Surviving off junk: low-energy prey dominates the diet of African penguinsSpheniscus demersusat Mercury Island, Namibia, between 1996 and 2009

Abstract

The diet of African penguins Spheniscus demersus in Namibia consisted mainly of sardine Sardinops sagax in the 1950s. Since the collapse of pelagic fish stocks in the 1970s, birds fed mainly on bearded (pelagic) goby Sufflogobius bibarbatus, a low-energy prey species. We present diet data for African penguins breeding at Mercury Island, the largest colony for this species in Namibia, between 1996 and 2009. Bearded goby was the main prey item throughout the study period, both in terms of frequency of occurrence (67.8%; SD 31.2) and in terms of mass (59.2%; SD 31.5). Diet composition varied throughout the year as well as between years; birds occasionally fed on a variety of fish species other than bearded goby. In Namibia, poor prey abundance is considered as a major factor contributing to the decline of penguin numbers after the collapse of the sardine stocks. However, bearded goby appears to be relatively abundant along Namibia's southern coast and low prey quality rather than low abundance appears to be a key factor influencing population dynamics of African penguins and other marine top predators in southern Namibia.

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

“…Sardines were the dominant prey in both regions. Since the late 1970s bearded goby has dominated penguin diet in the NB (Crawford et al 1985, Ludynia et al 2010, and as a result the energy density of the diet has declined from more than 6 kJ g −1 in the 1950s to 3.50 in the 1980s and 4.11 kJ g −1 since the mid-1990s (a decline of between 35% and 45% from values from early studies of both ecosystems; Fig. 6A).…”

“…By the late 1970s substantial stocks were found between Walvis Bay and Lüderitz, and the extension of the species range was documented (Cruickshank et al 1980). Since then, the bearded goby has become one of the major prey items of most top predators feeding on the shelf in the NB, from seabirds (Crawford et al 1985, Ludynia et al 2010) to marine mammals (Mecenero et al 2006a, b) and predatory fishes such as juvenile hake (Traut 1996) and snoek, Thyrsites atun (Euphrasen, 1791) (Ministry of Fisheries and Marine Resources, Namibia, unpubl data). The goby stock increased rapidly after collapse of the small pelagic stocks (Cruickshank et al 1980), but recent studies (Utne-Palm et al 2010) have demonstrated that the trophic position of the bearded goby is very different from that of a clupeoid.…”

“…The penguin population has declined at an average exponential rate of -2.55% yr −1 (based on counts of molting birds, updated from Kemper 2007), and the species is classified as Endangered ). The main reason cited for this trend is the low availability of energy-rich small pelagic fishes (Ludynia et al 2010).…”

“…6). The energetic values of the different prey taxa were determined by bomb calorimetry (Balmelli and Wickens 1994, K Ludynia unpubl data), and the diet energy density (in kJ g −1 ) of the different predators was calculated by the methods of Ludynia et al (2010).…”

The initial journey of an Endangered penguin: implications for seabird conservation

“…Heavy exploitation of lower trophic level fish precipitated a switch to poorquality prey in Namibia (Ludynia et al 2010), while a change in the relative abundance of adult sardine Sardinops sagax and anchovy Engraulis encrasicolus during the late-1990s and early-2000s (van der Lingen et al 2006) has resulted in a spatial mismatch between the majority of the spawner stocks and the foraging ranges of breeding seabirds on South Africa's west coast (e.g. Grémillet et al 2008).…”