Comparative ecology of seabirds in the Galapagos Archipelago

Harris, Michael

doi:10.1007/978-1-349-05226-4_7

Cited by 22 publications

(17 citation statements)

References 14 publications

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“…The coupling of food availability and the timing and success of reproduction may be very tight in tropical seabirds (Harris 1977, Nelson 1984, Diamond & PrysJones 1986. Hence if food availability changes significantly as a result of trawlers discarding bycatch, fluctuations in population sizes and success of reproduction may relate to changes in commercial fishing effort and location.…”

Section: Introductionmentioning

confidence: 99%

Trawl discards in the diets of tropical seabirds of the northern Great Barrier Reef, Australia

Blaber¹,

Milton²,

Smith³

et al. 1995

Mar. Ecol. Prog. Ser.

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ABSTRACT. The diets of 12 species of tropical seabirds were investigated in 2 areas of the far northern Great Barrier Reef, Australia, with particular reference to the effects of bycatch discarded from trawlers on diets and reproduction. The species whose diets included at least 20% discard taxa are Sterna bergii, S. dougallii and perhaps S. anaetheta; between 5 and 19% -Hydroprogne caspia, Anous stolidus, Sterna bengalensis, Sula leucogaster and Fregata ariel, less than 5 % -Sterna sumatrana; and none -Fregata minor, Sula dactylatra and Sula sula. There was a marked contrast in the diet of S. bergii in closed and open trawling seasons: in the closed season only 5% of the prey were benthic species, whereas in the open season they made up about 7 0 % of the diet. Differences in the diets of birds from areas open and closed to fishing were less marked, probably because birds from the closed zone can forage in adjacent areas open to trawling. About half the bycatch discarded by the trawlers is of a size suitable for one or more of the seabirds, and most of this half floats, some of it for up to 6 h. S. bergii, S. leucogaster and E ariel are actively opportunistic, feeding around trawlers, but S. anaetheta and S. dougallii may be passive discard feeders, feeding on floating discards away from the site of dumping. The provision of discards has not changed the size ranges of prey taken by any of the species. It may, however, have increased overlap in the diets of the various seabirds and changed some feeding strategies. There is little evidence that discards have directly affected breeding cycles or nesting periodicity. Nevertheless, most species breed mainly in late summer and the largest quantities of bycatch are discarded in March, immediately post-fledging. This may lead to juvenile birds becoming conditioned to feeding on discards and may also reduce juvenile mortality rates.

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Section: Introductionmentioning

confidence: 99%

Trawl discards in the diets of tropical seabirds of the northern Great Barrier Reef, Australia

Blaber¹,

Milton²,

Smith³

et al. 1995

Mar. Ecol. Prog. Ser.

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“…While emphasis on the local scale predominated (Ashmole 1963, 1971, Nelson 1970, Harris 1977 prior to the 1982-83 El NinoSouthern Oscillation (ENSO) event, the global effects of that event on seabirds prompted a reassessment of the role of regional processes. During late 1982 and the first half of 1983, seabirds over a wide area reared few or no young and deserted their colonies , Schreiber & Schreiber 1984, papers in Robinson & del Pino 1985, Ainley et al in press, La Cock 1986, Norton 1986, Hatch 1987.…”

Section: Introductionmentioning

confidence: 99%

Differential responses of boobies and other seabirds in the Galapagos to the 1986-87 El Nino-Southern Oscillation event

Anderson¹

1989

Mar. Ecol. Prog. Ser.

103

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The impact of the 1986-87 El Nido-Southern Oscillation (ENSO) event on seabirds in the Galapagos Islands was generally less severe than that of the previous ENSO in 1982-83. Sea surface temperatures (SST) rose to levels comparable to those of 4 ENSOs pnor to the 1982-83 event. SST became anomalous approximately in January and had returned to typical levels by July. Blue-footed booby Sula nebouxii reproductive attempts failed throughout the archipelago, and breeding colonies were deserted, shortly after SST became unusually warm in January. Masked boobies S. dactylatra, redfooted boobies S. sula and several other species were apparently unaffected by the anomalous conditions, or temporarily suspended breeding for several months. A gradient in both SST and in the ENSO's impact on some seabirds was evident, with populations nesting in the cooler south of the archipelago affected less than those in the warmer north. At one colony studied both before and during the ENSO, blue-footed booby failure was associated with apparent reductions in both availablllty and body size of their primary prey item.

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“…Th us, it has been noted (Croxall et al 1997) that ecological segregation between all the krilleating species at South Georgia appears to be maintained chiefl y by diff erences in foraging range and feeding methods (Methven and Piatt 1991). Th is situation is somewhat diff erent from studies of seabird communities elsewhere, where prey type and size are believed to be the main mechanisms of dietary segregation (Ashmole and Ashmole 1967, Harris 1977, Diamond 1983). Th us, it is plausible that species-specifi c foraging strategies, and inter-specifi c resource competition could lead to a re-assortment of krill dependent predators at South Georgia, particularly if some species are able to take advantage of diff erent krill aggregation states, or modify them, perhaps to their advantage.…”

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confidence: 99%

Ecological drivers of change at South Georgia: the krill surplus, or climate variability

Trathan¹,

Ratcliffe²,

Masden³

2012

Ecography

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Macaroni penguins Eudyptes chrysolophus are thought to be one of the most important mesopredators in the Southern Ocean having a greater impact on prey availability and abundance than any other seabird species. Their population centre has long been held to be South Georgia where populations were thought to comprise many million animals. Here we report the results of a recent census of the macaroni population at South Georgia undertaken using aerial survey methods. We report dramatic declines in numbers (∼1.0 million breeding pairs) compared to numbers observed in the late 1970s (∼5.4 million pairs), but show that these reductions have occurred principally at sites where numbers had previously been very large. During the breeding season, the main foraging grounds of birds from these sites overlap with the foraging grounds of Antarctic fur seals Arctocephalus gazella, a major competitor for their principal prey, Antarctic krill Euphausia superba. We suggest that the redistribution of the macaroni penguin population at South Georgia reflects the recent recovery of fur seal populations and thus the ongoing consequences of human intervention at South Georgia, a process which started more than 2 centuries previously. The implied resource competition and the observed population changes may also be exacerbated by recent reductions in Antarctic krill abundance which have been linked with reductions in seasonal sea ice following recent, rapid, regional warming in the Antarctic; however, the recovery of fur seal populations, and the ongoing recovery of krill‐eating whale populations argues that tropho‐dynamic interactions may be sufficient to explain the observed changes.

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Comparative ecology of seabirds in the Galapagos Archipelago

Cited by 22 publications

References 14 publications

Trawl discards in the diets of tropical seabirds of the northern Great Barrier Reef, Australia

Trawl discards in the diets of tropical seabirds of the northern Great Barrier Reef, Australia

Differential responses of boobies and other seabirds in the Galapagos to the 1986-87 El Nino-Southern Oscillation event

Ecological drivers of change at South Georgia: the krill surplus, or climate variability

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