Salinity induced physiological responses in juvenile dusky kob, Argyrosomus japonicus (Sciaenidae)
- Authors: Bernatzeder, Andrea Katinka
- Date: 2009
- Subjects: Osmoregulation , Argyrosomus -- Effects of salt on -- South Africa , Sciaenidae -- Effects of salt on -- South Africa , Argyrosomus -- Physiology , Sciaenidae -- Physiology , Fishes -- Physiology , Biological control systems , Marine ecology -- South Africa , Fishery management -- South Africa , Mariculture -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5318 , http://hdl.handle.net/10962/d1005163 , Osmoregulation , Argyrosomus -- Effects of salt on -- South Africa , Sciaenidae -- Effects of salt on -- South Africa , Argyrosomus -- Physiology , Sciaenidae -- Physiology , Fishes -- Physiology , Biological control systems , Marine ecology -- South Africa , Fishery management -- South Africa , Mariculture -- South Africa
- Description: Fisheries management regulations for dusky kob Argyrosomus japonicus, an important commercial and recreational fisheries species, have failed and the stock is considered collapsed. It is important to take an ecosystems approach to management which includes understanding the effect of environmental factors on recruitment, abundance and distribution. The distribution of early juveniles (20-150 mm TL) in the wild appears to be restricted to the upper reaches of estuaries at salinities below 5 psu. Food availability could not explain the distribution of early juveniles. The aim of this study was to investigate the role of salinity on the distribution of early juvenile dusky kob (<150 mm TL) by examining physiological responses of juveniles exposed to a range of salinities under laboratory conditions. The hypothesis was that the physiological functioning of early juveniles would be optimised at the reduced salinities which they naturally occur at. The objectives of this study were to investigate the effect of salinity on: i) plasma osmolality; ii) growth, food conversion ratio and condition factor; and iii) gill histology with emphasis on chloride cell size and number. A preliminary study was undertaken to determine whether the use of 2-phenoxyethanol had an effect on plasma osmolality. Juveniles pithed prior to blood sampling were used as the control. Plasma osmolality was not affected by exposure or duration of exposure (2, 4, 6, 8, 10 min) to 2-phenoxyethanol. The ability of teleosts to regulate plasma osmolality over a wide range of salinities indicates their degree of ‘physiological euryhalinity’. Plasma osmolality of juveniles exposed to 5, 12 and 35 psu was measured every two weeks over a total of six weeks. Although juveniles were able to regulate plasma osmolality over the duration of the experiment, plasma osmolality at 5 and 12 psu was significantly lower than in fish maintained at 35 psu. Growth is used as an indicator of the relative energy used for osmoregulation at different salinities, as the energy used for osmoregulation becomes unavailable for growth. A nineweek growth experiment was conducted on juveniles exposed to 5, 12 and 35 psu. Juveniles grew and survived at all three salinities. However, growth of juveniles at 5 psu was significantly lower than at 12 and 35 psu. Other than a significantly greater weight gain at 35 psu relative to 12 psu, there was no significant difference in specific growth and length gain between juveniles at 12 and 35 psu. Food conversion ratio and condition factor at 12 and 35 psu were not significantly different, but food conversion ratio and condition factor at 5 psu was significantly greater and lower than at 35 psu respectively. In fish, gills are considered the major organ involved in osmoregulation. Within the gills, chloride cells are the predominant site of ion exchange which is driven by the Na⁺, K⁺- ATPase enzyme. Gill samples of juveniles exposed to 5, 12 and 35 psu for six weeks were examined histologically using light microscopy. Chloride cells of juveniles maintained at 5 psu were significantly more abundant than in juveniles at 12 and 35 psu. Chloride cells of juveniles at 5 psu were significantly larger than in juveniles kept at 12 psu, but not significantly different to those of juveniles kept at 35 psu. The ability of the juvenile fish to regulate plasma osmolality indicates that they are 'physiologically euryhaline', but the reduced growth and proliferation of chloride cells at 5 psu suggests that energy expenditure for osmoregulation is increased at hypoosmotic salinities. Salinity induced physiological responses could therefore not explain the natural distribution of early juvenile dusky kob and it is proposed that other environmental factors (e.g. temperature) are also important. It is also hypothesised that the high conductivity of an estuary in South Africa, to which our understanding is limited, may negate the effect of reduced salinity. Although freshwater input into estuaries is an important factor, further investigations to explain the distribution and abundance of early juveniles is required to make management recommendations. Dusky kob is also becoming an increasingly popular aquaculture species in South Africa. In this regard, early juvenile dusky kob can be grown at salinities as low as 12 psu without negatively affecting growth and production.
