A contribution towards the taxonomy of the ichthyoplankton species community and an understanding of its dynamics along the south-east coast of South Africa
- Authors: Wood, A D (Aidan David)
- Date: 1999
- Subjects: Fishes -- Eggs Fishes -- Larvae Fishes -- Eggs -- Africa, Southern Fishes -- Larvae -- Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5342 , http://hdl.handle.net/10962/d1006124
- Description: This study was prompted by the need to remedy the situation that existed with respect to the poor status of our knowledge regarding the ichthyoplankton assemblage of the nearshore region along the south-east Cape coast of South Africa. The first chapter provides a brief introduction to the field of ichthyoplankton research and includes a summary of the status of research in southern Africa and an explanation of early life history terminology. The selection of all sample sites, times and strategies is also outlined. The study area along the south-east Cape coast with respect to its location, climate and physical oceanography is described in the second chapter, as is the gear used, bongo nets and an RMT1x6. A sampling protocol for the use of bongos from a small ski-boat, and the RMT from the research vessels, and for the handling and processing of samples was established. The selection of Middlebank as the main monthly sampling site within the Tsitsikamma National Park (TNP) was based on taxonomic diversity as well as logistical and safety constraints. The effect of mesh size and time of sampling with bongo nets on the catchability of ichthyoplankton was investigated in chapter three. Most data was accumulated during Sea Fisheries research cruises, with additional collections coming from the National Parks vessel. Although the differences were not significant, the 505 μ mesh nets captured larger larvae, with catches comprising higher percentages of flexion and postflexion larvae. Larval concentration and size were consistently greater in samples from periods of reduced light intensity, but significant differences were the exception. It was decided that sampling with 505 μ mesh nets during daylight would provide a representative sample of the available ichthyoplankton assemblage, while at the same time being the most practical and least time consuming with respect to handling, clogging and backflushing. In chapter four, the early life history stages of thiny of the seventy-five taxa sampled are described, reflecting the paucity of information which existed on the ichthyoplankton of the nearshore zone in the south-east Cape. These descriptions are seen as an important contribution towards any future research efforts in the region, but as many of these descriptions are based on few or single specimens, it is realised that the description of egg and larval stages will be an ongoing process. Based upon the data collected during this study, an ichthyoplankton species checklist was established in chapter five. Seventy-five taxa of fish larvae were identified to either family, genus or species level. A number of squid para-larvae were also encountered. Similarities and discrepancies with a previous survey in the region are presented. The temporal distribution of eggs and larvae between August 1993 and October 1996 was established, and the spatial distribution of ichthyoplankton along an offshore transect was determined between January 1995 and May 1996. Only 7 species from Middlebank and twelve from all stations combined displayed seasonal trends, with most of these being prevalent during winter months. Egg production, both over Middlebank and from all stations combined, appeared to be consistent, with no seasonal trends. Based upon the results from the offshore transect samples, it would appear that a single ichthyoplankton assemblage exists from Storms River out to fifteen nautical miles. Although a variety of statistical methods were applied to the data during this study, low egg and larval concentrations and a low sampling frequency meant that results had to be interpreted carefully. Chapter six describes the preliminary investigation into the vertical distribution of eggs and larvae. During two research cruises, discrete depth sampling using an RMT1x6 net was performed, with the majority of samples being accompanied by physical data provided by a CTD rosette sampler. No definite patterns could be seen as larval concentrations were low, and the short time scale did not allow for the identification of any diel migratory patterns. The relationship between total larvae and the physical environment was poor. The only possible relationship was that between plankton volume and total egg and total larval concentrations. The TNP may play an important role in the conservation of reef fish and the seeding of nearby fishing grounds through the export of pelagic eggs and larvae. Chapter seven describes a preliminary investigation into the dispersal potential of ichthyoplankton from the TNP. Based upon longshore currents determined from drogues, ADCP vectors and current meter readings, it was clear that if larvae were passive drifters, the potential for their dispersal from the TNP did exist. This pilot study showed that future work should concentrate not only on the oceanographic aspects, but on behavioural aspects of larvae which may enhance or retard dispersal. In the final discussion, it is emphasised that while this study went a long way to increasing our knowledge of the nearshore ichthyoplankton community, the true picture of the temporal and spatial dynamics of the species assemblage may only be revealed once more intensive sampling has been performed. The resolution of the early life history stages of the sparids and the identification of eggs to species level are seen as priorities for the near future. A complete species checklist for the TNP is provided based upon previous surveys (both on adult and larval fishes), this work and personal observations. A total of 171 species of fish from 70 families were identified, illustrating that the eggs and larvae of many species in the TNP have yet to be sampled. A brief description of the collaborative effort which is envisaged for the TNP over the next few years is provided.
