THE DIET AND IMPLICATIONS OF DIETARY CHANGE OF CAPE GANNETS ON BIRD ISLAND,ALGOA BAY

Batchelor, A. L. & Ross, G. J. B. 1984. The diet and implications of dietary change of Cape Gannets on Bird Island, Algoa Bay. Ostrich 55:45-63.

The diet of the Cape Gannet Sula capensis breeding on Bird Island, Algoa Bay, South Africa, was investigated. Approximately 2000 regurgitations representing 13300 prey items were examined. Twenty-three teleost and two cephalopod species were recorded in the diet. More than 90 % of the numerical composition consisted of the three species Pilchard Sardinops ocellata, Anchovy Engraulis capensis and Saury Scomberesox saurus. The proportions of these three species in the diet changed during the sampling periods. These changes were not related to the implied migration patterns of these species. Chick growth rates were investigated in conjunction with the dietary study. These results indicated that Cape Gannet chicks on a diet of Pilchard and/or Anchovy were more likely to fledge successfully than those on a diet of Stockfish Merluccius capensis or Saury. These observations were based on differences in growth rate, and peak and fledgling mass, The data suggest that Pilchard is a preferred prey species and its presence or absence in regurgitationsMayreflect a similar situation on the feeding grounds. The observed increase in the Bird Island gannet populationMaybe related to the availability of high-quality food and consequent reduced pre-breeding mortality. In the absence of historical dietary data, further detailed population dynamics studies are required to support this supposition.     

Evidence for fatal collisions and kleptoparasitism while plunge‐diving in Gannets

Plunge‐diving is a highly successful strategy for dealing with the challenges confronting birds feeding on pelagic prey. We tested for evidence of fatal injuries due to collision between conspecifics in plunge‐diving Australasian Gannets Morus serrator and Cape Gannets Morus capensis, respectively, by performing post‐mortem examinations of carcasses recovered from New Zealand waters and analysing video footage of Cape Gannet foraging events from South Africa. We found evidence of accidental collisions between Gannets and also observed a case of attempted kleptoparasitism, in which a diving Cape Gannet targeted a previously captured fish in the beak of a conspecific.     

The status and conservation of the Cape Gannet Morus capensis

The Cape Gannet Morus capensis is one of several seabird species endemic to the Benguela upwelling ecosystem (BUS) but whose population has recently decreased, leading to an unfavourable IUCN Red List assessment. Application of ‘JARA’ (‘Just Another Red-List Assessment,’ a Bayesian state-space tool used for IUCN Red List assessments) to updated information on the areas occupied by Cape Gannets and the nest densities of breeding birds at their six colonies, suggested that the species should be classified as Vulnerable. However, the rate of decrease of Cape Gannets in their most-recent generation exceeded that of the previous generation, primarily as a result of large decreases at Bird Island, Lambert’s Bay, and Malgas Island, off South Africa’s west coast (the western part of their range). Since the 1960s, there has been an ongoing redistribution of the species from northwest to southeast around southern Africa, and ～70% of the population now occurs on the south coast of South Africa, at Bird Island in Algoa Bay, on the eastern border of the BUS. Recruitment rather than adult survival may be limiting the present population; however, information on the seabird’s demographic parameters and mortality in fisheries is lacking for colonies in the northern part of the BUS. Presently, major threats to Cape Gannet include: substantially decreased availability of their preferred prey in the west; heavy mortalities of eggs, chicks and fledglings at and around colonies, inflicted by Cape Fur Seals Arctocephalus pusillus and other seabirds; substantial disturbance at colonies caused by Cape Fur Seals attacking adult gannets ashore; oiling; and disease.     

