Advances in Marine Biology (eBook)

4.2.1.2 Albatrosses

4.2.1.2.1 Black-Browed Albatross (Thalassarche melanophris)

• Location: South Georgia (breeding) and South Sandwich Islands (foraging).

• IUCN Red List Status: Endangered.

• Population size: 74,296 pairs (South Georgia).

• Population trend: 25% decrease in South Georgia in last 20 years.

• Diet: Krill, cephalopods, fish.

• Foraging range: Variable; early in breeding season north of South Georgia to the Antarctic Polar Front; during brooding Shag Rocks; post brooding, South Orkney Islands and Antarctic Peninsula; winter, Benguela current, Patagonian Shelf, off south coast of Australia.

• Breeding: Adults arrive in September. A single egg is laid in November to December. Eggs hatch late December to early January and chicks fledge April/May.

Foraging Ecology

Black-browed albatross (Figure 35) usually seize prey at the surface, but are also capable of diving to a maximum of 4.5 m depth (Prince et al., 1994a). They often associate in large numbers with fishing vessels (ACAP, 2010a), and sometimes also with large ocean predators, such as killer whales (Sakamoto et al., 2009). The diet of the black-browed albatross varies depending on location and inter-annually within a single location/region (ACAP, 2010a; Xavier et al., 2003). At South Georgia, between the years of 1996 and 2000, krill (E. superba) formed the majority of the diet of black-browed albatross (38% by mass), followed by squid (31%) and fish (27%; Xavier et al., 2003). However, in some years, fish were the most important component of the diet (1998, 1999; mainly C. gunnari and southern barracudina, Magnisudis prionosa) or cephalopods (1996, 1997; mainly Galiteuthis glacialis, Kondakovia longimana, Martialia hyadesi, Moroteuthis knipovitchi; Xavier et al., 2003). Breeding success in black-browed albatross appears to be positively correlated with the quantities of the notothenioid fish C. gunnari in the diet (Xavier et al., 2003). In previous years, such as 1986, other fish, P. guntheri (51% of estimated fish biomass, probably obtained from a commercial fishery) and Icichthys australis (40%), were the main prey species (ACAP, 2010a).

As is true of other seabird species, foraging distance varies depending on the time of year for black-browed albatross. At South Georgia, observations have indicated that during egg incubation, adults forage to the north of the islands in sub-Antarctic waters north of the Antarctic Polar Front (Phillips et al., 2008; Figure 36). During brooding, they forage mainly in areas around Shag Rocks, while at the post-brooding stage they forage mainly to the south at the South Orkney Islands and the Antarctic Peninsula (Phillips et al., 2008; 2009; see Figure 35). Over winter, black-browed albatross migrate mainly to the Benguela shelf region, although a small number of birds migrate to the Patagonian Shelf or to areas off the coast of Southern Australia (Phillips et al., 2005a; Figure 36).

Life History

Black-browed albatross nest in colonies. The entire breeding cycle lasts about 8 months. Birds begin to return to colonies in South Georgia in late September and egg-laying begins in late October to early November. A single egg is laid and is incubated for about 68 days before hatching in late December to early January. Chicks fledge April to May. Juveniles do not return to land until they are at least 2 years of age to 6 years of age. Birds begin to breed at a median age of 10 (range 8–13 years). Mortality of black-browed albatross chicks mainly arises from the abandonment of the nest following the failure of one of the adults to return from a foraging trip, or predation of chicks by brown skuas and giant petrels (nests on the periphery of colonies are particularly vulnerable; Forster and Phillips, 2009).

