Browsing by Author "Moseley, Christina"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Open Access
    Comparing body condition and foraging ecology of two populations of Cape Gannets on Bird and Malgas Islands
    (2010) Moseley, Christina; Grémillet, David; Pichegru, Lorien; Ryan, Peter G
    Cape gannets (Morus capensis) are one of the dominant seabirds in the Benguela current ecosystem and feed mainly on sardines (Sardinops sagax) and anchovy (Engraulis encrasicolus). Starting in the late 1990s the distribution of these fish shifted from the west coast of South Africa to the south-east coast. This has resulted in gannets on the west coast feeding extensively on fishery wastes, which slows the growth of chicks and decreases their fledging mass. I compared the foraging ecology, diet and body condition of adult Cape gannets from two colonies, one where individuals have been feeding on fishery wastes (hake) for several years (Malgas Island; west coast) and the other where individuals feed on natural prey (Bird Island; south coast). In October and November 2009, through the use of GPS loggers I compared the foraging behaviour of birds from the two colonies. I compared the diet of gannets at the two colonies, using stomach contents samples and an isotopic mixing model using stable isotope ratios of carbon and nitrogen in blood, feathers and potential prey. I also compared the body condition of adults by measuring pectoral muscle thickness and other morphological parameters. These results were analysed with data from a concurrent hydro-acoustic survey of the distribution and abundance of pelagic fish along the coast of South Africa. The hydro-acoustic survey showed that more than half of the sardine and anchovy stocks were present on the west coast for the first time in several years but that the overall biomass of these two species remained low in the southern Benguela. Stomach samples and isotopic analysis of blood showed that gannets at both colonies fed mainly on sardines during the study period. Long-term diet estimates from feather isotopes suggest that there was little hake in the diet of birds at Malgas Island, despite the direct diet samples showing that hake and saury dominated the diet over the preceding 10 months. This could be due to insufficient prey sampling or the diet samples not being representative of the gannet population as a whole. Gannets from Bird Island made longer foraging trips and flew further from their colony than did those from Malgas Island. Individuals from this colony had slightly greater pectoral muscle thicknesses and body masses (after controlling for size) than those from Bird Island, but 4 this was not significant. Despite gannets from Malgas Island relying on fishery wastes for a number of years, there has been little effect on body condition among breeding birds when compared with Bird Island gannets feeding on pelagic fish. Two possible reasons for this are that (1) when feeding on fishery wastes, adults decrease their reproductive effort to preserve body condition or (2) gannets on the west coast have regained body mass and pectoral muscle rapidly since the local recovery of sardines. It is likely that the gannets from Bird Island have greater foraging costs due to intra-specific competition for food as the colony has increased five-fold in size over the last 50 years. However, chick growth and adult body condition have been affected only marginally. Despite the presence of sardine and anchovy on the west coast, it is likely that Cape gannets are food limited, especially since there is also a strong spatio-temporal overlap of gannet foraging with the large commercial purse-seine fishery. Better spatial management of this fishery is necessary to ensure the persistence of seabirds and other top predators in the southern Benguela.
  • Thumbnail Image
    Item
    Open Access
    Tracking African penguins (Spheniscus demersus) outside of the breeding season: Regional effects and fishing pressure during the pre-moult period
    (2014-08-15) Harding,Craig Thomas; Wanless, Ross M; Ryan, Peter; Moseley, Christina; Pichegru, Lorien
    African penguins (Spheniscus demersus) have experienced a 60% population decline in the past 30 years due to an eastward shift in the relative abundance of their main food source, anchovy (Engraulis encrasicolus ) and sardine (Sardinops sagax). This shift may be exacerbated by pressure from the small-pelagic fishery targeting these species. A lack of knowledge regarding foraging areas when not breeding has been identified as a deficiency in management planning for African penguins. Satellite transmitters were deployed on 20 adults during the pre-moult foraging period from colonies on the west coast (Dassen Island, n=10) and south coast (Bird Island, Algoa Bay, n=10) of South Africa. Kernel density analyses were produced using nightly locations to create foraging range maps, which were compared to catches made by the small-pelagic fishery during September-December 2012. Birds from the two colonies differed in their foraging strategies. Compared to penguins from Bird Island, those from Dassen Island spent more than six times the number of nights beyond the foraging range used during the breeding season (40 km from their colony). Birds from Dassen Island typically made long, looping trips more than 300 km away from the colony, and travelled further and at higher daily rates to foraging areas than individuals from Bird Island, feeding outside areas where fishing activity was highest. Kernel density analysis showed the foraging range of Dassen Island penguins was between Cape Columbine and the central Agulhas Bank. Individuals from Bird Island continued to central place forage, typically staying within 35 km of the colony. Penguins from Dassen Island that remained on the west coast had to compete with higher levels of fishing pressure than penguins from Bird Island. More than 70% of the fleet-wide, 2012 purse-seine catch occurred within the area where Dassen Island penguins spent 50% of their time. These colony-specific characteristics suggest that management plans for African penguins need to be regionally targeted and incorporate larger foraging ranges during the pre-moult period for birds from western colonies. The larger foraging ranges and effort demonstrated by birds from the west coast suggest that a combination of the low relative fish abundance and higher commercial fishing pressure may force pre-moulting birds to seek food sources farther from the colony, putting them at higher risk of not surviving the annual moult. This project requires more years of data to ensure these foraging patterns are representative, and to more accurately provide management suggestions directed to alleviate stress on African penguins for long-term protection of this endangered seabird.