Conservation biology and management of the Twee River redfin, Barbus erubescens (Pisces : Cyprinide)
- Authors: Marriott, Michael Stephen
- Date: 1999
- Subjects: Barbus , Cyprinidae
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5248 , http://hdl.handle.net/10962/d1005091 , Barbus , Cyprinidae
- Description: Agricultural practices and increasing levels of pollution, water abstraction and numbers of predatory exotic fishes gave rise to mounting concern for the survival of the already endangered Twee River redfin, Barbus erubescens. Numbers were believed to be dwindling and an urgent re-assessment of the species' conservation status and major threats was called for. Distribution and estimated numbers of B. erubescens were detennined from a census conducted in the Twee River catclunent, and baseline knowledge of age and growth, reproduction, diet and habitat preferences was gained from biological work on collected specimens. Such understanding was necessary to formullate management ideas. Although an estimated 40% reduction in area of occupancy has occurred in the middle reaches of the Twee River system, the known distribution of B. erubescens was extended into the upper Suurvlei River. Despite fragmentation, surviving redfin populations remained healthy and total population size was estimated at 8400 individuals, 4100 of which were considered mature. Adults preferred pools with water exceeding 1 m in depth and tended to be associated with sandy or boulder substrates. Due to low numbers no such study was conducted on juveniles, although observations indicated a preference for the upper 50 cm of the water column, in or around marginal vegetation. Barbus erubescens spawn in summer and follow an asynchronous, iteroparous pattern. Males and females reach a maximum age of six years, maturing after two years at calculated SL of 45 mm in males and 42 mm in females. The diet was dominated by simulid and ephemeropteran larvae. Although the combined pressures of pesticide and fertiliser use, predation and competition from exotic species have undoubtedly had a negative impact on B. erubescens, the species has maintained viable populations. With continued expansion of these threats the focus of conservation attention may change but, at present, it is most likely agricultural water use, amounting to 7.43 x 10⁶ m³y⁻¹, 15% of the entire catchment production, which poses the greatest immediate threat to the survival of the species. Based on a deteriorating habitat and restricted area of occupancy, B. erubescens was rated Critically Endangered, and it is recommended that a conservation management plan be implemented in the Twee River catchment. The focus of such action should be on genetic preservation, with immediate projects including gamete cryopreservation and captive breeding and rearing programmes. Catchment management, including education and rehabilitation programmes, must be the long-term aim of conservation, to ensure the survival of B. erubescens.
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- Date Issued: 1999
Studies on captive rearing of spotted grunter, pommadasys commersonnii (Pisces : Haemulidae) under ambient conditions
- Authors: Bacela, Ntobeko
- Date: 1999
- Subjects: Grunts (Fishes)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5322 , http://hdl.handle.net/10962/d1005167 , Grunts (Fishes)
- Description: The effects of stocking density, size grading, feeding frequency and ambient temperature on the growth performance and size variation of spotted grunter, Pomadasys commersonnii were investigated. The time that would be required to rear the species to market size was modelled. An area where maximum growth rate could be achieved under ambient temperature conditions, and therefore the location of a commercial farm within the distribution range of spotted grunter along the coast of South Africa, was predicted. The growth performance of spotted grunter was not significantly affected by stocking density. Growth performance seemed to improve with increasing stocking density. Competitive behaviour was absent among fish in the various stocking densities. Growth in terms of fork length and body weight was not significantly different between stocking densities. The highest specific growth rate, best food consumption, food conversion ratio and protein efficiency ratio of the fish were recorded at a density of 6.4 kg/m³, whereas the best condition factor was recorded at a density of 3.8 kg/m³. The lack of significant difference in many of the growth parameters between the various stocking densities suggest that juvenile spotted grunter could be reared at densities higher than 6.4 kg/m³. Further investigations are needed to determine the optimal initial stocking density of juvenile spotted grunter under ambient temperature and photoperiod conditions. Replacing the largest fish with average sized fish did not have a significant effect on the specific growth rate and competitive behaviour based on the relationship between the coefficient of variation and average size. Feeding frequency had a sIgnificant effect on food consumption, food conversion and