Lanternfish (Myctophidae) in the diet of the Cape petrel Daption capense at the South Shetland and South Orkney Islands, Antarctica

All of the fish identified in stomach contents and regurgitations of breeding and chick Cape petrels collected during January and February 1996 at Fildes Peninsula and Harmony Point, both in the South Shetland Islands and at Laurie Island, South Orkney Islands, were myctophids, a family never previously reported in the diet of breeding Cape petrels. Electrona antarctica was the most important fish prey, followed by Electrona carlsbergi at Fildes Peninsula, Krefftichthys anderssoni at Harmony Point and Gymnoscopelus braueri at Laurie Island. The absence of Pleuragramma antarcticum in the diet of this petrel, which is considered a P. antarcticum-feeder, is discussed. Received: 22 December 1997 / Accepted: 6 June 1998     

Responses by four seabird species to a fluctuating availability of Cape Anchovy Engraulis capensis off South Africa

In recent years, Cape Anchovy Engraulis capensis has been the most important food for four seabirds breeding in South Africa–African Penguin Spheniscus demersus, Cape Gannet Morus capensis, Cape Cormorant Phalacrocorax capensis and Swift Tern Sterna bergii. Between 1984 and 1992, biomass of spawning anchovy fluctuated between about 0.5 and 1.75 million tons. Abundance of anchovy was significantly related to numbers of chicks fledged by African Penguins, occurrence of anchovy in the diet of Cape Gannets and numbers of Cape Cormorants and Swift Terns that attempted to breed. Numbers of African Penguins and Cape Gannets that attempted breeding probably also were influenced by abundance of anchovy. African Penguins and Cape Cormorants abandoned nests when anchovy were scarce and deferred breeding until anchovy became more plentiful. Survival of immature African Penguins in a period of anchovy scarcity was enhanced by availability of South African Sardine Sardinops sagax as an alternative food. When anchovy abundance was low, Cape Gannets fed on sardine.     

4. BIRD PROTECTION POLICIES AND LAWS

Walter, C. B., Cooper, J. &; Suter, W. 1987. Diet of Swift Tern chicks in the Saldanha Bay region, South Africa. Ostrich 58:49-53.

The diet of the Swift Tern Sterna bergii was investigated over a ten-year period off the west coast of South Africa by collecting regurgitations from chicks during ringing operations. A total of 1311 prey items of 25 identifiable species (20 of which were fish) was collected. Fish formed 86% of all the prey items. Other prey species included cephalopods, crustaceans and insects. 60% by number of prey taken consisted of pelagic shoaling fish, of which Cape Anchovy Engraulis japonicus occurred most frequently and was the most abundant prey species in seven of the nine years in which samples were collected. Weekly collections in 1984 confirmed that pelagic shoaling fish, in particular Cape Anchovy, were the most abundant prey taken during the chick-rearing period. Prey size varied from 7 to 138 mm in length and from 0,l to 30,0 g in mass.     

BREEDING NOMADISM IN SOUTHERN AFRICAN SEABIRDS-CONSTRAINTS,CAUSES AND CONSERVATION

Crawford, R.J.M., Dyer, B.M. & Brooke, R.K. 1994. Breeding nomadism in southern African seabirds-constraints, causes and conservation. Ostrich 65:231-246.

Fourteen species of seabirds representing three orders and five families breed in southern Africa. Nomadism in the breeding populations of 13 of these seabirds is examined. African Penguins Spheniscus demersus, Cape Gannets Morus capensis. Bank Cormorants Phalacrocorax neglectus, Whitebreasted Cormorants P. carbo and White Pelicans Pelecanus onocrotalus show strong fidelity to specific localities. Caspian Terns Sterna caspia change breeding localities at a low frequency; Cape Cormorants P. capensis, Hartlaub's Gulls Larus hartlaubii and Swift S. bergii and Roseate S. dougallii Terns at a higher frequency. At some localities, Bank Cormorants remain at the same breeding sites for extended periods, whereas Crowned Cormorants P. coronatus, Whitebreasted Cormorants, Kelp Gulls L. dominicanus, Hartlaub's Gulls, Damara Terns S. balaenarum, Swift Terns and Roseate Terns all alter breeding sites. Where space is not limiting, African Penguins may also change sites. Constraints on nomadism include strong attachment to traditional breeding localities, unwillingness to roost at non-breeding localities, and lack of suitable alternative nesting localities. Amongst causes of nomadism are excessive disturbance by humans and South African Fur Seals Arctocephalus pusillus and competition for breeding space, which have resulted in even the least nomadic of the species changing their breeding locations. Regularly nomadic species are likely to respond to environmental cues that enable them either to reduce cost of breeding or to increase reproductive output. Therefore, they are potentially useful indicators of ecological health. Nomadic breeders additionally have the ability to establish new breeding colonies. Some species have used artificial structures to expand their breeding range, and probably also to increase their overall populations. However, nomadic tendencies complicate the conservation of breeding activities. By contrast, breeding species with strong fidelity to traditional localities are more easily protected.     

