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Doctoral Degrees (Marine Biology)

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Browsing Doctoral Degrees (Marine Biology) by Author "Beckley, Lynnath E."

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    Comparative life histories and stock assessments of rockcods (family Serranidae) from the east coast of South Africa.
    (2000) Fennessy, Sean Thomas.; Beckley, Lynnath E.; Sadovy, Yvonne.
    The family Serranidae is a diverse group of fishes, of which the genus Epinephelus (rockcods or groupers) is the largest. Serranids are commonly caught in reef fisheries in tropical and warm-temperate latitudes, and are targeted because of their tasty flesh and high value. In South Africa, epinepheline serranids mainly occur in hook and line fisheries in the province of KwaZulu-Natal. Mostserranids are caught by the boat-based (skiboat) fishery, and the commonestspecies are the endemic catface rockcod (Epinephelus andersoni), thehalfmoon rockcod (E. rivulatus), the yellowbelly rockcod (E. marginatus) and the endemic white-edge rockcod (E. albomarginatus). Although serranids contribute about ten percent to catches by the commercial and recreational skiboat sectors in KwaZulu-Natal, representing an estimated total catch of around 200 mt per year, little is known about these fishes in South Africa. From the mid-1980s to the mid-1990s, the mean lengths of E. marginatus and E. albomarginatus in the region declined significantly. Over this period, lengths of E. andersoni remained the same, while those of E. rivulatus increased. Lengths of E. marginatus and E. albomarginatus from Mozambique, where fishing effort was low at the time of sampling, were significantly greater than in KwaZulu-Natal. Monthly biological data were mostly collected from commercial skiboat catches on the northern and southern coast of KwaZulu-Natal. Additional data for E. marginatus and E. albomarginatus were also collected irregularly from commercial catches made in Mozambique. Unless the fish had ripe ovaries, all gonads had to be sectioned to establish sex and stage. Histology revealed that all gonads had a female-like appearance, with lamellae and a central lumen. In E. andersoni, there was a complete overlap of male and female length frequencies, and their meanlengths were not significantly different. Some males and inactive bisexuals were both smaller and younger than the female size and age at first maturity. Together with the occurrence of mature bisexual fish (transitionals), these observations indicate that males are derived from immature or mature females, hence this species is a diandric protogynous hermaphrodite. The other three species exhibit typical signs of monandric protogynous hermaphroditism. Males and females had significantly different mean lengths, and age and length frequencies by sex werebimodal. Transitional individuals were recorded in E. rivulatus. E. andersoni and E. rivulatus matured at small sizes and early ages relative to E. marginatus and E. albomarginatus. Ripe ovaries were much larger than ripe testes in all four species. E. andersoni, E. marginatus and E. albomarginatus spawned in spring and summer, while E. rivulatus spawned in winter and spring. There were no indications of spawning in E. andersoni in the southern sampling region, and few ripe individuals of E. albomarginatus were encountered in KwaZulu-Natal samples. Size at maturity of this species was much smaller in Mozambique samples. Large, reproductively inactive individuals of E. andersoni were frequently observed in the spawning season. The lack of reproductive activity of E. andersoni and E. albomarginatus in KwaZulu-Natal may be because this area represents thesouthernmost limit of the distribution of these species. Ageing of the four species was undertaken using sectioned otoliths. Age validation was undertaken by a combination of tetracycline marking in captive fishes, and analysis of the marginal zone in otoliths. All four species are relatively long-lived, although estimates of maximum age may be under-estimated because of long-term harvesting. In all four species, fish from the southern sampling region were larger than fish from the northern region at the same age. Only in the case of E. rivulatus were these significant enough to warrant the fitting of two growth curves to the northern and southern populations. Males in all four species tended to be larger than females at the same age, suggesting that there may be a growth spurt following sex change, or that faster-growing females changed sex. A logistic growth curve was fitted to the age-length data for E. andersoni, while von Bertalanffy curves produced the best fit for the other species. Based on the rates at which L∞ attained in these four species, E. marginatus and E. albomarginatus are slow-growing species, while E. andersoni and, particularly, E. rivulatus arefaster growing. Rates of total mortality and natural mortality were estimated using length-converted catch curves and the Rikhter and Efanov equation, respectively. Stock assessments undertaken by yield per recruit and spawner biomass per recruit analyses indicate that E. andersoni in KwaZulu-Natal is currently optimally exploited, while E. rivulatus is lightly exploited. Both E. marginatus and E. albomarginatus are over-exploited. The potential problems in applying standard per recruit models to species with complex life histories are discussed. Support for the reduced stock status of the latter two species is provided by the observed changes in lengths of these species over a ten-year period, and their relatively small size in KwaZulu-Natal compared to the lightly-fished Mozambique populations. Local fishers in KwaZulu-Natal have also reported declines in sizes and reduced catches of these two species.The life history styles and other features of the four species are compared and discussed with reference to the resilience of these species to harvesting. Two of the species (E. marginatus and E. albomarginatus) are monandric protogynous hermaphrodites, which grow slowly, mature late and attain large sizes. E.andersoni and E. rivulatus grow faster, mature earlier and are smaller species. The normally deleterious effects of fishing on sex-changing species are not manifested in these two species, possibly because E. rivulatus is so small, that males are not selectively removed. In contrast, E. andersoni is a diandric protogynous hermaphrodite, and hence, does not rely on sex-change as a source of males. The current management methods for serranids in KwaZulu-Natal are presented, and suggestions for future approaches are discussed.