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- Date Issued: 1999
Aspects of the ecology of fishes associated with salt marshes and adjacent habitats in a temperate South African estuary
- Authors: Paterson, Angus William
- Date: 1999
- Subjects: Estuarine fishes -- Ecology -- South Africa , Salt marsh ecology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5365 , http://hdl.handle.net/10962/d1013010
- Description: There is a paucity of published information on fish utilization of salt marshes outside North America. This dissertation represents the first intensive examination of the ichthyofauna associated with salt marshes in southern Africa and examines the species composition, diversity, size structure, distribution and estuarine dependence of fishes that utilize salt marshes in the Kariega Estuary. The research was concentrated on Taylor's salt marsh, with comparative studies being conducted on other salt marshes and habitats within the marine dominated Kariega Estuary. The possible importance of salt marshes as feeding and refuge areas for fishes was examined, as was the role of salt marshes in the food web of the estuary. The fishes frequenting salt marshes in the Kariega Estuary were predominantly the juveniles of marine species, with Mugilidae being the dominant family. The ichthyofauna was distributed primarily in the intertidal creeks with very few specimens captured on the vegetated Spartina maritima and Sarcocomia perennis flats. The different reaches of the intertidal creek were characterised by distinct fish assemblages. The fish assemblages associated with the intertidal salt marsh creeks were significantly different from those found in the eelgrass beds, the other dominant intertidal habitat in the Kariega Estuary. The eelgrass beds were dominated by estuarine fish species and had a higher density and standing stock of fishes when compared to the salt marsh creeks. The diversity of fishes in the two habitats was however similar. The intertidal salt marsh creek ichthyofauna also differed significantly from that found in the main estuary channel. The creek ichthyofauna was dominated by 0+ juveniles while the main channel had many subadult and adult fishes. The channel habitat also had numerous large piscivorous fishes which were absent from the salt marsh creeks. The low number of piscivorous fishes, together with limited fish predation from other sources, may be the reason why salt marshes provide a refuge for juvenile fishes that frequent these habitats. Unlike previous studies on North American, Australian and European salt marshes, the dominant fish species that frequented Taylor's marsh were not recorded feeding extensively on the marsh, and those that did had a limited distribution. The role of fishes in the transfer of energy off the Kariega salt marshes is therefore likely to be minimal. A stable carbon isotope study on the dominant primary producers and consumers within the Kariega Estuary revealed that detritus originating from the high lying salt marsh plants Sarcocornia perennis and Chenolea diffusa was not utilized by fishes in the Kariega Estuary. Preliminary results indicated that the cord grass Spartina maritima may be an important energy source to the fishes in the Kariega Estuary, but further research is needed to confirm this.