Lumbering the gauntlet: Cape Gannet fledglings killed by African Penguins

This account presents the first known observations of Cape Gannet Morus capensis fledgling mortalities as a result of aggressive nest defence behaviour by African Penguins Spheniscus demersus. Observations were conducted in 2013 on Bird Island, Algoa Bay, South Africa – the world's largest breeding colony of Cape Gannets. Twentyeight attacks were witnessed between 21 March and 18 May 2013 of which 16 resulted in mortality. The absence of previous observations of this phenomenon may be due to an unusually high proportion of African Penguin chicks being present relatively early on in the breeding season, associated with a potential for increased nest defence at this stage, and a larger number of gannet fledglings that year compared to previous years over the same period.     

Gill Memorial Medal Address Holding the mirror up to nature

Since the early 1980s, there have been large decreases in numbers breeding in South Africa of three seabirds that compete with fisheries for food: African Penguin Spheniscus demersus, Cape Cormorant Phalacrocorax capensis and Bank Cormorant P. neglectus. By contrast, two other species that compete with fisheries but which have a larger foraging range when breeding or are nomadic between breeding localities, Cape Gannet Morus capensis and the nominate race of Swift Tern Thalasseus bergii bergii, have increased. Five seabirds that do not compete with fisheries for prey have maintained stable populations or increased. The decreases in the three species have led to deterioration in the overall conservation status of South Africa's seabirds, of which 10 of 15 species (67%) are presently classified as Threatened or Near-threatened, including six of the seven species that are endemic to the Benguela ecosystem off western southern Africa.     

The diet of the Imperial Shag Phalacrocorax atriceps at a colony on New Island, Falkland/Malvinas Islands combining different sampling techniques

The diet of the Imperial Shag Phalacrocorax atriceps was studied on New Island, Falkland/Malvinas Islands during the 2008/2009 breeding season, with some additional data from 2007/2008. The diet comprised a large variety of prey, mainly fish, crustaceans and squid. In contrast to other species of the blue-eyed shag complex, prey not only consisted of benthic organisms but also included pelagic prey. Different sampling techniques were combined in order to obtain a comprehensive overview of the diet. Pellets, regurgitations and stomach contents yielded different results. We discuss the causes for these variations including different sample availability over time. In particular, the stomach analyses seem to overestimate the importance of squid based on the occurrence of squid beaks. For the pellet analyses, lobster krill accounted for the majority of the prey remains except during the second half of December (i.e. when young chicks were being attended), when fish was more important.     

What is out there: diversity in feeding of gentoo penguins (Pygoscelis papua) around the Falkland Islands (Southwest Atlantic)

During austral spring 2000, the diet of Gentoo penguins, Pygoscelis papua, was studied and compared to prey availability, assessed by trawl and plankton surveys, in waters adjacent to a number of colonies along the coast of West Falkland. There was good agreement in size and abundance of key prey species in the penguin diet and from the plankton and trawl surveys. The diet of birds breeding adjacent to shallow sheltered waters was dominated by demersal species, in particular crustaceans, Munida spp. whilst birds breeding adjacent to deeper exposed waters generally preyed upon pelagic species of fish, Falkland herring Sprattus fuegensis and squid, Moroteuthis ingens. Gentoo penguins generally foraged at depths of up to 30 m, relatively close to the coast, and illustrated a certain degree of selection of prey species where possible, thus providing evidence that they are not fully opportunistic predators.     

