Browsing by Author "Kirkman, S P"

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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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    Potential of a trap-fishery for the spiny lobster Palinurus delagoae off eastern South Africa : has the stock recovered?
    (2007) Boucher, Monique; Branch, George M; Groeneveld, Johan Conrad; Kirkman, S P
    An experimental trap fishery for the deep-water lobster spiny lobster Palinurus delagoae commenced in 2004, and was run until 2006. Data collected by on-board scientific observers were used to explore fishing effort, catches and catch per unit effort. The recent data were compared with those for a previous experiment conducted in 1994-1997, which apparently contributed to the collapse of the stock, to determine whether the stock had recovered and could sustain a trap fishery. There was a demonstrable recovery of the P. delagoae stock after the previous stock collapse, according to the catch rates and the size composition of catches between 2004-2006. However, catches of P. delagoae declined between 2004-2006. This was related to decreases in effort and in catch per unit effort over the three years. This, and a simultaneous decline in the by-catch of P. delagoae in the prawn trawl fishery off KwaZulu-Natal, suggests that the P. delagoae stock is extremely vulnerable to concomitant trawl and trap fishing. It is recommended that trap fishing should either be permitted at low levels of effort and catch only, or once more be suspended to allow the resource to recover.
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    Temporal variation of cephalods in the diet of Cape fur seals in Namibia
    (2003) De Bruyn, P J N; Bester, M N; Mecenero, S; Kirkman, S P; Roux, J-P; Klages, N T W
    Cape fur seal (Arctocephalus pusillus pusillus) scats were sampled over a period of eight years (1994–2001) at Atlas and Wolf Bay seal colonies in order to assess the cephalopod component of the diet of these seals and cephalopod diversity off the coast of Namibia. The temporal variation within the cephalopod component was investigated. A low diversity of cephalopods, only six species, are preyed upon, with Todarodes angolensis being the most important component both in numbers and wet weight in all years. Its lowered weight contribution during winter coincided with a greater diversity of other cephalopod species in the diet, which showed higher proportional weight contribution relative to Todarodes angolensis. Scat sampling was found to be an unreliable method of providing estimates of total prey weight consumption by seals, but was considered an acceptable method for proportional comparisons, especially given the ease of scat collection over extended periods.