Black-footed albatross (Phoebastria nigripes) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

The U.S. Fish and Wildlife Service generates estimates of the annual breeding population for Black-footed Albatross by using standardized techniques that include direct counts of active nests and estimates from chick production. The total world population of breeders does not include breeders that did not lay an egg, did not return to breed, or birds not yet of breeding age (USFWS unpubl. data 2005). A band-reading project underway at Tern Island, French Frigate Shoals since 1979 will be used to calculate the segment of the population not previously included in breeding bird counts (Flint pers. comm. 2005).

Since 1980, direct counts of active nests have been done at French Frigate Shoals and they have been done at Midway Atoll since the 1991/1992 breeding season (hatch year (HY) 1992). Estimates have been conducted at Laysan Island since 1992, but direct counts only began there in 1997/1998 (Naughton pers. comm. 2006).

In the years immediately following World War II, biologists working in the Hawaiian archipelago, and particularly on Midway Atoll, generated early population estimates for Black-Footed Albatross colonies (e.g., Fisher and Baldwin 1946; Rice and Kenyon 1962b; Robbins pers. comm. 2006). Methods used for these counts varied.

Abundance

The current global population size for Black-footed Albatross is estimated at 278,000 (BirdLife 2004a,b) to 300,000 individuals (Cousins and Cooper 2000). Over 95% of breeding birds nest in the Hawaiian Islands chain (Cousins and Cooper 2000; USFWS 2005a; Table 1) and the majority of Black-footed Albatrosses nest on two islands, Laysan Island and Midway Atoll.

In 2000, an estimated 62,000 pairs were breeding at 12 colonies (Cousins and Cooper 2000). In 2005, the total number was estimated at 61,141 pairs, with 21,006 and 21,829 pairs nesting at Laysan Island and Midway Atoll, respectively. The Japanese population consists of approximately 2,450 breeding pairs (Table 1).

Table 1. Best estimates of the number of breeding pairs of the Black-footed Albatross at all breeding sites to 2005/06 ( USFWS unpubl. data; Tershy pers. comm. 2006 for Revillagigedo Is.; Hasegawa pers. comm. 2006 for Japanese populations). See text for details.
Breeding site Number of pairs Last census year
Kure Atoll, Hawaii
2,020 table notea
2000
Midway Atoll, Hawaii
21,829
2004
Pearl & Hermes Reef, Hawaii
6,116 table notea
2003
Lisianski Island, Hawaii
3,737 table notea
2002
Laysan Island, Hawaii
21,006
2004
French Frigate Shoals, Hawaii
4,259
2004
Necker Island, Hawaii
1995
Nihoa Island, Hawaii
31
1994
Kauai, Hawaii
0
2004
Lehua Island, Hawaii
2002
Niihau, Hawaii
?
Kaula, Hawaii
0
1998
Oahu, Hawaii
0
2002
Izu Island (Torishima), Japan
1,900 table noteb
2005
Senkaku Islands, Japan
2002
Bonin Islands, Japan
2005
Guadalupe Island, Mexico
0
2003
Revillagigedo Islands, Mexico
0
2002
Total
61,570
 

In Mexico, a single nest was recorded on Guadalupe Island in 1998 and another on San Benedicto Island (Revillagigedo Group) in 2000 (Pitman and Ballance 2002). During the most recent survey (2002) of the Revillagigedos, no breeding Black-footed Albatross were recorded at San Benedicto but a prospecting bird was observed at Clarión Island. As pigs and sheep have recently been eradicated from the latter, Black-footed Albatross may now be breeding there (Tershy pers. comm. 2006). In January 2006 field crews recorded a pair of Black-footed Albatrosses exhibiting breeding behaviour at Guadalupe (Henry pers. comm. 2006). It is believed that these Mexican birds represent a new colonization event rather than re-colonization of an extirpated colony. In Japan, the small breeding population has increased in number and expanded in extent over the past 20–50 years (Hasegawa pers. comm. 2006).

Fluctuations and trends

The French Frigate Shoals colony, while only one quarter the size of the two largest colonies (Midway and Laysan) has the longest continuous time series of direct nest counts and is thus extraordinarily valuable when assessing long-term changes in this species with respect to concomitant anthropogenic factors, oceanographic conditions and changes in prey populations. The total number of breeding Black-footed Albatross on French Frigate Shoals has fluctuated annually from 2,760 to 5,067 pairs (Figure 10). Comparing annual counts to a 25-year average, anomaly values fluctuated from -1,195 to + 3,872 pairs/year, with 54% of years having positive anomalies. However, the number of active nests declined steadily from hatch years 1987 to 1996, following one of the lowest counts at this colony and ending with the second lowest count. In 1996, Whale-Skate – one of the islands at French Frigate Shoals – was lost to erosion; at its peak in 1984, 2,046 Black-footed Albatrosses nested at this site. After 1996, however, numbers at French Frigate Shoals steadily increased to reach pre-1985 values in 2005.

