Ancient murrelet (Synthliboramphus antiquus) COSEWIC assessment and status report: chapter 9

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Population sizes and trends

• Population estimates

Population declines have been observed at six colonies and eleven have been abandoned. Decreases have been observed on Langara, Lyell and Kunghit Islands because of introduced rats (Harfenist 1994; Bertram and Nagorsen 1995). Declines on Helgesen, Saunders and Limestone Islands have been attributed to raccoons, Procyon lotor (Gaston and Masselink 1997). Population increases of between 2.6% and 9.5% annually have been recorded on Reef, George, Ramsay and Lihou (Lemon and Gaston 1999, Table 1). Using data from colonies with surveys in both the 1980s and 1990s, an overall decline rate of approximately 18 percent over 10 years can be calculated.

Several small colonies in the southeast Moresby area have disappeared since the early 1970s. Murrelets were present on Low Island and the Skedans Islands in 1970, but were gone by 1983 (Summers 1974; Rodway et al. 1988). It is unlikely that either of these sites would have supported large populations because the islands are very small. Other colonies that have disappeared include Lucy, Cox, Instructor, Boulder, Sea Pigeon, Arichika, Bischof, Tar and High Islands (Rodway 1991). 

Population estimates

There are an estimated 256 000 pairs of Ancient Murrelets nesting on 31 colonies in the Queen Charlotte Islands (Rodway 1991; Gaston 1992; Vermeer et al. 1997). The largest colonies are found on Frederick, George and Langara islands. Since the time that surveying began in the early 1970s, 6 colonies have decreased in size, 4 have increased, while 11 have been abandoned (Table 1, and section on Canadian range).

* Counts or estimates of burrows: other figures are breeding population estimates in pairs of birds, except (+), colony area in ha.
Numbers for Frederick Island in 1998 are provisional, pending reanalysis of colony area.
Data from 1980 - 1988 for whole colony censuses are from Rodway et al. 1988, 1990 and 1994.
All estimates suggest increases ranging from 0,2 - 9,5% annually except where there are/were rats (Langara, Lyell, Kunghit), or raccoons (East Limestone, Helgesen).

Though there is evidence that birds can move between colonies that are close together, there is no evidence that individuals in abandoned colonies moved to another location.

Changes in population size of colonies recently surveyed in the Queen Charlotte Islands are summarized below. Many islands have not been surveyed in nearly 20 years and therefore that information has not been included as it offers no additional insight into population trends. Estimates made on Reef, East Limestone, Langara, Ramsay and George islands are based on counts or estimates of burrow numbers; Lyell, George, Helgesen and Lihou islands are estimates of breeding pairs of birds. Data from Frederick Island are provisional, pending re-analysis of the colony area. Overall, recent increases of between 0.2 and 9.5% have occurred on all islands except on those where rats or raccoons are found (Lemon and Gaston 1999). All colonies with alien predators decline, often dramatically.

Langara Island

Based on early surveys, historic levels of the Langara Island population were estimated at approximately 200 000 birds (Gaston 1994b). In 1971, initial estimates indicated a population of between 80 000 and 90 000 breeding pairs. Subsequent studies found that the population had declined to 25 700 pairs by 1981 (Rodway et al. 1983) and 24 100 pairs in 1988 (Bertram 1989). The area occupied by Ancient Murrelets had also contracted between 1981 and 1988. These data indicate a dramatic reduction in population size of what was probably the largest colony in the Queen Charlotte Islands and perhaps in the world (Nelson 1990).

Earlier studies showed that the colony on Langara Island was not only declining, but also becoming more dense, from a density of 840 burrows/ha in 1981 (Rodway et al. 1983) to 1358 burrows/ha in 1988 (Bertram 1989). Estimates of average density are obviously affected by the choice of colony boundary. For example, if some unoccupied areas are included, the density will be lower, but compensated for in the population size estimate by the increased area of the colony. However, a difference in where the colony boundary was does not explain the difference in density, because the mean density found by Bertram (1989) was greater than the maximum density found by Rodway et al. (1983). One reason for this increase in density may have been the presence of rats on the island.

