Browsing by Author "Govender, Anesh"

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    Comparative age and growth of juvenile dusky kob (Argyrosomus japonicus) in three South African estuaries, with notes on diet and temperature effects
    (2003) Mafwila, Samuel Kakambi.; Govender, Anesh; Griffiths, M H
    The dusky kob Argyrosomus japonicus is an important commercial and recreational linefish species in South Africa. It has a wide distribution range from Cape Agulhas to the Mozambique border. Juveniles A. japonicus <1070 mm) occur inshore and in estuaries where surf-zone and estuarine anglers target them. Adult A. japonicus (> 1070 mm) are caught beyond the surf zone by line fishermen. In this study, 380 juvenile dusky kob were caught by hook and line from the Breede River, Fish River, and Keiskamma River estuaries. Age of juvenile A. japonicus in these three South African estuaries was estimated by counting growth rings in sectioned sagittal otoliths read under a dissecting microscope using reflected light. The reproducibility of otoliths readings was described by the average percentage error (APE) index and was found to be 0.44% (Breede), 2.78% (Fish), 0.25% (Keiskamma) for an experienced reader and 0.5% (Breede), 4.37% (Fish), 0.53% (Keiskamma) for an inexperienced reader. Comparisons of A. japonicus otoliths by an experienced and an inexperienced reader indicated a reasonable between-reader consistency, however, the otoliths from the Fish River estuary were more difficult to read than from the other two estuaries. The Schnute growth model was used to determine which sub-model would best describe the growth of juvenile A. japonicus. Linear regression lines were fitted to the age-at-length growth curves of juvenile A. japonicus for each estuary to determine whether there were significant differences in growth rate. It was found that growth was the highest in the Breede River estuary (113.7 mm/yr), followed by the Keiskamma River estuary (92.1 mm/yr) and the Fish River estuary (83.9 mm/yr). Variable growth rates of juvenile A. japonicus in the three estuaries may be related to food availability and prey item types and to a lesser extent to water temperature in the estuaries. The comparisons of estuarine diets of A. japonicus indicate that juvenile dusky kob in different estuaries have different dominant prey species. There were one or two taxa dominant in each estuary with Galeichthys feliceps (61.9%), and the mysids (71.8%) being the most important prey items in the Breede River and the Great Fish River estuaries respectively, while teleost Pomadasys commersonnii (73.8%) were the most important prey species in the Keiskamma River estuary.
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    Does the implementation of a closed Fishing Season during the breeding Season benefit a Species? A per-recruit-based approach using Cymbula Granatina as an Illustration
    (2004) Arendse, Clement; Govender, Anesh; Branch, George
    Closed seasons are generally implemented on the presumption that they increase reproductive output of fished populations. This is.based on the assumption that the imposition of a closed season during the breed~· season allows more individuals to reproduce, as they are not being harvested.I evaluated the validity of imposing closed seasons during the breeding season creating a simulation model using the limpet Cymbula granatina as a test case and the compared following four scenarios: 1) an unharvested population; 2) no closed season imposed; 3) a closed season imposed during the breeding season; and 4) a closed season imposed outside the breeding season, to determine the effects of the latter three situations on the reproductive output and yield. From the outputs of the model it was determined that closed seasons do not significantly affect the reproductive output of the population and that the timing of closed seasons made no difference to the reproductive output of the population, as the output was the same for populations with closed seasons during the breeding season or outside the breeding season. Survivors, catch in numbers and yield in biomass were affected by the timing of closed seasons, with higher outputs for each obtained when the closed season was closer to the month when individuals become of harvestable size. It was thus concluded that the imposition of a closed season at any time of the year is an effective management measure if imposing the closed season can reduce annual fishing, but the imposition of a closed season specifically during the breeding season with a view to increase reproductive output brings no benefits relative to closure at any other time of the year. Moreover, any closure will be ineffective if it does not also bring about a reduction of annual fishing effort. The reasons for advocating closure of a fishery during the breeding season are thus based on false ground.