On Laysan Island, direct nest counts from hatch year 1998 to hatch year 2005 (n = 8 years) resulted in 19,900 to 23,297 breeding pairs (Figure 11; mean ± SD: 21,079 ± 1,382 pairs). The percent annual change (n = 7) varied from + 7.3 percent to - 17.1 percent. On Midway Atoll the number of breeding pairs from 1998 to 2005 was 17,617 to 21,829 (19,709 ± 1,346) (Figure 11). The percent annual change on Midway Island (n=7) varied from + 6.6 percent to -16.4 percent. Both colonies show the greatest annual decline from HY 1999 to HY 2000.

The number of breeding pairs on Laysan Island prior to HY 1998 was estimated using the density of eggs in quadrats (representing 5% of the total area) and multiplied by the total nesting area. The number of pairs by egg density estimates is not comparable to direct count methods adopted in 1998. On Laysan Island, where both techniques are still employed, the egg density method always produced a higher number of pairs than direct counts (Table 1).

Figure 10. Total count of active nests on French Frigate Shoals with 25-year average to examine inter-annual variation (USFWS unpubl. data 2005).

Figure 10.  Total count of active nests on French Frigate Shoals with 25-year average to examine inter-annual variation (USFWS unpubl. data 2005).

Figure 11. Black-footed Albatross population trends on Laysan Island (indirect active nest counts HY 1992–1997, direct counts 1998–2005) and Midway Atoll (direct counts HY 1992 – present; USFWS unpubl. data).

Figure 11.  Black-footed Albatross population trends on Laysan Island(indirect active nest counts HY 1992–1997, direct counts 1998–2005) and Midway Atoll (direct counts HY 1992 – present; USFWS unpubl. data).

Decades-long data gaps preclude an analysis of trends at the two largest Black-footed Albatross colonies over a period of three generations (60 years). A 1945 survey estimated 26,500 nesting pairs based on transect counts of surviving chicks (Fisher and Baldwin 1946). This yields a count of 35,333 pairs when the USFWS assumption of 75% nesting success is applied. Subsequent counts between 1954 and 1967 show a dramatic drop in number of pairs breeding at Midway, presumably due to post-war military operations (Rice and Kenyon 1962b; Robbins pers. comm. 2006). In contrast, birds on Laysan Island were less disturbed; in their 1956-57 survey of albatrosses in the Hawaiian archipelago, Rice and Kenyon (1962b) estimated that 34,000 pairs were nesting there (Figure 12).

Figure 12. Black-footed Albatross pairs at Laysan Islandand Midway Atoll, 1945–2005 (Fisher and Baldwin 1946; Rice and Kenyon 1962b; Robbins pers. comm. 2006; USFWS unpubl. data).

Figure 12. Black-footed Albatross pairs at Laysan Islandand Midway Atoll, 1945–2005 (Fisher and Baldwin 1946; Rice and Kenyon 1962b; Robbins pers. comm. 2006; USFWS unpubl. data).

Figure 13. The total number of breeding pairs on the three monitored Black-footed Albatross colonies: French Frigate Shoals, Midway Atoll and Laysan Island, 1992–2005. Prior to 1998, active nests on the latter colony were estimated by indirect methods and 95% confidence intervals indicate uncertainty in those years (USFWS unpubl. data).

Figure 13. The total number of breeding pairs on the three monitored Black-footed Albatross colonies: French Frigate Shoals, Midway Atoll and Laysan Island, 1992–2005. Prior to 1998, active nests on the latter colony were estimated by indirect methods and 95% confidence intervals indicate uncertainty in those years (USFWS unpubl. data).

Apparent declines in the number of Black-footed Albatrosses at their major colonies during the late 1990s, coupled with increasing awareness of the bycatch of albatross in longline fisheries, highlighted a conservation concern for this species (Naughton pers. comm. 2006). However, in 2000, when the number of breeders attending the colony was at its lowest, Pyle (2000) reasoned in a non-peer reviewed article that the cause for the decline was unclear and could be the result of changes in ocean productivity, an increase in adult mortality, a change in the breeding interval, or a combination of all three.

