an evolutionary approach to fisheries management and the importance of spatial protection in maintaining larger and more resilient populations while providing the raw material essential for adaptation. , Thesis (PhD) -- Faculty of Science, Ichthyology and Fisheries Science, 2022
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- Date Issued: 2022-04-08
Movement behaviour and reproductive biology of adult spotted grunter (Pomadasys commersonnii) in the Breede Estuary
- Authors: Ziko, Bantony Alford
- Date: 2021-06
- Subjects: Fishery management , Freshwater fishes , Fishes--Nutrition
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/20970 , vital:46870
- Description: Movements of the adult spotted grunter (Pomadasys commersonnii), an overexploited estuary-dependant fishery species, were investigated in relation to its reproduction behaviour and selected environmental influences. The South African P. commersonnii population is known to spawn at sea along the KwaZulu-Natal (KZN) coastline, whereas there no published records on spawning in the Eastern and Western Cape Provinces exist. However, due to the occurrence of ripe running fish that were reported in the Breede Estuary (Western Cape - WC), an investigation into its reproductive behaviour was necessary, as was gaining a greater understanding of its movements between the estuary, the sea, and other connected habitats. Seven adult P. commersonnii (610 – 690 mm fork length, FL) were captured and implanted with long-life acoustic transmitters and their movements were logged on a linear array of 16 acoustic receivers in the Breede Estuary (WC) and six acoustic receivers in the adjacent marine environment. Movements of the tagged fish to other marine and estuary habitats were also determined using data collected by the Acoustic Tracking Array Platform (ATAP) which is an extended network of acoustic receivers along the South African coastline with receivers at sea and in selected estuaries. Furthermore, adult P. commersonnii tagged in the Goukou (n = 6) (WC) and Sundays (n = 8) (Eastern Cape - EC) estuaries, approximately 50 km and 470 km away, respectively, were also monitored to facilitate an improved understanding of inter-habitat connectivity. Fish tagged in Sundays and Goukou estuaries were monitored by the Acoustic Telemetry Array Platform (ATAP). Each individual fish tagged in the Breede Estuary was monitored between 356 and 1166 days and selected environmental parameters, including water temperature, river flow rate and rainfall, were measured during the study period (November 2016 – March 2020) to assess their influence on the movements of the tagged fish. Furthermore, to determine time of spawning and duration, gonads of 112 adult fish (383 – 700 mm FL) were opportunistically collected from recreational anglers in the Breede Estuary to determine the gonadal maturation cycles by using a combination of Gonado-somatic index (GSI) and histology. The tagged adult P. commersonnii spent significantly more time in the estuary (83.5 percent) than the marine (16.5 percent) environment, with all of the tagged P. commersonnii making sea trips of varying frequency and duration. While in the estuary, fish showed a strong preference for the lower reaches of the estuary compared to the middle and upper reaches. Area use varied seasonally, with the mouth area being heavily utilized during winter, then followed by summer. Sea trips in all three tagging estuaries also fluctuated seasonally, with more frequent movements during summer which lasted for longer periods. This coincided with the peak of the spawning period inferred from the gonad developmental cycle of fish captured in the Breede Estuary. Results from a generalized linear mixed model showed that the presence of Breede tagged P. commersonnii in the sea, where the species is known to spawn, was significantly influenced by water temperatures in the estuary and river flow rate with fish more likely to occur at sea during warmer months (summer) and periods of high river flow (winter). Furthermore, circular analysis showed that season (month) had a significant influence on the departure from and arrival in the Breede Estuary, with most departures and arrivals occurring during January. Moreover, P. commersonnii tagged in the Sundays Estuary also showed similar results where both departures and arrivals were influenced by season, mostly taking place in December and January, respectively. The increased frequency of sea trips by adult P. commersonnii in summer suggests regional spawning occurring in the WC. Based on the similar movement behaviour displayed by tagged fish in the Sundays Estuary, it is possible that spawning also occurs in the EC. Most of the P. commersonnii tagged in all the three estuaries never left their tagging estuaries (57 percent Breede, 66 percent Goukou, 38 percent Sundays), only making short sea trips, while others displayed use of multiple habitats. On average, the tagged P. commersonnii that left their tagging estuaries travelled a maximum distance of 377 km (Breede), 63 km (Goukou) and 300 km (Sundays). Tagged fish were more likely to visit nearby estuaries and offshore marine habitats (< 130 km) compared to distant ones (> 130 km). These results suggest that there are high levels of localised connectivity between P. commersonnii populations in different habitats although connectivity between distant habitats appears to be rare. Connectivity will