Does reservoir trophic status influence the feeding and growth of the sharptooth catfish,Clarias gariepinus (Teleostei: Clariidae)?

The diet and growth of sharptooth catfish, Clarias gariepinus, in an oligotrophic system (Kat River Reservoir, Eastern Cape, South Africa) were compared to those in a eutrophic system (Laing Reservoir, Eastern Cape) to determine if the trophic status of a waterbody had an effect on the growth rate of the species. In order of importance, the diet of catfish in Kat River Reservoir consisted of fish, insects, zooplankton, plant material and other items, while the diet of catfish in Laing Reservoir consisted of fish, plant material, zooplankton, other vertebrates and insects. The diets of catfish in the two reservoirs had a similarity index of 68.1% and there was no significant difference in their nutritional value. Fish prey was the most important dietary component in both reservoirs. Temperature regime and zooplankton and zoobenthos density were similar in both systems. However, fish prey density was significantly higher in the eutrophic Laing Reservoir and catfish grew significantly faster in that system. The slower growth rate in Kat River Reservoir was attributed to the higher energy costs associated with the capture of fish prey, which was less abundant than in Laing Reservoir. Trophic status therefore had an indirect effect on catfish growth by influencing the availability of fish prey.     

On a wing and a prayer: the foraging ecology of breeding Cape cormorants

The Cape cormorant Phalacrocorax capensis is unusual among cormorants in using aerial searching to locate patchily distributed pelagic schooling fish. It feeds up to 80 km offshore, often roosts at sea during the day and retains more air in its plumage and is more buoyant than most other cormorants. Despite these adaptations to its pelagic lifestyle, little is known of its foraging ecology. We measured the activity budget and diving ecology of breeding Cape cormorants. All foraging took place during the day, with 3.6 ± 1.3 foraging trips per day, each lasting 85 ± 60 min and comprising 61 ± 53 dives. Dives lasted 21.2 ± 13.9 s (maximum 70 s), attaining an average depth of 10.2 ± 6.7 m (maximum 34 m), but variability in dive depth both within and between foraging trips was considerable. The within-bout variation in dive depth was greater when making shallow dives, suggesting that pelagic prey were targeted mainly when diving to <10 m. Diving ecology and total foraging time were similar to other cormorants, but the time spent flying (122 ± 51 min day⁻¹, 14% of daylight) was greater and more variable than other species. Searching flights lasted up to 1 h, and birds made numerous short flights during foraging bouts, presumably following fast-moving schools of pelagic prey. Compared with the other main seabird predators of pelagic fish in the Benguela region, Cape gannets Morus capensis and African penguins Spheniscus demersus , Cape cormorants made shorter, more frequent foraging trips. Their foraging range while feeding small chicks was 7 ± 6 km (maximum 40 km), similar to penguins (10–20 km), but less than gannets (50–200 km). Successful breeding by large colonies depends on the reliable occurrence of pelagic fish schools within this foraging range.     

Aspects of the foraging behaviour of the Antarctic Tern Sterna vittata gaini at Harmony Point, South Shetland Islands

During January and February of 2002 and 2003, we studied the diet of the Antarctic Tern Sterna vittata gaini at two colonies in Nelson Island, South Shetland Islands, by identifying the prey fed to chicks by breeders. The fish Notothenia coriiceps was the main prey in both seasons, followed by the myctophid Electrona antarctica, Antarctic krill Euphausia superba and gammarid amphipods. The contribution of fish to the diet increased as chicks grew older. Fish and amphipods were brought to chicks during the day, whereas adults brought Antarctic krill at sunrise and sunset. Both the duration of the feeding trips and the number of trips per foraging bout varied according to the type of prey caught. Preliminary information suggests that, among other causes, the foraging strategy is strongly influenced by the predation pressure of skuas on chicks. Results are compared with the only two previous study on the diet of the Antarctic Tern at the South Shetland Islands.     

