African Journal of Marine Science 2006, 28(1): 73–88 Printed in South Africa — All rights reserved Copyright © NISC Pty Ltd AFRICAN JOURNAL OF MARINE SCIENCE ISSN 1874–232X Diet of Cape fur seals Arctocephalus pusillus pusillus at three mainland breeding colonies in Namibia. 2. Temporal variation S Mecenero 1 *, J-P Roux 2 , LG Underhill 1 and SP Kirkman 1 1 Avian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch 7701, South Africa 2 Lüderitz Marine Research, Ministry of Fisheries and Marine Resources, PO Box 394, Lüderitz, Namibia * Corresponding author, e-mail: smecener@adu.uct.ac.za Introduction The distribution of the Cape fur seal Arctocephalus pusillus pusillus population coincides with areas that sustain prof- itable fishing industries (Wickens et al. 1992), and their diet consists of several species that are of commercial impor- tance (David 1987, Mecenero et al. 2006). This, and the dramatic increase in the size of the population in recent decades (Butterworth et al. 1995), has heightened concerns that increased predation on fish by seals might cause certain fisheries catches to decline (Shaughnessy 1985, David 1987, Wickens et al. 1992, Punt and Butterworth 1995). According to Harwood (1987), models developed to assess consumption of fish stocks by seals should attempt to incorporate the extent of temporal and spatial variation in parameters such as diet composition, if they are to be used as predictive management tools. This applies to the Cape fur seal population, the bulk of which inhabits the cold and nutrient-rich upwelling Benguela system (Currie 1953), which is subject to considerable spatio-temporal variability in environmental conditions and marine resources (Shannon et al. 1992). Several studies have shown spatial shifts in the diet composition of Cape fur seals (e.g. David 1987, Lipin / ski and David 1990, De Bruyn et al. 2005, Mecenero et al. 2006), but only De Bruyn et al. (2003), using scat analysis, investigated changes in diet over time. That study focussed on the dietary importance of cephalopods, and it was found that the importance of some species, from a single location in Namibia, varied both annually and seasonally. The pres- ent study focusses only on teleost fish, which comprise the major proportion of the Cape fur seal’s diet (Rand 1959, Shaughnessy 1985, David 1987, Castley et al. 1991, Punt et al. 1995), and is based on scats collected at three Nami- bian breeding colonies over a period of eight years. Mece- nero et al. (2006) show that the importance of teleost prey varies spatially between these colonies. In this study, diet composition is assessed for each colony at annual, sea- sonal and monthly time-scales. Material and Methods Study sites The location of the three study sites: Cape Cross (CC), Atlas Bay and Wolf Bay combined (AWB) and Van Reenen Bay (VRB), are described in Mecenero et al. (2006). The most intense upwelling centre of the Benguela system, the Lüderitz upwelling cell, is just north of VRB in the vicinity of the AWB colony. In that region, the winds are strong and the continental shelf is narrowest and deepest (Shannon 1985). Cold, upwelled water filaments from this upwelling Scat analysis was used to assess temporal variability in the diet composition of Cape fur seals Arctocephalus pusillus pusillus from three breeding colonies in Namibia (January 1994 to April 2002). The diet displayed significant inter- and intra-annual variation in composi- tion at each of the study colonies. The diet was not predictable on an annual pattern, and little consistent seasonality was apparent at each of the study sites. However, some prey species tended to display some seasonality, possibly as a result of their life-history patterns. At the Cape Cross colony, small Cape hake Merluccius spp. in the diet coincided with their spawning season. At the Atlas-Wolf Bay and Van Reenen Bay colonies, lanternfish Lampanyctodes hectoris were most prevalent in the diet during the upwelling season (August–January). Outside that period, pelagic goby Sufflogobius bibarbatus were most abundant in the diet at those colonies. This pattern may be a result of the gobies’ southwards migration from northern Namibian waters where smaller goby are dominant in the diet at the Cape Cross colony in the upwelling season. The prevalence of Cape horse mackerel Trachurus trachu- rus capensis in the diet at Cape Cross was coincident with the pelagic phase of juvenile horse mackerel. Keywords: Arctocephalus pusillus pusillus, breeding colony, Cape fur seal, diet, Namibia, scat analysis, temporal variation