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Fishes in the Mngazi and Mngazana estuaries, with particular emphasis on the community structure and primary carbon sources
- Authors: Mbande, Sekiwe
- Date: 2004
- Subjects: Fishes -- South Africa -- Mngazi Estuary , Fishes -- South Africa -- Mngazana Estuary , Fishes -- Effect of habitat modification on -- South Africa -- Mngazi Estuary , Fishes -- Effect of habitat modification on -- South Africa -- Mngazana Estuary , Estuarine fishes -- South Africa -- Eastern Cape , Estuarine ecology -- South Africa -- Eastern Cape , Fishes -- Physiology , Fishes -- Effect of water quality on , Fishes -- Effect of temperature on , Fishes -- Food , Fishes -- Effect of turbidity on
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5700 , http://hdl.handle.net/10962/d1005386 , Fishes -- South Africa -- Mngazi Estuary , Fishes -- South Africa -- Mngazana Estuary , Fishes -- Effect of habitat modification on -- South Africa -- Mngazi Estuary , Fishes -- Effect of habitat modification on -- South Africa -- Mngazana Estuary , Estuarine fishes -- South Africa -- Eastern Cape , Estuarine ecology -- South Africa -- Eastern Cape , Fishes -- Physiology , Fishes -- Effect of water quality on , Fishes -- Effect of temperature on , Fishes -- Food , Fishes -- Effect of turbidity on
- Description: The fish community structure of two contrasting estuaries, one with a well developed mangrove forest (Mngazana) and the other without mangroves (Mngazi) was compared. Both the Mngazi and Mngazana estuary fish communities were dominated by marine species, reflecting the importance of these systems as nursery areas for marine fishes. The Mngazi Estuary contained 18% more estuarine fishes in terms of catch per unit effort (CPUE) than the Mngazana Estuary. The reduced tidal influence due to the narrow mouth opening is a possible reason for the heightened CPUE of estuarine species in the Mngazi estuary. The recorded higher diversity of fish species in the Mngazana Estuary when compared with the Mngazi Estuary was attributed to the greater influence of the marine environment due to the wide permanently open mouth, as well as the presence of a variety of habitats in this system. In both estuaries tropical and temperate species were captured, confirming the transitional nature of their biogeographic location which is situated close to the boundary between the subtropical and warm temperate regions of the Southern African coastline. Contrary to previous studies, which recorded seasonal changes in the proportions of tropical and temperate species, the proportions of tropical species remained unchanged at approximately 70% during the January and June sampling occasions. Global warming as a possible reason for the increased dominance of tropical species is discussed. Although several studies in southern Africa have investigated estuarine food web structure, none have compared mangrove and non-mangrove estuaries. In this study, the primary sources of carbon utilised by the fish fauna in the Mngazi and Mngazana estuaries was investigated. The carbon isotopic values of fishes in both estuaries displayed a continuum rather than a tight clustering around particular energy sources. Most detritus feeders of the family Mugilidae (mullets) from both estuaries were relatively more enriched than other fish taxa. The isotopic values of the mullet species suggest a diet derived from relatively enriched carbon sources such as benthic microalgae, the eelgrass Zostera capensis and associated epiphytes. Based on the isotopic values, piscivorous fishes from both estuaries could not be linked to specific prey fish taxa, but clearly the mullet species were not their main food source. The invertebrate feeders that were found in both estuaries showed greater isotopic variations in the Mngazana Estuary than in the Mngazi Estuary, probably reflecting the higher diversity of habitats (carbon sources) and invertebrate prey species in the Mngazana system. Generally the isotopic signatures of fishes from the Mngazi Estuary were more enriched than those from the Mngazana Estuary, thus indicating the possible effect of δ¹³C depleted mangrove derived carbon in the latter system.
