Conservation genetics of North Pacific Albatrosses caught in longline fisheries nets
Black-footed Albatrosses (Phoebastria [Diomedea] nigripes) are one of the many species of pelagic seabirds that are caught inadvertently by fisheries operations in their attempts to catch large numbers of fish. Such birds and other fauna caught in this way are termed incidental "bycatch". We are using mitochondrial DNA sequences to determing the geographical origin of black-footed albatrosses caught in fisheries nets in the north Pacific, in and around the Gulf of Alaska. Mitochondrial DNA (mtDNA) is a small circular molecule found in several copies per mitochondrion, and hence in thousands of copies per cell. Some parts othe mtDNA molecule change very fast, hence there is much genetic diversity within natural populations. Monica C. Silva, a graduate student in the lab, has been using the polymerase chain reaction (PCR) to study mtDNA sequence variation in bycatch albatrosses, and to compare their variation with mtDNA sequences of birds known to be breeding on the two main breeding sites, in the northwest Hawaiian Islands and in islands south of the main Japanese islands. The following is a synopsis of Silva's unpublished research, conducted in collaboration with scientists at the Yamashina Institute for Ornithology in Abiko, Japan, and soon to be published in a volume resulting from a symposium at the 1999 Pacific Seabird Group meetings on problems and prospects of seabird bycatch organized by Julia Parrish and Ed Melvin.
The origin of black-footed albatrosses caught in North Pacific fisheries revealed by mtDNA analysis: preliminary results
Most pressures threatening seabird populations today are anthropogenic and require multidisciplinary measures for their effective mitigation. Causes of mortality of Black-footed albatrosses Phoebastria (Diomedea) nigripes include the North Pacific commercial fisheries (Flint, 1998).
The Black-footed albatross is a socially monogamous species and, as most Procellariiformes, exhibits extreme demographic characteristics; it has low reproductive rates coupled with high life expectancy (Warham 1990). The adults (at least over 5 years old) breed on isolated oceanic islands, often in dense colonies (Whittow 1993) and the current population of breeding birds has been estimated to be approximately 60000 pairs (Cousins and Cooper 1999). Current geographic distribution for the species comprises two main discrete populations: a western group, including colonies from the Izu, Bonin and Senkaku islands (off the coast of Japan) and an eastern group, comprising the Northwestern islands of Hawaii, which holds approximately 95% of the world’s population. Like all seabird species belonging to this group, Black-footed albatrosses show high levels of nest fidelity, returning to the same colony (often the colony where they were born) year after year to breed, frequently with the mate from previous seasons. Such philopatry suggests that this species may be genetically structured across its breeding range. Also, there is geographic variation of some morphological traits, Japanese birds being smaller (Pers. comm., Hiroshi Hasegawa, Toho University, Chiba, Japan), which may indicate population differentiation.
A few thousand Black-footed albatrosses are caught every year in longline fisheries operating in the Gulf of Alaska, Bering Sea and also in waters north of the Hawaiian archipelago. Although Laysan and Short-tailed Albatrosses are also caught by longliners, some evidence suggests that Black-footed Albatrosses may be being caught in disproportionately large numbers relative to their population size (Cousins and Cooper 1999). This potential threat to the viability of Black-footed albatross populations motivated our genetic study, which had two goals: 1 - assess levels of DNA sequence divergence and thus genetic uniqueness of the two major groups of Black-footed albatrosses; 2 - determine the geographic origin of Black-footed albatrosses caught in pelagic longliners operating in the N. Pacific Ocean.
Thus far a total of 45 individuals have been screened for sequence variation in a short fragment, domain I, of the mitochondrial control-region (ca. 350 bp; Fig. 1). Included in this sample were 16 individuals from Tern island, 18 individuals from Torishima island, Izu group, and 11 individuals salvaged from longlines in the North Pacific and of unknown geographical origin in the Burke Museum Collection. MtDNA was extracted and analyzed following general protocols suitable for this type of analyses (Sambrook et al. 1989, Rozas & Rozas 1997). Preliminary results indicate that Japanese individuals have very low levels of uncorrected average percent nucleotide difference: 0.58± 0.28%. Hawaiian birds, on the other hand, show levels of percent nucleotide difference almost six times larger than the Japanese population (3.32± 1.69%) and among the highest percent nucleotide differences reported for an avian population for this gene segment (Edwards 1993, Wenink 1993; Baker and Marshall, 1997). We also constructed a phylogenetic tree of all the mitochondrial haplotypes identified among the 45 birds sampled (Fig. 2). In this tree Japanese and Hawaiian birds cluster according to geographic origin, indicating population differentiation. The relationships of the bycatch birds suggests that birds being caught in longlines derive mainly from Hawaiian populations. Consistent with banding data, the tree also suggests that possibly two bycatch samples probably are of Japanese origin.
In vertebrates, phylogenetically related mtDNA genotypes often constitute geographic assemblages separated by major genetic (and often geographic) breaks due to limited dispersal and high levels of philopatry in relatively discontinuous environments (Avise et al. 1987). More than a decade ago, Quinn & White (1987) proposed that if in birds, as in other vertebrates, one could detect assemblages of closely related mtDNA genotypes, then those assemblages could be genetically characterized and subsequently used to identify the geographic origin of any captured birds. According to our preliminary data, Black-footed albatrosses cluster according to their geographical region and the two main breeding groups seem to have been at least historically isolated. Such lack of gene flow, in part probably due to the high levels of colony fidelity (Warham 1990), permits the development of population-specific genetic markers which allows the assignment of captured birds to specific geographic regions.
The Hawaiian Islands hold about 20 times more breeding birds than the Japanese islands (Cousins and Cooper 1999) which is a likely explanation for such a difference of genetic diversity within each group. Also, a bottleneck can results in a decrease of genetic variability and population heterozygosity (Menotti-Raymond & O’ Brien 1993), with the resulting decreases related to the bottleneck's severity and duration (Nei, 1987). Black-footed albatross populations from both geographic groups suffered throughout their history periodical events of human exploitation (for feathers, oil and meat) and environmental-caused viability threats. Namely, numbers of Japanese (especially in Torishima) albatrosses declined and remained in extremely low numbers until the end of 1950’s (Hasegawa unpubl. in Cousins and Cooper 1999). It is therefore not surprising that Torishima birds display lower levels of genetic diversity.
In the future it will be important to sample not only more birds from these populations but more importantly other populations. With a thorough sampling protocol that would include the complete breeding range of this species, we should be able to describe more completely the genetic diversity within and among colonies of this species. Analyzing more mtDNA sequence and nuclear loci will help identify population-specific markers, but the frequency of these markers and the extent to which they aid in identifying source populations of bycatch albatrosses will ultimately depend on the extent of gene flow among colonies and regions. Sequence variation in the control region of mtDNA holds valuable information for the study of genetic population structure and migration of seabird species like the Black-footed albatross and therefore provides useful information for demographic models for seabird species.