Population Trends

Breeding populations of the black-browed albatross have declined in both South Georgia and the Falkland Islands, the latter being the most important breeding area for the species. In South Georgia, comparisons between 1985/1986 and 2003/2004 show a decrease of 17.5% in the number of pairs (1.1% per annum) in single species colonies, and of 32.8% (2.2% per annum), in mixed colonies, with larger decreases indicated for individual colonies over a similar time period (e.g. Bird Island, 44%; Poncet et al., 2006). Overall, population size in South Georgia has declined from approximately 101,488 pairs in the late 1980s/early 1990s (Prince et al., 1994b), to 74,296 pairs in the 2003/2004 breeding season (Poncet et al., 2006). This decline (about 25% of the population size) is similar in order to the decline of black-browed albatross in the Falkland Islands (Arnold et al., 2006).

It is likely that declines in population sizes of black-browed albatross are occurring for a number of reasons, but accidental mortality from fisheries in the southern hemisphere is probably the most important factor. The distribution of black-browed albatross coincides with the major pelagic and demersal longline and trawl fisheries that are known sources of mortality for the species (see Figure 36). Longlines are recognized as a significant source of mortality for black-browed albatrosses that dive on the baited hooks and are caught or entangled and drowned. Off South Africa, longline fisheries for tuna and swordfish between 1998 and 2000 were estimated as killing between 19,000 and 30,000 seabirds per year, including black-browed albatross (Ryan et al., 2002). Pelagic longline fisheries managed by the International Commission for the Conservation of Atlantic Tuna (ICCAT) overlap substantially with the distribution of black-browed albatrosses in the South Atlantic. Estimates between 2003 and 2006 suggest up to 3900 individual black-browed albatrosses per annum were killed in ICCAT-managed longline fisheries during this period, and that the species made up the highest proportion of identified seabird by-catch (Tuck et al., 2011). Other longline fisheries with a by-catch of black-browed albatross include the swordfish, Xiphias gladius, fishery off Chile, the Argentine longline fisheries targeting toothfish and kingclip, Genypterus blacodes, the Uruguayan pelagic longline fishery and artisanal fisheries in Brazil targeting tuna (ACAP, 2010a; Azocar et al., 2011; Goya et al., 2011; Jimenez et al., 2009). Historically, longline fisheries off Australia, the Falkland Islands and South Georgia were also sources of mortality for black-browed albatross.

Trawl fisheries are also a source of mortality for black-browed albatross, including aerial collisions with trawl warps, drowning in nets as they are deployed, or, more commonly, becoming entangled in trawl warps and subsequently drowning during trawling operations (Sullivan et al., 2006; Watkins et al., 2008). Such impacts have been identified in finfish fisheries in the Falkland Islands where 1411 birds (predominantly black-browed albatross) were killed in 2002–2003 (Sullivan et al., 2006). In the deep-water hake fishery off South Africa where, based on observations in 2004 and 2005, an estimated 18,000 birds (range of 8000–31,000) were killed annually in 2004 and 2005, of which an estimated 39% were black-browed albatrosses (Watkins et al., 2008). Significant mortality of black-browed albatross also occurs in the trawl fishery in Golfo San Jorge off Argentina (ACAP, 2010a).

Mortality of seabirds from fishing operations can be dramatically decreased with the implementation of technical measures within a fishery, or other mitigation measures such as seasonal closures. For longline fisheries, such measures include changes in the timing of fishing operations, technical measures such as deployment of streamer lines over longlines during deployment to deter birds from diving on baited hooks, and the deployment of longlines through shoots (ACAP, 2010a, 2011a, b; Phillips et al., 2010). Such measures have been successful, and in South Georgia by-catch of albatross in the toothfish longline fishery fell from 5755 individuals in 1997 to zero by 2006 (Croxall, 2008). An identification scheme allowing hooks to be attributed to specific vessels has also been introduced by the Government of South Georgia and the South Sandwich Islands. Effective measures have also been developed to reduce bird mortality in trawl fisheries and include banning the discharge of offal during net deployments, binding nets to allow them to sink rapidly with minimal risk of entanglement of birds, and the employment of streamers on the trawl warps, bird scaring lines over nets (e.g. Tori lines) or booms around the stern of the...