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    Mark-recapture models for determination of mortality, migration and growth in Pomatomus saltatrix (Teleostei)
    (1996) Govender, Anesh.; Beckley, Lynnath E.; Cochrane, Kevern.
    This study primarily attempts to develop models to estimate population dynamic parameters from mark-recapture data. Model implementation is illustrated using data collected from the South African Pomatomus saltatrix fishery . The models developed allow for the estimation of mortality, survival and migration rates in exploited fish stocks. A growth model is also developed which simultaneously estimates growth parameters as well as validates the hard structure banding using age-length and markrecapture data. There are number of advantages to these models . Given appropriate datasets the mark-recapture models developed in this study can be applied to others species of interest. The models can be modified easily e.g. the growth model can incorporate growth functions other than the von BertalanfIy model. The models can be programmed into a spreadsheet which facilitates the estimation of parameter variances using likelihood profile or bootstrapping methods and allows the testing of model assumptions based on simulations. A general mortality model is developed and is illustrated with mark-recapture data from the P. saltatrix fishery. The model provides an estimate of the average fishing mortality for the Cape and KwaZulu-Natal and is then extended to include movement between the Cape and KwaZulu-Natal. It utilises mark-recapture data from the Sedgwick's-ORI Tagging Programme as well as effort and catch data from the National Marine Linefish System (NMLS). Estimates of annual fishing mortality rates in KwaZulu-Natal are derived from the model which takes into account immigration of P. saltatrix into KwaZulu-Natal from the Cape as well as emigration from KwaZulu-Natal to other areas including the Cape. The average fishing mortality rate was estimated to be 0.27 year" between 1984 and 1993 in the Cape and KwaZulu-Natal combined. This is likely to be underestimated because of non-reporting of tags, shedding of tags and tag-induced mortality. The model is shown to be robust for estimating the average fishing mortality rate and exploitation rate only when annual variability in fishing mortality is small during the study period. The second model to quantify migration into and out of KwaZulu-Natal waters suggested that the whole adult Cape stock migrates into KwaZulu-Natal during winter. Further, this whole stock is available to fishing in KwaZulu-Natal although there is probably large exchange between inshore and offshore areas and, in the latter zone, P. saltatrix is inaccessible to shore-based fishing. Large fishing mortality rates for the years 1987 to 1993 were estimated in KwaZulu-Natal. These large fishing mortality rates may prevent the return migration of P. saltatrix to the Cape and the model predicts that possibly less than 4% actually return to the Cape. An age and growth study based on otolith readings was also undertaken. Validation of the growth banding as annual was confirmed by developing a model that estimated growth parameters using age-length data and simultaneously estimating times-at liberty of tagged individuals based on arbitrarily chosen band deposition periodicities. It is shown that the assumption of annual banding led to the best prediction of periods of liberty of tagged individuals with small coefficients of variations in the parameter estimates. However, since only a few tagged animals were used in the analysis more research is needed to verify the robustness of this technique for use on other fish. The growth of P. saltatrix in the present study was found to be faster than that of a previous study in South Africa. A modified delay-difference model was developed to estimate relative biomass and relative catch based on observed mean body weights and effort indices. For the period 1956 to 1972 the model predicts that there was a decline in P. saltatrix abundance with corresponding declines in mean weight of the catch. Although during this period there was a general decline in fishing mortality, the fishing mortality was sufficiently high for growth overfishing to occur. It was found that during the 17 year period there was a 44% reduction in biomass which is similar to an estimate in another study. Catch during the period was annually variable but generally declined with time especially in the later years. The decline in average weight harvested and the variable but lower catches during this period are consistent with observations by fishers . An evaluation of the present closed season for P. saltatrix in conserving egg production was performed. It showed that better conservation of egg production is possible by shifting the present closed season (September to November) to extend over the October to December season but this may adversely affect the tourism industry in KwaZulu-Natal. Shortening the present closed season by one month (September) does not affect egg production but increases present yield levels. This study suggests that the closed season may not be useful in terms of reducing the fishing mortality rate on P. saltatrix as fishers may be encouraged to fish harder in the open season to make up for the lost yields of the closed season. Moreover, lengthy closed seasons may also increase fishing mortality because fishers tend to fish harder in the months open to fishing. Assuming no large annual recruitment variations the P. saltatrix stock is presently optimally exploited as current fishing mortality rates are just below the MSY or optimum yield levels.