protein efficiency ratio, and not on size increase, specific growth rate and condition factor. Its effect on competitive behaviour could not be conclusively explained. The best food conversion ratio recorded when feeding once a day showed that although the fish consumed a limited amount of food, they utilised the food that was fed most effectively. It is suggested that the fish be fed three times a day. The survival of juvenile spotted grunter was 95.6 % when feeding three times a day compared to 90 and 90.2 % when feeding once and five times a day, respectively. Fluctuating ambient temperature had a dramatic effect on specific growth rate and food consumption of spotted grunter. Growth modelling showed that the fish could be reared to a market size of 550 g (270 mm FL) in 19 months under ambient temperature conditions (23.2°C) at Richard's Bay. The optimal predicted rearing period of 19 months is approximately seven months less than that calculated for fish in the wild, and can possibly, be reduced further by feeding a balanced diet. The overall food consumption (on a dry weight basis) in the three size classes ranged from 0.15 ± 0.16 to 0.38 ± 0.35 % body weight per day. Food conversion ratio improved with increasing fish size. This relationship was attributed to diet quality and more specifically, the protein : energy ratio. As a result, fish in the large size class had the best overall protein efficiency ratio. Maximum specific growth rates of1.5, 0.84 and 0.74 % body weight per day were recorded from the small, medium and large size classes in the peak of summer with average daily temperature ranging from 21 to 22°C. Positive slopes in the coefficient of variation against fish size in the large size class indicated the presence of competitive behaviour which was attributed to the onset of adolescence. The information from this study can be used for pilot production of spotted grunter. Further research should be undertaken to investigate captive reproduction of the species.
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- Date Issued: 1999
The effect of diet and age-at-weaning on growth and survival of clownfish Amphiprion percula (Pisces : Pomacentridae)
- Authors: Gordon, A. K. (Andrew K.)
- Date: 1999
- Subjects: Anemonefishes , Pomacentridae , Fishes -- Growth
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5270 , http://hdl.handle.net/10962/d1005114 , Anemonefishes , Pomacentridae , Fishes -- Growth
- Description: The aim of this study was to improve the rearing of common clownfish Amphiprion percula, by weaning juveniles from a live feed to a formulated feed as early as possible, while still maintaining good growth and survival. The growth response of A. percula to a formulated dry feed was initially investigated. There was no difference in growth rate of juveniles fed a formulated feed, and the formulated feed supplemented with either Artemia or a Donax serra / Penaeus indicus combination. Amphiprion percula readily consumed the formulated feed, and the fishmeal/casein combination appeared an acceptable protein source. As the amount of protein included in a diet can have a profound effect on growth, the optimal dietary protein level for juvenile A. percula was investigated by feeding semipurified diets containing graded levels of protein, ranging from 40-65%. There was no difference in the growth rates of juveniles fed the various diets, however all diets promoted good growth with an average weight gain of 419%, and thus for the purposes of this study the diet formulation was deemed adequate. The histological study of the digestive system of larval A. percula revealed that the alimentary canal was advanced at hatching and that larvae start exogenous feeding immediately. Three days after hatch (DAH) the yolk sac is completely absorbed. In the hind-gut epithelium of 5-day-old larvae small supranuclear inclusion vacuoles appear, suggesting pinocytotic digestion, and by 7 DAH gastric glands are established in the epithelium of the stomach. Nine DAH supranuclear inclusion vacuoles appear in the epithelium of the mid-gut, indicating extracellular digestion and absorption across the lumen. As pinocytotic digestion of protein is less efficient than extracellular digestion, especially in the case of formulated feeds, it was hypothesised that the digestive system of A. percula could only effectively digest formulated feeds 9 DAH onwards. The two weaning experiments designed to test this hypothesis revealed that A. percula was able to utilise the formulated feed, without reduction in survival, from 7 DAH onwards. However, in terms of growth, the optimal time to wean juveniles from the live feed to the formulated dry feed was between 15 to 20 DAH. As A. percula accept a formulated feed and can benefit nutritionally from it, the dependence of larvae and juveniles on live feed can be reduced. This study has shown that the rearing of A. percula can been simplified and improved by weaning from 7 DAH with no reduction in survival, and from 15 to 20 DAH with no reduction in growth.
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- Date Issued: 1999