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Masters Degrees (Marine Biology)

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Browsing Masters Degrees (Marine Biology) by Author "Fennessy, Sean Thomas."

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    An assessment of abundance trends and biology of langoustines (Metanephrops mozambicus) and pink prawns (Haliporoides triarthrus) from the deep-water trawl fishery off eastern South Africa.
    (2013) Robey, James.; Groeneveld, Johan Conrad.; Fennessy, Sean Thomas.
    Deep-water trawling (>200 m deep) for crustaceans in the South West Indian Ocean (SWIO) yields catches of several species, including prawns (Haliporoides triarthrus, Aristaeomorpha foliacea, Aristeus antennatus and Aristeus virilis), langoustine (Metanephrops mozambicus), spiny lobster (Palinurus delagoae) and geryonid crab (Chaceon macphersoni). Infrequent deep-water trawling takes place off Tanzania, Kenya and Madagascar; however, well-established fisheries operate off Mozambique and South Africa. Regular trawling off South Africa started in the 1970’s, mainly targeting M .mozambicus and H. triarthrus. Catch and effort data for the South African fishery were regularly recorded in skipper logbooks over a 23 year period (1988 – 2010); this database was obtained from the Department of Agriculture, Forestry and Fisheries (DAFF) in order to assess abundance trends of M. mozambicus and H. triarthrus. Generalized linear models (GLM) were used to quantify the effects of year, month, depth and vessel on catch per unit effort (CPUE). By year, the standardized CPUE of M. mozambicus increased, and three factors (or a combination of them) could explain the trend: reduced effort saturation, improved gear and technology, or an increase in abundance. By month, CPUE peaked in July and was highest between depths of 300 and 399 m. The standardized CPUE of H. triarthrus fluctuated more by year than for M. mozambicus, possibly because it is a shorter-lived and faster growing species. The monthly CPUE peaked in March, and was highest between depths of 400 and 499 m. Totals of 2 033 M. mozambicus (1 041 males and 992 females) and 5 927 H. triarthrus (2 938 males and 2 989 females) were sampled at sea between December 2010 and March 2012, during quarterly trips on-board a fishing trawler. A GLM framework was used to explore their size composition, sex ratio variability, size at maturity and reproductive cycles. Male and female M. mozambicus size distributions were similar, but varied by month and decreased as depth increased. Female H. triarthrus were significant larger than males; size structure varied by month, but showed no change over depth. The sex ratio of M. mozambicus favoured males (1 : 0.89), but was close to parity in all months, except November when males predominated. H. triarthrus exhibited parity (1 : 1.002) with no significant variations in sex ratios by month. The proportion of egg-bearing M. mozambicus in the population declined between March and August (hatching period) and then increased until December (spawning period). The L₅₀ (length at 50% maturity) of M. mozambicus was estimated to be 49.4 mm carapace length (CL), and the smallest and largest observed egg-bearing females were 33.5 and 68.6 mm, respectively. No reproductively active female H. triarthrus were recorded during the sampling period. Growth parameter estimates for M. mozambicus (male and female combined) using Fabens method were K = 0.48 yearˉ¹ and L∞ = 76.4 mm CL. Estimates for the von Bertalanffy growth formula (VBGF) were: K = 0.45 yearˉ¹ and L∞ = 76.4 mm CL. H. triarthrus male and female growth parameter were estimated separately. For males they were K = 0.5 yearˉ¹ and L∞ = 46.6 mm CL using Fabens method, and K = 0.76 yearˉ¹ and L∞ = 46.6 mm CL using the VBGF. For females they were K = 0.3 yearˉ¹ and L∞ = 62.9 mm CL using Fabens method, and K = 0.47 yearˉ¹ and L∞ = 62.9 mm CL using the VBGF. CL to total weight regressions were calculated for both species; no significant differences were found between male and female M. mozambicus, although H. triarthrus females became larger and heavier than males. Comparisons with three earlier studies (Berry, 1969; Berry et al., 1975; Tomalin et al., 1997) revealed no major changes in the biology of either species off eastern South Africa. Stocks appear to be stable at current levels of fishing pressure, although some factors are not yet fully understood. Disturbance caused by continual trawling over a spatially limited fishing ground may affect distribution and abundance patterns, especially in M. mozambicus, which was less abundant in the depth range trawled most frequently. The absence of reproductive H. triarthrus in samples suggests that they occur elsewhere, and there is some evidence of a possible spawning migration northwards to Mozambique; this suggests that H. triarthrus is a shared stock between South Africa and Mozambique. The results from this thesis will add to the knowledge of M. mozambicus and H. triarthrus in the SWIO, and provide a basis for developing sustainable management strategies for the deep-water crustacean trawl fishery off eastern South Africa.
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    An assessment of changes in the Ichthyofaunal bycatch of the Tugela Bank prawn trawlers in KwaZulu-Natal.
    (2006) Mkhize, Mbali.; Fennessy, Sean Thomas.; Van der Elst, Rudy P.
    Bycatch refers to the portion of the catch that is captured incidentally to the target species and is one of many growing problems facing the world's marine fisheries in recent years. Prawn trawling is one of the world's most lucrative marine fisheries, contributing about 3% to the total annual production of the world's marine fisheries. It is also one the most wasteful because it is associated with large quantities of bycatch. This is because of the high diversity and abundance of other organisms in areas where prawn trawling occurs, and the non-selective nature of the otter trawl used to land prawns. The South African shallow water prawn trawl fishery is typical of penaeid fisheries, and operates on the Tugela Bank off northern KwaZulu-Natal. The bycatch of penaeid prawn trawlers operating on the Tugela Bank was analysed between March 2003-July 2004 as a follow up to an initial study between May 1989-June 1992. The aim of this study was to determine if there has been any change in bycatch species composition over the 13-years and to determine the impacts of trawling on the environment. Catch composition data were recorded from 168 trawls processed onboard trawlers and on-shore. A total of 122 species was identified with teleosts contributing more than 60% to relative abundance by number. Comparison between the 1989-1992 and 2003-2004 data sets showed that although the species compositions were similar, there was an increase in the relative abundance of pelagic species. It is not clear if the change is due to trawling impacts or due to fluctuations in recruitment of these taxa. The annual discarded bycatch was estimated at 88 tonnes in 2003 and the prawn catch was estimated at 1 1 tonnes, both substantially lower than in 1 989-1992. There was no difference in discard rate between trawls from shallow and medium depths. However, there were significant differences in discard rates between large and medium gears, and warm (December-April) and cool (May- November) seasons. Kolmogorov-Smirnov tests revealed that there were significant differences between the 1990s and 2000s length frequency distributions of three sciaenid species (Otolithes ruber, Johnius dorsalis and J. fuscolineatus), which all showed an increase in fish size for the 2003- 2004 data. This assessment, however, is confounded by a gear effect as mesh size differs between the two sample periods. Based on the results of the study, management recommendations were made to improve the management of the Tugela Bank prawn trawl fishery by reducing bycatch. It was recommended that the observer programme continue for further investigation into the changes in species dynamics. Further research on BRDs (Bycatch Reducing Devices) and gear types that reduce bycatch is also recommended.