likely occur on habitats located close to the boundaries of the WC, EC and KZN. The findings from this long-term monitoring study have shown that adult P. commersonnii rely heavily on the estuarine environment, undertaking short sea trips, thus demonstrating the importance of estuaries to the adult population. Due to various threats faced by estuaries such as habitat degradation and overexploitation, management of these systems is paramount to conserve such fishery resources. Due to the resident behaviour of the P. commersonnii within estuaries, Marine Protected Areas (MPAs) and area closures, in conjunction with Estuarine Protected Areas (EPAs) will be significant in the recovery and management of P. commersonnii populations. Since P. commersonnii appears to reach peak spawning activity during summer when fishing pressure is usually highest, improved enforcement during this period is necessary to conserve the spawning stock. As connectivity is an important aspect in the life of the P. commersonnii, ensuring such connectivity is maintained by protecting all these habitats is important. , Thesis (MSc) (Zoology) -- University of Fort Hare, 2021
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- Date Issued: 2021-06
The genetic stock structure and distribution of Chrysoblephus Puniceus, a commercially important transboundary linefish species, endemic to the South West Indian Ocean
- Authors: Duncan, Murray Ian
- Date: 2014
- Subjects: Sparidae , Fishes -- Indian Ocean , Fish populations , Fishery management , Fish stock assessment -- South Africa , Fish stock assessment -- Mozambique , Overfishing , Habitat conservation , Fishes -- Genetics , Fishes -- Climatic factors , Fishes -- Variation , Fishes -- Migration
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5360 , http://hdl.handle.net/10962/d1011868 , Sparidae , Fishes -- Indian Ocean , Fish populations , Fishery management , Fish stock assessment -- South Africa , Fish stock assessment -- Mozambique , Overfishing , Habitat conservation , Fishes -- Genetics , Fishes -- Climatic factors , Fishes -- Variation , Fishes -- Migration
- Description: Chrysoblephus puniceus is an over-exploited linefish species, endemic to the coastlines off southern Mozambique and eastern South Africa. Over-exploitation and habitat loss are two of the biggest threats to the sustainability of fisheries globally. Assessing the genetic stock structure (a prerequisite for effective management) and predicting climate related range changes will provide a better understanding of these threats to C. puniceus which can be used to improve the sustainability of the fishery. Two hundred and eighty four genetic samples were collected from eight sampling sites between Ponta da Barra in Mozambique and Coffee Bay in South Africa. The mitochondrial control region and ten microsatellite loci were amplified to analyse the stock structure of C. puniceus. The majority of microsatellite and mtDNA pairwise population comparisons were not significant (P > 0.05) although Xai Xai and Inhaca populations had some significant population comparisons for mtDNA (P < 0.05). AMOVA did not explain any significant variation at the between groups hierarchical level for any pre-defined groupings except for a mtDNA grouping which separated out Xai Xai and Inhaca from other sampling sites. SAMOVA, isolation by distance tests, structure analysis, principle component analysis and spatial autocorrelation analysis all indicated a single population of C. puniceus as being most likely. The migrate-n analysis provided evidence of current driven larval transport, with net migration rates influenced by current dynamics.Two hundred and thirty six unique presence points of C. puniceus were correlated with seasonal maximum and minimum temperature data and bathymetry to model the current distribution and predict future distribution changes of the species up until 2030. Eight individual species distribution models were developed and combined into a mean ensemble model using the Biomod2 package. Winter minimum temperature was the most important variable in determining models outputs. Overall the ensemble model was accurate with a true skills statistic score of 0.962. Binary transformed mean ensemble models predicted a northern and southern range contraction of C. puniceus' distribution of 15 percent; by 2030. The mean ensemble probability of occurrence models indicated that C. puniceus' abundance is likely to decrease off the southern Mozambique coastline but remain high off KwaZulu-Natal. The results of the genetic analysis support the theory of external recruitment sustaining the KwaZulu Natal fishery for C. puniceus. While the high genetic diversity and connectivity may make C. puniceus more resilient to disturbances, the loss of 15 percent; distribution and 11 percent; genetic diversity by 2030 will increase the species vulnerability. The decrease in abundance of C. puniceus off southern Mozambique together with current widespread exploitation levels could result in the collapse of the fishery. A single transboundary stock of C. puniceus highlights the need for co-management of the species. A combined stock assessment between South Africa and Mozambique and the development of further Marine Protected Areas off southern Mozambique are suggested as management options to minimise the vulnerability of this species.
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- Date Issued: 2014