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    Estimates of numbers of White sharks (Carcharodon carcharias) in Eastern and Southern South Africa : a post-moratorium assessment
    (2004) Tress, Mary Elizabeth; Govender, Anesh
    A moratorium was placed on fishing for white sharks (Carcharodon carcharias) in South Africa in 1991 prior to knowing the population status of this species. A mathematical model was developed to estimate the total numbers of white sharks from 1950 to 2050 in the presence of fishing (F = 0.055 year-I) and without fishing (moratorium). The total number of white sharks from Richard's Bay to Struis Bay, South Africa was estimated to be 1,954 (range= 1,855-2,050) for the year 2004. If the moratorium was to be lifted and fishing was to resume, the results of the model indicate that the instantaneous fishing mortality rate F of 0.035 year-l or less allows the population to increase in numbers from 2004 to 2050. A sensitivity analysis determined that the instantaneous natural mortality rate M (tested range=0.070-0.l90 year-I) and the average number of pups per mature pupping female (tested range=4-14 pups) were highly sensitive parameters of the model. The maturity age span (maturity minimum age=13 and 16 years; longevity=23, 36, and 60 years) and timeframe of the female reproductive cycle (two or three years) were the least sensitive parameters of the model.
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    Loggerhead (Caretta caretta) and Leatherback (Dermochelys coriacea) nesting activity along the Maputaland coast (South Africa) : 1965 - 2002
    (2004) Wright, Ruth Vivienne; Nel, Roel; Govender, Anesh; Branch, George M
    From 1963 until the present, loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) female turtles nesting on a 56-km stretch of beach in Maputaland, on the northeast coast of KwaZulu-Natal, South Africa have been tagged and monitored. In this study, population trends, size, preference for timing of nesting and nesting sites, and seasonality in nesting activity and nesting area were analysed and the effectiveness and efficiency of the monitoring programme assessed. Since the programme's inception, approximately 46893 loggerhead and 11509 leatherback females have nested on the Maputaland beaches. A non-linear relationship was found between the numbers of nesting females of both species over time, with three distinct phases. large interannual variations in nesting numbers were evident for both species, as were synchronized seasons of elevated or depressed nesting activity. While Maputaland leatherbacks appear to be approximately the same size as in populations elsewhere, the loggerhead females were significantly smaller compared to other populations. Peak nesting activity occurred from November to January and was associated with warmer sea temperatures, as was the subsequent emergence of hatchlings. Marginal temporal shifts in the nesting numbers of both species were evident during peak nesting-activity seasons, with the peak nesting period beginning slightly earlier than usual. Peak nesting-activity seasons appeared to follow EI Nino events with a lag period of between 0 and 2 years. Separate 'preferred' nesting areas were found for each species, largely associated with the offshore seabed topography. The loggerheads exhibited a fairly narrow nesting-area distribution in the north, while the leatherbacks utilised a far wider area in the southern region. The length of the 'preferred' nesting area (km) was strongly correlated with the total number of nesting loggerhead and leatherback females. No major nesting-area shifts were evident over the programme's duration. However, distinct spatial shifts, with increases in the length of the 'preferred' nesting area, were evident for both species during peak nesting-activity seasons, and was possibly related to the increased influx of nesting females. Interspecific competition appeared to be of little importance in Maputaland, as both populations displayed strong synchrony in nesting declines and increases over time, and while both populations have increased, no major shifts in the 'preferred' nesting area of either species have occurred. No correlations were found between the Southern Oscillation Index (SOl) values and nesting activity or changes in nesting area. However, a negative correlation was found between the average curved carapace lengths (CCl) of both species and the SOl values, most likely related to sea surface temperatures and the resultant changes in food availability that occur during EI Nino events. Disturbance incurred during monitoring caused, at most, only a 5% reduction in the nesting rates of both species. To date, the conservation efforts of the monitoring programme appear to have come to fruition, with both the loggerhead and leatherback nesting populations healthy and in a more-or-Iess stable state. The importance and value of continuing the current monitoring programme was emphasized, and several management and sampling strategies proposed for future seasons.
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    Seasonal and lunar trends in catch rates of sharks caught in the protective gill nets off KwaZulu-Natal, South Africa
    (2006) Lundemo, Ola Andreas; Govender, Anesh; Griffiths, Charles L; Wintner, Sabine
    A retrospective statistical analysis of catch and effort data for sharks captured in protective gill nets along the KwaZulu-Natal coastline was carried out with the objective to establish if the frequency of shark catches is related to different phases of the moon and secondly to season.