Are Cape Cormorants (Phalacrocorax capensis) losing the competition? Dietary overlap with commercial fisheries

Master Thesis

2018

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University of Cape Town

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Characterisation of the diet of pelagic feeders can be regarded as key to development of ecosystem-based management plans, conservation of predators as well as understanding of ecological and trophic interactions. Therefore, long term studies on dietary changes provide insights into the nature of competition and overlap between seabirds and fisheries. The Cape Cormorant Phalacrocorax capensis breeds in the Benguela upwelling region of southern Africa. Its population has decreased by over 50% in the three most recent generations resulting in the species being listed as Endangered in the IUCN Red Data List. Its decline has previously been postulated to result from reduced prey availability. In this thesis, I examined and reported on the spatio-temporal distribution of Cape cormorant diet obtained from different colonies over two decades (1988 – 1997 and 1998 – 2007). I further investigated the extent of overlap between the diet composition and sizes of fish eaten by Cape Cormorants and those caught by the purse-seine fishery. Epipelagic fish (Anchovy, Sardine, Horse Mackerel and Redeye) dominated the diet of Cape cormorants at all sites. Anchovy was the most important in the diet in all years except in 1992 and 1993, when Sardine dominated the diet, and in 2007 when ‘other’ fish species (mainly Cape Silverside Atherina breviceps and Southern Mullet Chelon richardsonii) became the most frequently eaten fish. There were decadal variations in the relative numerical abundance of different prey species. Over both decades investigated, the diet was largely dominated by Anchovy and Sardine, with Horse Mackerel contributing more in the second than first decade. There was, however, an increase in the contribution of Anchovy relative to Sardine from the first decade to the second. At all colonies, Cape Cormorants mostly preyed on Anchovies of sizes between 5 and 11cm. Sardines eaten were larger than Anchovies (mostly 11 – 23 cm), with a bimodal distribution in the first decade. However, the few Sardine eaten in the second decade were smaller (4 – 7 cm) perhaps reflecting the length of fish available. There was an overlap in the distribution of fish sizes caught by the fishery and those found in the diet of Cape Cormorants especially in the first decade. However, increased sampling of Cape Cormorant diet is required to more fully understand the extent of competition and overlap with fisheries.
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