Concern for the low returning number of breeders in the late 1990s triggered several population projection estimates to evaluate the potential future status of the species. In 2000, a workshop was held to estimate the total mortality of Black-footed Albatross in longline fishing and participants calculated a 20% decline in three generations using demographic data from Laysan Albatross (because demographic data for the Black-footed Albatross were not available) (Cousins and Cooper 2000). In 2003, Lewison and Crowder published a similar population projection of the Black-footed Albatross based on bycatch rates for the observed portion of the US fisheries, and an unpublished estimate of the total annual fishing effort by international pelagic longline vessels. Lewison and Crowder (2003) used a basic age-structured matrix model and the same demographic parameters as Cousins and Cooper (2000), that is, those for Laysan Albatross. Both of the population projection estimates assumed unaffected demographics, i.e., that adult survival rates of (Laysan) albatross were not currently affected by incidental mortality in longline fisheries. The publication of the model by Lewison and Crowder (2003) and the workshop report by Cousins and Cooper (2000), in concert with the uncertainties surrounding total bycatch in the North Pacific, were the primary basis for listing the Black-footed Albatross as Endangered by the World Conservation Union (IUCN; BirdLife International 2004a,b; see Status designations).

In an attempt to understand the potential effect of the current estimates of fisheries mortality on future populations of the Black-footed Albatross, Wiese and Smith (2003), in an unpublished report, estimated growth rates using an age-structured matrix model based on the same demographic parameters (because these are still the only values available) but assumed that the demographic data were affected, that is, incidental mortality was already incorporated in the observed adult survival rates since longline fishing has occurred in the North Pacific since the mid-1900s. The result of the Wiese and Smith (2003) model showed population growth rates of 0.98-1.04 in the next 20 years, or in other words, no substantial decline. When they used their model to hindcast to population levels in 1998, the model successfully bound real data.

Since 2004, there has been growing controversy around the reliability of the published population projection estimates (and the IUCN listing) because current demographic data for the Black-footed Albatross were not used, the total fishing effort in the North Pacific, by all countries, was not known, and the bycatch rates for all foreign and US fisheries were not known because of the lack of adequate observer coverage and reporting. As well, mark/recapture information for banded birds was generally not sufficient to generate precise estimates of annual survival (Naughton pers. comm. 2006).

Early in 2006, the Patuxent Wildlife Research Center of the US Geological Survey (USGS) calculated demographic parameters for the Black-footed Albatross breeding in US territories (results not yet publicly available). A new USFWS monitoring program for Black-footed Albatross was initiated in 2005 as a result of the USGS research, which should provide survival estimates in the future (Naughton pers. comm. 2006).

In January 2006, a draft status assessment for Laysan and Black-footed albatrosses was submitted to the USFWS, and will be available for public review sometime in the future (Arata et al. 2006; Sievert pers. comm. 2006). J. Arata and others constructed a population projection estimate using an age-structured matrix model with the not-yet-available USGS Patuxent demographic data, all historical and current pelagic and longline fishing effort data available, spatially explicit bycatch rates based on observer coverage in the US and Canada, and estimated bycatch rates for foreign vessels based on FAO data (Sievert pers. comm. 2005). After acceptance and release by the USFWS, it will provide the most up-to-date assessment of population trends for both Laysan and Black-footed albatrosses.

In summary, the long-term population trends for this species are unclear despite a published estimate of a 60% decline in three generations (Lewison and Crowder 2003; also see Limiting Factors and Threats – Cumulative effects section). There is no doubt that Black-footed Albatross are caught in high numbers in North Pacific longline fisheries, but the recent expansion of breeding adults to Mexican islands and Wake Atoll is not necessarily in accordance with the behaviour of a dramatically declining population. Current data suggest that the interannual variability in counts of active nests is extremely high in this species and thus an assessment of the status of the population would vary depending on what time period was examined. BirdLife International based their recent listing on population models using data from the 1990s but since that assessment, the number of breeders has increased at all colonies. Some population projection model results showed weak to dramatic declines (Cousins and Cooper 2000 (non-peer-reviewed workshop proceedings); Lewison and Crowder 2003 (published, peer-reviewed paper)) while others show a relatively stable population (Wiese and Smith 2003 (non-peer-reviewed report, currently being revised for publication)). Inadequate demographic parameters, as well as limited information on fisheries effort and bycatch rates in domestic and international fisheries, are serious problems for all population projection models.

There is no specific trends analysis available for the Black-footed Albatross in Canadian waters.

Rescue effect

The concept of rescue effect does not apply to this species in Canada since it does not breed in this country.

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