Since the main rat eradication program was initiated on Langara Island in 1995, the population has not increased in size, though the colony area has expanded. By 1999, the breeding population had decreased from 14 630± 2 060 pairs in 1993, to 10 365 ± 2 011 pairs, but the difference was not statistically significant (Drever 2002). Preliminary analysis of 2004 data continues to show no statistically significant recovery for this population (Hipfner 2004, pers. comm.). However, in 1999, there was a significant decrease in burrow density (1800 ± 160 burrows/ha to 765 ± 104 burrows/ha). There was also an increase in overall colony size from 22.9 ha in 1993 to 35.6 ha in 1999, with colony expansion towards the shoreline where the rats used to be (Drever 2002). This supports the suggestion that the presence of the rats was influencing the burrow density in the colony. There may be other factors (e.g. changes in zooplankton production, mortality from commercial fishery) limiting the population on the island. (Drever 2002)

East Limestone Island and Reef Island

Studies of the Limestone Islands reveal that between 1974 and 1989, the population fell from an estimated 5000 pairs (Summers 1974), to approximately 1500 pairs (Rodway et al. 1988; Gaston et al. 1989). Only a small number of birds remained on the west island, occupying a small portion of the northeast corner. The population on East Limestone has either remained stable or decreased slightly (1% annual decrease) since the removal of the raccoons between 1985 and 1995, and presently (1999) is relatively stable (Gaston and Lemon 1996; Gray 2001). Reef Island, only 6 km away, experienced a population increase of 30% (annual increase of 2.9%) during that same time (1974-1989), based on census and chick-trapping results (Gaston and Lemon 1996). The presence of raccoons on East Limestone may explain the difference between colonies and may also explain the recruitment of Limestone birds to Reef Island (Gaston and Lemon 1996).

Rankine Islands

No discernable change in population has been noted on Rankine Island. The colony was surveyed in 2000, revisiting burrow density plots that had been set up in 1984. An analysis comparing the numbers of burrows in the plots in 1984 and 2000 revealed no change in numbers (M.J.F. Lemon, unpubl. data).

Frederick Island

Frederick Island was re-surveyed in 1998. There was a slight increase in population size, from 68 407 pairs in 1982 to 70 321 in 1998, an annual increase of 0.20% (M.J.F. Lemon, unpubl. data). There are no introduced predators on Frederick Island.

Other Islands

Introduced predators have caused reduction in populations on several islands, including Langara (rats), Helgesen (raccoons; annual decrease of 23% (Gaston and Masselink 1997), and may have caused the extirpation on others. The presence of rats has caused populations on other islands, such as Kunghit and Lyell, to decrease dramatically or disappear altogether, Kunghit Island experienced a decrease in colony size from 44.2 ha to 11.1 ha between 1986 and 1993 (annual decrease of 18%, Harfenist 1994, Bertram and Nagorsen 1995), and it now appears that the population was extirpated by 2004, (Hipfner 2004, pers. comm.). On Lyell Island, the population decreased from 10 656 burrows in 1982 to 8 332 in 1992 (25% decrease, or 2.5% per annum) with a 30% decline in colony area (Lemon 1993a). The impacts of predators appears to be long-lasting, as the populations on Helgesen, Little Helgesen and Saunders have shown no signs of recovery since raccoons were exterminated in the mid 1990’s (Hipfner 2004, pers. comm.).

On islands with no introduced predators, there has been an annual increase in population of between 0% and 9.5% (Lemon and Gaston 1999). Annual increases of 9.5% were seen on Lihou Island between 1986 and 1993 (Gaston and Masselink 1997), an island with no introduced predators, and this population continues to remain stable today (Hipfner 2004, pers. comm.)Some of these population increases may represent a shift in the distribution of birds away from islands with alien predators (Harfenist et al. 2002).

Monitoring plots

In 1985 and 1986, Canadian Wildlife Service (CWS) field crews set up permanent monitoring plots on several Ancient Murrelet colonies in the Queen Charlotte Islands, including George and Ramsay Island. The plots were mapped and clearly marked on the ground and the numbers of burrows counted:

George Island - Number of burrows in 8 monitoring plots increased from 258 burrows in 1985 to 327 in 1991, an increase of 27%. In 1996, a total of 367 burrows were present, a further 12% increase (Lemon 1997). Overall, the population is estimated to have increased from 11 614 pairs in 1985 to 17 384 in 1996, an annual increase of 3.7% (Lemon and Gaston 1999).

Ramsay Island - A 1992 survey of the 1985 plots showed that burrow numbers on 11 monitoring plots had also increased (Lemon 1993b). A survey in 2002 showed an overall increase of 58% between 1984 and 2002 (M. Hipfner unpubl. data). These data indicate that the numbers of Ancient Murrelets are increasing in the Gwaii Haanas area. Both George and Ramsay Islands have no introduced predators.