Browsing by Subject "Robben Island"

Now showing 1 - 6 of 6
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    Open Access
    Analyses of the results from the island closure feasibility study for the Dassen/Robben and St Croix/Bird Island Pairs
    (2014) Robinson, William M L; Butterworth, Doug S; Furman, Liam B
    The results from the Island Closure Feasibility Study are analysed using the GLMs as set out at the 2010 international stock assessment workshop. Estimates of residual variance for a random year effects GLM for the various penguin response variables are considered to be sufficiently precise to enable power analyses to be conducted to contribute to the evaluation of whether to transition to a full experimental closures programme, so that the feasibility study may be considered successfully concluded. For the Dassen and Robben Islands about 80% of the estimates of the fishing effect parameter λ are positive, with this same proportion maintained for those (about one sixth) of these estimates which are significant at the 5% level. Thus the preponderance of the evidence from these analyses is that the impact of fishing around these islands has been positive. The rather fewer instances available to analyse for the Eastern Cape colonies suggest a weakly positive effect at Bird Island, but a somewhat stronger negative effect at St Croix. The power analyses suggest that in cases for Dassen and Robben Islands where further data collection might render currently non-significant λ estimates significant at the 5% level within the next two decades, the likely period required for such further collection would typically be in the vicinity of five years. The advantage provided by continuing the closure programme itself seems however to be slight, as the natural variation over time in normal catches would be sufficient to provide the contrast to achieve such results with only typically two years’ extension to those five years. For the two Eastern Cape islands, it seems that results which are statistically significant at the 5% level seem unlikely to be achieved in the foreseeable future – a result which may be a consequence of the relatively low levels of sardine catches typically taken close to those islands.
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    An analysis of the small scale surveys of anchovy abundance around Robben and Dassen islands from 2009 to 2013
    (2014) Robinson, William M L; Butterworth, Doug S
    The results from the small scale hydroacoustic surveys of the abundance of anchovy around Robben and Dassen islands over the 2009-2013 period are analysed under the assumption of a Gaussian form for the trends in density at each island over the course of the winter months. Based primarily on AICc, the model selected from amongst a number of variants has the same trend in abundance with year for the two islands, compatible with the assumption used by Robinson (2013) in his GLM analysis of the impact of closures to pelagic fishing around these islands on penguin recovery, though the data have limited power to distinguish deviations from that assumption. The abundance estimates from the island surveys, though compatible also with the May recruitment survey trends, show appreciably larger variance. This raises the question of whether these small scale surveys merit continuation, unless it is possible to increase their frequency considerably during the winter months each year to improve the overall precision of the integrals over local abundance which they can provide.
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    Aspects of modelling Robben Island African Penguin Spheniscus demersus populations
    (2006) Plagányi, Éva E; Butterworth, Doug S
    Crawford et al. (2006) present relationships between the breeding success of Robben Island African penguins Spheniscus demersus and the abundances of both anchovy Engraulis encrasicolus and sardine Sardinops sagax. Based on their analysis, they suggest that the management of the purse-seine fishery should ensure adequate escapement of fish to maintain the combined biomass of anchovy and sardine above two million tons. Given that this potentially has important repercussions for the management of the pelagic fishery, their analyses have highlighted the importance of further and more detailed investigations into this issue. This paper provides a brief summary of some further considerations in this regard.
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    Open Access
    Comments on the benefit to penguins of fishing restrictions around Robben Island predicted by Weller et al. Robben Island penguin model simulations
    (2014) Robinson, William M L; Butterworth, Doug S
    The following comments relate to results in a recent publication (Weller et al., 2014) that are pertinent to the discussions of the Island Closure Task Team, specifically the impact of fishing around Robben Island on the penguin population. Three scenarios were explored in that paper, each starting with 3500 adult penguins.
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    Open Access
    Full description of the Robben Island penguin-fish interaction model and base case Bayesian results
    (2011) Robinson, William M L; Butterworth, Doug S
    Work on the penguin–fish interaction model has progressed to the point where a reasonable base case for Robben Island has been achieved.
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    Update to the Robben Island moult count series for 2011
    (2011) Robinson, William
    The 2010/2011 penguin moult season conventionally runs from 1 July 2010 to 30 June 2011. During this period, counts of moulting birds were made on 20 occasions (Table 1). These counts have now been converted to aggregates for the year using the technique of fitting the sum of two Gaussian curves and calculating the area under the function (Robinson and Butterworth 2010).