Browsing by Author "Durholtz, Deon"

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    Age, growth, reproductive biology and diet of Spondyliosoma emarginatum (Cuvier 1830) in the West Coast National Park, South Africa
    (2005) Fairhurst, Lucinda; Moloney, Coleen; Attwood, Colin; Durholtz, Deon
    Comprehensive morphometric measurements and counts, external colouration, reproductive behaviour and diet of steentjies (Spondyliosoma emarginatum) in Langebaan Lagoon were recorded between September 2004 and February 2005. Sagittal otoliths were extracted and age estimates recorded from both whole and sectioned otoliths in order to estmate growth and mortality parameters. A total of 302 steentjies (139-312 mm fork length) was sampled of which 243 were females, 55 were males and four were individuals with hermaphroditic gonads. Macroscopic and microscopic examination of gonads and monthly gonadosomtic indices suggest a spring spawning season and that steentjies are protgynous hermaphrodites with males significantly than females (t = 64.75, df = 301, p < 0.001). During the reproductive season, males develop a blue colouration. All males with this colouration had testes classified as either active or ripe and had significantly larger gonad masses than those without mating colours (U = 3.5, Z = 6.48, p $lt; 0.001). The length-at-50%-maturity for females was 171 mm (FL) and for males was 204 mm (FL). The size at 50% sex change was estimted as 250 mm (FL). Analysis of foregut contents showed steentjies to be opportunitistic ominivores, changing from plant to animal food as they age. The average percent error and coefficient of variation values for whole otoliths were 4.59% and 6.00% respectively, and 10.89% and 14.53% for sectioned otoliths. Although the result of the whole otolith study provided more precise age estimates, there was a strong indication of bias so the sectioned otolith results were used for calculating the growth model. Growth (by sex) was modelled using a model developed for sex changing sparids that allows for accelerated growth after sex change. Steentjies appear to have shorter life spans than most sparids; the oldest individuals sampled attaining less than 7 years of age. Growth rates appear to be relatively high. The smallest, youngest fish were all female, with males only present in the larger size classes. The estimated mortality for steentjies was 0.7y[-1]. Such a high mortality rate implies that further increase in fishing pressure on this species may not be sustainable in Langebaan Lagoon.
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    Aspects of the respiratory metabolism of juvenile rainbow trout (Oncorhynchus mykiss) at sustained swimming speeds
    (1993) Durholtz, Deon; Cook, Peter
    A detailed description of the design of an apparatus suitable for investigations into the respiratory physiology of small nektonic organisms at various swimming speeds is presented. Juvenile rainbow trout (Oncorhynchus mykiss) were used to evaluate the performance of the apparatus. Twenty-four hour experiments indicated no detectable diurnal variation in the respiration rates of juvenile rainbow trout. Experiments investigating aerobic energy expenditure at sustained swimming speeds, through measurements of oxygen consumption rates, are described. Oxygen consumption was found to increase exponentially with increasing swimming speed. The influence of body size on this relationship was examined. The contribution of anaerobic metabolism at sustained swimming speeds was investigated.
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    Bridge-building between the 2017 and 2018 hake RC assessment models
    (2018-05) Ross-Gillespie, Andrea; Butterworth, Doug S; Durholtz, Deon
    A step-by-step set of results is given to indicate the various impacts of various changes made to the hake assessment Reference Case model over the past 12 months.
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    Considerations on management of the chokka squid jig fishery with reference to the pending FRAP 2020 allocation of fishing rights and implementation of Small Scale Fisheries
    (2018-08) Mwicigi, Jean; Durholtz, Deon; Glazer, Jean; Butterworth, Doug S
    The South African squid jig fishery is primarily managed using effort control. The management objective for the fishery is to cap effort at a level that secures the greatest catch, on average, in the longer term without exposing the resource to the threat of reductions to levels which future recruitment success might be impaired or catch rates drop below economically viable levels. Assessments of the dynamics and status of the resource, and subsequent scientific advice on the management of the jig fishery targeting the resource (primarily expressed as a Total Allowable Effort recommendation), are based on an effort metric of person-days. The current TAE is set at 270 000 person-days. Fishing Rights are, however, allocated in terms of the number of fishers that are permitted to operate in the fishery (2 451 fishers). This mismatch has required that within the current structure of the fishery an additional closed season has had to be implemented to ensure that the management objective for the fishery is not compromised. The implications in terms of the pending 2020 allocation of fishing rights and establishment of a Small Scale Fishery are discussed, and alternative management scenarios are explored.
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    Examination of redeye roundherring size and age data - towards developing a growth curve for the redeye stock assessment
    (2010) Durholtz, Deon; de Moor, Carryn; Geja, Yonela
    Redeye roundherring (Etrumeus whiteheadi) in South African waters is exploited by the pelagic purse seine fishery, which primarily targets anchovy (Engraulis encrasicolus) and sardine (Sardinops sagax). In spite of a relatively large adult biomass (averaging 1.3 million tons over the last 10 years), redeye catches have been low, averaging 50 000 tons per annum over the last 10 years. The redeye fishery is currently managed with an annual Precautionary Upper Catch Limit (PUCL) of 100 thousand tons. Furthermore, the Operational Management Procedure for sardine and anchovy (de Moor and Butterworth 2008) makes provision for some by-catch of adult sardine [3500 t] with redeye. Only once in the history of the fishery, has the industry managed to come close to this PUCL (92 thousand tons in 1997). In view of the under exploitation of the redeye resource and possibility of expanding the fishery, work has been directed at assessing the status of the redeye to support the management of the resource. Implicit in such an assessment is the requirement for information on the age and growth of the species.
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    Updated calculations related to the additional closed season.
    (2019) Glazer, Jean; Butterworth, Doug S; Durholtz, Deon
    The length of the additional closed season which would need to be applied to the squid jig commercial sector to keep effort within the TAE has been re-calculated taking into account that the TAE recommendation for the 2019/20 fishing season by the Squid Scientific Working Group is 295 000 person-days, and further information about effort levels applied in the fishery over recent years. These calculations suggest that the additional closed season should remain of three months duration.