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- Date Issued: 1999
Fish population dynamics in a temporarily open/closed South African estuary
- Authors: Cowley, Paul D (Paul Denfer), 1964-
- Date: 1999
- Subjects: Fish populations Estuarine ecology -- South Africa -- Eastern Cape Estuarine fishes -- Ecology -- South Africa Fish populations -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5354 , http://hdl.handle.net/10962/d1008543
- Description: The primary aim of this study was to investigate the population dynamics of the fishes associated with a small (17.5 hectares) temporarily open/closed estuary on the south east coast of South Africa. The results are based on the findings of an intensive sampling programme conducted over a period of four years in the East Kleinemonde estuary (33° 32' S : 27° 03' E). By adopting a quantitative approach, this study addresses the need for information on estuarine-associated fish population sizes, standing stock (biomass) estimates and productivity. The ichthyoplankton assemblage in the surf zone adjacent to the mouth of the estuary was dominated by postflexion larvae representing at least 21 taxa in 14 families. Rhabdosargus holubi of sizes ranging between 9 mm and 21 mm BL was the most abundant species with a mean density of 7.3 individuals per 100 m'. This species, which accounted for 77.6% of the catch composition, was recorded throughout the year but revealed a distinct peak in abundance in spring (August - September). The ichthyofaunal community within the East Kleinemonde estuary was dominated by juvenile marine-spawning species and typical of a warm temperate southern African estuary. A total of 30 species in 17 families was recorded, including the endangered estuarine pipefish Syngnathus watermeyeri. Multivariate analyses (classification and ordination) of the catch assemblages revealed a high degree of similarity (> 70%) throughout the estuary, with two distinct groups being identified on the basis of substratum type. The sampling stations near the mouth with a sandy substratum were distinguished from all other sampling sites in the estuary. The dominant estuarine-spawning species were represented by all life-history stages, suggesting that they bred successfully in the estuary. This group was numerically and gravimetrically dominated by the two zooplanktivorous shoaling species Gilchristella aestuaria and Atherina breviceps with density extrapolated population size estimates of 420 973 and 198 275 individuals, and biomass estimates of 1.6 and 0.6 g m⁻² respectively. The total population size of all estuarine-spawning species with a mean biomass 00.4 g m⁻² was estimated at 754 217 individuals. Population size estimates of the marine-spawning species were calculated using data obtained from three independent mark-recapture experiments. The assumptions for the mark-recapture analyses were adequately met and it was concluded that the techniques provided reliable estimates of population size. However, estimates obtained from density extrapolation revealed enormous variability and were considered to be unreliable. The total population size was estimated at 63 342, 18 592 and 13 5 192 during the three mark-recapture experiments respectively. The numerically dominant species during all three experiments was Rhabdosargus holubi. Biomass production of the marine-spawning species was evaluated over a 123 day census period when population sizes and estimates of growth rates were known. Productivity for all fishes with a standing stock of 26.2 g m⁻² was calculated at 4.5 g m⁻² month01 Rhabdosargus holubi accounted for more than 75% of the total marine fish productivity. This study draws attention to the success of Rhabdosargus holubi in the East Kleinemonde estuary, which is ascribed to aspects of its biology. These include an extended breeding season, the ability to recruit into the estuary under adverse open mouth conditions and its omnivorous food habits. The dominance of this migratory species suggests that it plays an important role in the transfer of energy to the coastal marine environment when the mouth of the East Kleinemonde estuary opens. Predation by birds and a dominant piscivorous fish (Lichia amia) was quantitatively assessed over a period of two years. Monthly food consumption by all piscivorous birds revealed large temporal variability, ranging from 32 to 466 kg month-I An unusual invasion of Cape cormorants during the winter of 1994 accounted for large scale mortality (2246 kg of fish) over a relatively short period. The predatory impact of this episodic event was reflected in the findings of the fish mark-recapture experiments, which revealed a 70% reduction in the total population of marine-spawning fishes (above a certain minimum size) in the estuary subsequent to this invasion. Monthly food consumption by the Lichia amia population in the estuary was calculated at 68 and 58 kg month-I for two distinct time periods when the population size was known. These findings suggest that this species is the top piscivorous predator in the East Kleinemonde estuary. Finally, the findings of this study highlight the temporal variability of fish populations within a single estuary. It is suggested that predation and estuary mouth conditions are the main factors influencing the abundance (and its variability) of individual species in the East Kleinemonde estuary.