Geographical and temporal variation in the diet of Bank Cormorants Phalacrocorax neglectus in South Africa

The Bank Cormorant Phalacrocorax neglectus is endemic to the Benguela upwelling ecosystem off southwest Africa and is classified as Endangered owing to a recent large reduction in its number. It is thought that food scarcity, including a decreased abundance of West Coast rock lobster Jasus lalandii, has been a major driver of the decrease, yet its diet in South Africa is poorly known. We collected 941 pellets regurgitated by Bank Cormorants, at 18 South African breeding colonies during 1975–1985, and 1 523 pellets at 17 colonies during 1995–2002. The species composition of the diet (% numbers) was significantly different between the two periods, with widespread decreases in proportions of rock lobster in the west and of octopus and cuttlefish Sepia spp. at most localities. These taxa were replaced in the diet by fish, including Gobiidae and Clinidae. The pelagic goby Sufflogobius bibarbatus, an important prey of Bank Cormorants in Namibia, was absent from pellets collected in 1975–1985 but common at northern localities from 1995–2002. Composition of the diet by frequency of occurrence was only determined for 1995–2002, when rock lobster was present in 67% of all samples collected, cuttlefish in 39%, and Clinidae in 32%. Data for 1975–1985 and 1995–2002 showed that carapace lengths of rock lobsters eaten by Bank Cormorants averaged 56 mm (range 22–82 mm) and 50 mm (range 22–75 mm), respectively, which compares to the minimum legal size of 75 mm for fisheries in South Africa. This energy- rich prey item was an important constituent of the diet in the winter breeding period.     

Species‐specific foraging strategies and segregation mechanisms of sympatric Antarctic fulmarine petrels throughout the annual cycle

Determining the year‐round distribution and behaviour of birds is necessary for a better understanding of their ecology and foraging strategies. Petrels form an important component of the high‐latitude seabird assemblages in terms of species and individuals. The distribution and foraging ecology of three sympatric fulmarine petrels (Southern Fulmar Fulmarus glacialoides, Cape Petrel Daption capense and Snow Petrel Pagodroma nivea) were studied at Adélie Land, East Antarctica, by combining information from miniaturized saltwater immersion geolocators and stable isotopes from feathers. During the breeding season at a large spatial scale (c. 200 km), the three species overlapped in their foraging areas located in the vicinity of the colonies but were segregated by their diet and trophic level, as indicated by the different chick δ¹⁵N values that increased in the order Cape Petrel < Southern Fulmar < Snow Petrel. During the non‐breeding season, the three fulmarines showed species‐specific migration strategies along a wide latitudinal gradient. Snow Petrels largely remained in ice‐associated Antarctic waters, Southern Fulmars targeted primarily the sub‐Antarctic zone and Cape Petrels migrated further north. Overall, birds spent less time in flight during the non‐breeding period than during the breeding season, with the highest percentage of time spent sitting on the water occurring during the breeding season and at the beginning of the non‐breeding period before migration. This activity pattern, together with the δ¹³C values of most feathers, strongly suggests that moult of the three fulmarine petrels occurred at that time in the very productive high Antarctic waters, where birds fed on a combination of crustaceans and fish. The study highlights different segregating mechanisms that allow the coexistence of closely related species, specifically, prey partitioning during the breeding season and spatial segregation at sea during the non‐breeding season.     

Stable isotope analysis as a tool to monitor dietary trends in little penguins Eudyptula minor

