Browsing by Subject "trends"

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    Making sense of censuses and dealing with missing data: trends in pup counts of Cape Fur Seals Arctocephalus pusillus pusillus between 1972-2004
    (2007) Kirkman, S P; Oosthuizen W H; Meÿer, M A; Kotze, P G H; Roux, J-P; Underhill, L G
    Trends in the population of Cape fur seals Arctocephalus pusillus pusillus were estimated from counts of pups on aerial photographs of colonies taken between 1972 and 2004 to determine trends in the overall population and subpopulations. Incomplete coverage resulted in missing data in some years. Various methods of determining proxy values for missing data were assessed, and it was concluded that different methods were applicable to Namibian and South African colonies. This reflected variation in trends of pup counts between the countries, which was associated with differences in productivity between the southern and northern Benguela ecosystems. In Namibia, temporal changes in pup numbers were non-linear in some years and there was correspondence in fluctuations at most colonies. This appeared to be on account of an effect of periodic, wide-scale prey shortages that reduced birth rates. There was a northward shift in the distribution of seals in the northern Benguela system. In South Africa, pup counts were much less variable between years, probably on account of a relative stability of food supply. A linear approach was therefore suitable for determining proxy values for missing data at South African colonies. Pup counts suggest that there has been little change in the overall population of the Cape fur seals since 1993, when it was estimated at about two million animals.
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    Open Access
    Results for the updated trends in policing effort and the number of confiscations for West Coast rock lobster using the “old” database for the period 2008 to 2017
    (2018-08) Brandão, Anabela; Butterworth, Doug S
    GLM methods are applied to compliance data (the “old” database) on confiscations (and abandonments) and on policing effort to estimate recent trends in the amount of rock lobster that is poached. Data for the period in 2016 when Operation Phakisa was launched with associated greatly enhanced levels of marine policing has been omitted from the analyses. The policing effort “sea patrols” has been omitted from the analyses as it has been deemed not to be relevant to rock lobster. The impact of omitting this policing effort type is minimal. The net effect of adding two further years of data suggests that the extent of poaching has increased slightly for Super-area 8+, but dropped more in the Super-areas 3+4+5+6+7.