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- Date Issued: 1999
The dynamics of a subtropical lake fishery in central Mozambique
- Authors: Weyl, Olaf L F
- Date: 1999
- Subjects: Fisheries -- Chicamba, Lake Fisheries -- Mozambique
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5212 , http://hdl.handle.net/10962/d1004785
- Description: Fisheries in African reservoirs are typically multi -species and in most cases the fish resource is harvested with a number of gears. These characteristics complicate their management and the development of management procedures. Typically, long time series of data on catch and effort and length- or age-based catch are not available for these fisheries. This precludes the use of data intensive methods such as multi-species virtual population analysis. The principal aim of this thesis was to develop a management procedure for African reservoir fisheries that takes into account the pertinent biological characteristics of the target species and accounts for the multi-species and multi-gear irIteractions in such fisheries. An opportunity availed itself to undertake this work on Lake ChicaIllba (19°08'S 33°08'E) a man-made hydroelectric dam in subtropical Mozambique (Manica province). The specific objectives of this study were: to obtain locality specific biological parameters for the target species in Lake ChicaIllba; to assess gear utilisation trends in the fishery through the determination of gear-selectivity, catch rate and effort for each of the principal gears used in the fishery; to assess the fishery using traditional per-recruit models and to test existing and new per-recruit models that account for the multi-species and multi-gear nature of the fishery and to determine the adequacy of each of these approaches in the determination of suitable target reference point (TRP) exploitation rates. The three principal specIes in Lake Chicamba are the introduced largemouth bass, Micropterus salmoides, and two cichlids the Mozambique tilapia, Oreochromis mossambicus and the redbreast tilapia, Tilapia rendalli. Sectioned otoliths were used for age and growth determination. Marginal zone analysis showed that annulus formation in all three species occurred during winter. The maximum-recorded age was 5 years for M. salmoides, 16 years for T. rendalli and 10 years for O. mossambicus. Growth of the three species was best described by the 3 parameter von Bertalanffy growth model as ℓa = 465.51 (1 - e⁻ₑ·ₑ⁷⁵⁽a⁺⁰·⁰⁰⁹⁾ mm FL for M salmoides; ℓa = 238.74 (1 - e⁻⁰⁶³⁶⁽a⁺⁰·⁹⁰⁵⁾) mm TL for T. rendalli; and ℓa = 266.06 (1 - e⁻⁰⁷⁹⁰⁴⁽a⁺⁰·²⁶⁹⁾) mm TL for 0. mossambicus. Female T. rendalli attained 50%-maturity at 2.89 years, while O. mossambicus matured at 2.83 years and M. salmoides at 0.9 years. Both cichlid species spawned throughout summer while M. salmoides had a very short spawning season from August to September. The total annual mortality rate (Z) for M. salmoides in Lake Chicamba was 1.27 yr⁻¹, the mean empirical estimate of natural mortality (M) was 0.73 yr⁻¹, and fishing mortality (F) was calculated at 0.54 yr⁻¹. For T. rendalli Z = 0.31 yr⁻¹, M = 0.20 yr⁻¹, F = 0.11 yr⁻¹ and for 0. mossambicus Z= 0.62 yr⁻¹, M= 0.38 yr⁻¹, F= 0.24 yr⁻¹. The three species exhibited reproductive traits, which implied a high reliance of recruitment on spawner stock (nest guarding in T. rendalli and M. salmoides and mouthbrooding in O. mossambicus). For this reason it was decided that the cichlid fisheries should be managed using TRPs which maintained the spawner biomass-per-recruit at 50% (FSB50) of pristine levels. However, based on good evidence it was hypothesised that the high rate of fishing mortality helped to maintain the fast growth rate of M. salmoides. It was, therefore, decided to manage this species at a TRP of F SB40. The three most important fishing sectors were the gill-net, seine-net and hook-and-line fisheries. The total catch for 1996 was 223 t. The gill nets selected all three species at a size/age approximating 50%-maturity while the seine-net and hook-and-line fisheries selected mainly juvenile fishes. There was strong evidence to suggest that seine net fishing also disrupted spawning. It was shown that the 'traditional' single-species per-recruit models were unsuitable to assess multi-species and multi-gear reservoir fisheries. Since existing multi-species/multifishery yield-per-recruit models were not capable of defining FsB(x) TRPs, a new multispecies/ multi-fishery spawner-biomass-per-recruit approach was developed. This approach allowed for the simulation of the response of spawner biomass-per-recruit to changes in effort in the three fishery sectors, simultaneously. The models showed that the spawner biomass-per-recruit, at current effort levels, was higher than the suggested TRP for the three species. However, it was shown that an increase of 10% in current total effort would reduce spawner biomass-per-recruit to below the recommended TRP levels. With the closure of the seine-net fishery, gill-net effort could be increased to 338 fishers (340 for management purposes) and effort in the hook-and-line fishery could be increased by 30% before the TRP was reached. To maintain the fish stocks above TRP levels, effort control was considered to be the most effective management method. The main recommendations for Lake Chicamba were to close the seine-net fishery, to limit the gill-net fishery to 340 fishers (using 137-m long x 3-m deep gill nets) and to maintain the open access nature of the hook-and-line fishery. The multi-species/multi-fishery per-recruit approach allows for the meaningful simulation of various scenarios and provides relatively robust management options. In the absence of long time series of effort and age- or length-based catch data, this approach was considered as the most suitable assessment method for multi-species/multi-gear African reservoir fisheries.
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- Date Issued: 1999