Browsing by Author "Somhlaba, S"
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- ItemOpen AccessFurther results when interactions between the presence of observer and month are taken into account for steel vessels(University of Cape Town, 2005) Somhlaba, S; Brandão, Anabela; Butterworth, Doug SInteractions between the presence of an observer and month are investigated for both anchovy and sardine fisheries. The aim is to ascertain whether the effect of the presence of an observer still remains significant after these interactions have been taken into account, and also how this effect varies over a twelve month period. The evaluation of these interactions and approach gives a clear indication of a trend in the effect of the observer on catch rates for each month. Using the trend that emerged from the interactions, the observer factor was redefined with three levels, two levels indicating the presence of an observer for each six month period of the year and the other level indicating absence of an observer.
- ItemOpen AccessAn indirect investigation of the possible influence of port on the estimation of observer effects on catch rates in the pelagic fishery(University of Cape Town, 2006) Somhlaba, S; Brandão, Anabela; Plagányi, Éva E; Butterworth, Doug SEarlier GLM analyses of catch per hour data for sardine and anchovy to determine observer effects have suggested statistically significant increases in this catch rate with observers on the vessels over January-to June for the sardine fishery, and over May to October for the anchovy fishery (Somhlaba et al. 2006). This in turn has raised the possibility that this may reflect (and provide quantification of the extent of) slippage in the catching operations conducted without observers present.
- ItemOpen AccessOverall results for the effect of the presence of an observer on catches made by the pelagic fleet(2005) Somhlaba, S; Brandão, Anabela; Butterworth, Doug SThe analysis of the observer data for the pelagic fishery is taken further for the complete data base from 1999 to 2004. Previously the analysis had been based on steel vessels (Somhlaba et al., 2005); those findings showed that there were statistically significant positive effects on catches given the presence of an observer on the steel vessels. The results below include the analysis of the combined data for all three categories of vessels: bait, ordinary and steel vessels. Sardine and anchovy data were analysed in detail using one model but with two different error structures: one lognormal (henceforth termed LogCPUE) and the other Poisson (henceforth termed catch).
- ItemOpen AccessPreliminary results of observer effect on pelagic fleet taking into account observer data from 1999 to 2011(University of Cape Town, 2014) Somhlaba, S; Brandão, Anabela; Butterworth, Doug S; van der Westhuizen, JA detailed update of the study of the data collected by scientific observers onboard the pelagic vessels is currently underway. The use of small pelagic commercial data base and an observer data base to investigate the effect of observers onboard commercial vessels has been an ongoing study since the mid-2000s (e.g. see Somhlaba et al, 2005, 2006, 2007). This paper updates those studies by including data from the more recent years 2008 to 2011.
- ItemRestrictedRevised results on the effect of observers on board pelagic fleets(University of Cape Town, 2005) Somhlaba, S; Brandão, Anabela; Butterworth, Doug SThis is a revised report on pelagic data analyses which aim to quantify the amount of dumping, or confirm the absence thereof, that is happening in pelagic fisheries. This study includes the analyses of anchovy data which was not included in the previous papers (Somhlaba et al, 2004 a,b) and two extra years’ data for sardine that were not included previously. Furthermore the effects of additional factors not included in the previous papers are investigated. Some diagnostic testing of residuals is also conducted for the chosen models.