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The biology of commercially important fish species and a preliminary assessment of the fisheries potential of Katse Dam, Lesotho
- Authors: Nthimo, Mokitinyane Francis
- Date: 2000
- Subjects: Fishes -- Lesotho -- Katse dam , Fishes -- Physiology , Rainbow trout , Barbus aeneus , Labeo , Katse dam (Lesotho) , Fisheries -- Lesotho -- Katse dam , Fishery management -- Lesotho
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5238 , http://hdl.handle.net/10962/d1005081 , Fishes -- Lesotho -- Katse dam , Fishes -- Physiology , Rainbow trout , Barbus aeneus , Labeo , Katse dam (Lesotho) , Fisheries -- Lesotho -- Katse dam , Fishery management -- Lesotho
- Description: The construction of the Katse dam was completed in 1996. The dam started filling in 1995 and reached full capacity in early 1998. It forms part of Phase 1A of the Lesotho Highlands Water Project (LHWP). The main aim of the project is to provide revenue to Lesotho, by transferring water from the catchment of the Senqu (Orange) River in Lesotho to South Africa’s major industrial and population centres. During the implementation of the project, an estimated 130 people were displaced. However, the most serious impact has been the loss of the traditional way of life in the form of arable and grazing land as a consequence of inundation. One of the obligations of the project is to ensure that the standard of living of those affected by the project is not impaired. The Lesotho Highlands Development Authority identified a number of rural development projects, which included fisheries development. This study is an integral part of fisheries development in Lesotho. The aim of this study was to gain an understanding of the biology and the demographics of the fish species in the lake. This information would serve as the basis for the development of a management plan for the sustainable utilisation of the fisheries resources. To achieve this aim, the following specific objectives were addressed: 1. Investigation of the biology of the three principal species. 2. Description of the key population parameters (growth, mortality & recruitment). 3. Investigation of the distribution and relative abundance of the three species. The three principal species in the Katse dam are rainbow trout, Oncorhynchus mykiss, and the two cyprinids, Barbus aeneus and Labeo capensis. Sectioned otoliths were used to age O. mykiss while both otoliths and scales were used to age B. aeneus and L. capensis. Marginal zone analysis revealed that slow growth was experienced in winter for the three species. The maximum-recorded age for both B. aeneus and L. capensis was 12 years while O. mykiss reached 4 years. There was no significant difference in growth rates of the different sexes and growth was best described by the 3 parameter Von-Bertalanfy growth model as Lt = 603[1-e⁻°·¹⁵⁽t ⁺ °·°³⁾] for B. aeneus, Lt = 526[1-e⁻°·²¹⁽t ⁺ °·⁹⁾] for O. mykiss and Lt = 521[1-e⁻°·¹⁷⁽t ⁺ °·²¹⁾] for L.capensis. Male and female O. mykiss attained 50% sexual maturity (Lm₅₀) at 235 and 275mm FL, respectively. There was no difference in Lm₅₀ for male and female B. aeneus and L. capensis. B. aeneus reached sexual maturity at 285mm FL while L. capensis reached sexual maturity at 244mm. Both cyprinid species spawned in summer (November to January) while O. mykiss spawned in winter (May to August). The mean total mortality rate (Z) estimated from catch curves and Butterworth et al (1989) equation was 0.72 yr⁻¹ for B. aeneus, 0.62 yr⁻¹ for L. capensis and 1.32 yr⁻¹ for O. mykiss. The estimate of natural mortality was 0.41 yr⁻¹ for both B. aeneus and L. capensis and 0.81yr⁻¹ for O. mykiss.
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