Long‐term dietary monitoring of seabirds can be used to relate population fluctuations to at‐sea events. Stomach flushing is a conventional dietary monitoring technique, but has a number of disadvantages. Stable isotope analysis (SIA) is a less invasive method that provides unbiased dietary information over a longer period. We evaluated stable isotope analysis as a potential tool for monitoring long‐term little penguin Eudyptula minor diet. We determined diet composition during the chick feeding stage using stomach flushing and SIA at three separate colonies, using spatial variation in diet as a surrogate for potential temporal variation. Bayesian isotopic mixing models were generated for blood and feathers to evaluate their ability to discriminate broad‐scale (fish, squid, crustaceans) and fine‐scale (individual prey species) diet composition. Differences in stable carbon and nitrogen isotope ratios were found between colonies: broad‐scale isotopic mixing models predicted different proportional contributions of broad taxa (fish, cephalopod, crustacean) to diet than was indicated by stomach samples, reflecting the bias incurred by one‐off stomach contents analysis. Fine‐scale isotopic mixing models predicted proportional contributions of prey items with less certainty. Blood isotopic mixing models had narrower confidence intervals than models for feathers, but trends in δ¹⁵N for feathers mirrored those for blood. Our results suggest that relying on stomach contents analysis to detect shifts in prey consumption in little penguins could be very misleading, resulting in a less‐than‐complete idea of total prey consumption. SIA of little penguin tissues could be used to monitor dietary shifts across dissimilar taxa that may affect population numbers, but would fail to detect shifts between fish species.     

Intra-seasonal variation in zooplankton availability,chick diet and breeding performance of a high Arctic planktivorous seabird

Different phenological responses to climate changes by species representing preys and predators may lead to mismatch between functionally dependent components of an ecosystem, with important effects on its structure and functioning. Here, we investigate within-season variation in zooplankton availability, chick diet composition and breeding performance of a small planktivorous seabird, the little auk (Alle alle) in two large colonies in Hornsund and Magdalenefjorden, Spitsbergen, differing in synchrony of breeding (11-day vs. 22-day hatching period, respectively). Assuming similar zooplankton phenology and existing differences in duration of the little auk breeding period, we expected lower availability of the preferred food in the less synchronized colony in Magdalenefjorden and in consequence a negative effect on nestling body mass and survival. We found that in both colonies Calanus glacialis (copepodite stage CV) was the most important prey item in the chick diet making up 68–87 % of the biomass and energy of all prey items. The only exception was the end of the chick-rearing period in Magdalenefjorden, when contribution of this prey item was significantly lower (24–26 %). Thus, late breeders in Magdalenefjorden were apparently mismatched regarding C. glacialis CV availability. However, the hatching date did not affect birds fitness (reproductive output and chick pre-fledging mass) significantly. Results of our study indicate that little auks breeding on Spitsbergen can respond to a wide range of environmental conditions and prey availabilities through the plasticity of their foraging behaviour, which may help them to maintain their optimum fitness level in changing and unpredictable environments.     

The diet of Common Guillemot Uria aalge chicks provides evidence of changing prey communities in the North Sea

Local differences in feeding conditions have been suggested as a cause of regional variation in seabird demography but multi‐colony comparisons of diet are rare. In UK waters the main fish eaten by seabirds during the breeding season belong to three families: Ammodytidae, Clupeidae and Gadidae. Climate change and fishing are affecting these fish stocks and so probably impact on predators such as seabirds. We used standardized observations of prey brought in for chicks to make the first integrated assessment of the diet of Common Guillemot Uria aalge chicks at a UK scale. Chick diet varied markedly among the 23 colonies sampled between 2006 and 2011. Sandeels (Ammodytidae), probably Lesser Sandeels Ammodytes marinus, were the commonest prey. Their contribution to the diet varied both latitudinally and among marine regions, with the proportion significantly higher for a given latitude on the west coast compared to the east. The non‐sandeel component of the diet showed latitudinal changes, with small clupeids, probably Sprats Sprattus sprattus, predominant at southern colonies whereas juvenile gadids were the main alternative to sandeels in the north. Comparison of our Guillemot chick diet with data collected 15–30 years earlier suggests that the proportion of sandeels in the diet has decreased at colonies bordering the North Sea. No significant change was apparent in Atlantic colonies but historical data were limited. The early years of our study coincided with a population explosion of Snake Pipefish Entelurus aequoreus in the Northeast Atlantic and North Sea. Pipefish were recorded in Guillemot chick diet at several northern and northwestern colonies in 2006 and 2007 but have been absent since 2009. Spatial and temporal variation in chick diet accorded broadly with patterns expected as a result of rising sea temperatures and impacts of fishing. Guillemot chick diet could potentially be a useful indicator of changes in the distribution and abundance of forage fish.