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Diet Dynamics and Trophic Relations of Laysan and Black-footed Albatrosses

Progress Reports (PDF): FY 2005

Abstract:
Laysan and Black-footed albatrosses are experiencing drastic declines in their breeding populations at their most important colonies. Because of their scavenging nature, the diet of these albatrosses may to some extent be dependant on fisheries such as drift-net and pelagic longline as a source of food. Project researchers will investigate the relationship between two North Pacific albatrosses and fisheries by analyzing the digestive tract contents and stable isotopes of albatrosses salvaged recently from the Hawaii longline fishery and breeding colonies of the Northwest Hawaiian Islands. Results from their findings will be compared to similar data collected from albatrosses salvaged from the squid and large-mesh drift net fishery a little over 10 years ago (Gould et al 1997). Understanding the relationship between these seabirds and fisheries is essential to the conservation and management of both the albatrosses and fisheries.

Breeding populations of Laysan Albatrosses (Phoebastria immutabilis) and Black-footed Albatrosses (P. nigripes) show disturbing rates of decline. At Midway Atoll, French Frigate Shoals, and Laysan Island (Northwestern Hawaiian Islands), where almost 90% and 67% of all Laysan and Black-footed albatrosses respectively nest, the US Fish and Wildlife Service (USFWS) estimated a 9.8% decline in the breeding population of Black-footed Albatrosses over the last 6 years, and a 32% decline in Laysan Albatross breeding attempts over a 10-year period. It is unknown whether these declines are due to natural fluctuations or are anthropogenic.

Reasons for the decline of seabird species may include changes in climate and interactions with fisheries (Duffy 1993, Furness 2002, Gjerdrum 2003). Both climate and fisheries interactions may alter colony sizes of seabirds because of changes in the amount, location, and type of food available to seabirds. It is possible that the decline of Laysan and Black-footed albatrosses may in part be due to changes in their trophic relations, namely, where on the food chain they feed.

The number of trophic levels on a food chain is limited by primary productivity and energy lost through respiration, so that during times of low productivity and availability of food, it might be expected that the natural diet of albatrosses would reflect shifts to lower trophic positions; however, if scavenging from fisheries supplements diets, then there should be shifts to higher trophic positions. Using a combination of digestive tract content analysis (DTCA) and stable isotope analysis (SIA), project researchers will study the trophic dynamics of Laysan and Black-footed albatrosses associated with pelagic longline and high seas drift-net fishing in the Central North Pacific; to see both whether the pelagic longline fishing affects their diet, and whether the closure of the high seas drift gill-net fishery affected their diet. Also, as an alternate theory for changes in diet of these albatrosses, researchers will explore the possibility that changes in climate may affect their diet, thus exploring whether the potential changes in diet, because of climate change and fisheries interactions, correlate with the declines.

Comparing data collected from this research to the data collected by Gould et al. (1997) provides a unique opportunity to reexamine the trophic dynamics of Laysan and Black-footed albatrosses and project researchers should be able to determine if the demise of the drift net fishery affected the trophic positions of North Pacific albatrosses. Extending this data set will also provide a means to determine whether the Hawaii longline fishery affects the trophic positions of the albatrosses, determine whether evidence suggesting Black-footed Albatrosses scavenge more than Laysan Albatrosses do is consistent with the conclusions of Gould et al. (1997) (even after the closure of the drift net fishery), and whether there are differences in the trophic relations between juveniles and adult albatrosses that may result from their use of different foraging strategies.

Hypotheses
H0 1: There is no difference between the current trophic positions of North Pacific albatrosses and those of a decade ago.
H0 2: Pelagic longline fisheries do not cause an increase in the trophic positions of North Pacific Albatrosses.
H0 3: There is no difference between the diets of Laysan and Black-footed albatrosses.
H0 4: There are no difference between the diets of juvenile and adult Laysan and Black-footed albatrosses.

Methodology
Sampling
. Project researchers have access to approximately 100 albatrosses salvaged from either pelagic longline fishing vessels (NMFS Observer Program), previous longline fishing experiments and from breeding colonies at the Northwestern Hawaiian Islands Refuge (through special permits from the USFWS). It is anticipated that up to an additional 50 albatrosses from the breeding colonies and 100 albatrosses from the Hawaii longline fishery may be salvaged for research purposes.

Age determination. Age classing will be based on the degree of involution of the bursa of fabricius (Broughton 1994). Specifically, each albatross will be divided into 3 age classes depending on the size of the bursa of Fabricius: <50 mm² = breeding-age; >75 mm² and < 500 mm² = prebreeding; and >600 mm² = newly-fledged.

Digestive tract content analysis. Methods for digestive tract content analysis will follow methods previously described by Gould et al. (1997). Digestive tract content analysis will be performed on all salvaged birds that suffered no damage to their digestive tract at the time of death. Stomachs will be examined and prey items will be identified, weighed, sorted into taxonomic groups, and classified as one of the following: cephalopods, non-cephalopod invertebrates, fish, and inorganic items. Inorganic items will be described but excluded from the analysis. For each level of precision in identification, the percent frequency of occurrence (F), percent wet mass (M), and the percent number of items (N) will be calculated (Gould et al. 1997). From these, an index of relative importance (IRI) will be calculated using the following formula: IRI = %F (%N + %M). For each food group, a %IRI will be calculated by dividing the IRI for the group by the total IRI.

Isotope analysis. Muscle tissue from the salvaged albatrosses and a selection of 50 food items will be dried, ground, lipid-extracted, and ground again to a homogenous fine powder. Isotopic ratios will be determined using an online stable isotope ratio mass spectrometer located at the University of Hawaii Stable Isotope Biogeochemistry Laboratory. Researchers will run 5 of the muscle tissue samples 10 additional times in order to demonstrate reproducibility of this method.

Funding for this 2-year project to be awarded in mid-2004.

Literature cited:
• Broughton, J.M. 1994. Size of the bursa of fabricius in relation to gonad size and age in Laysan and Black-footed albatrosses. Condor 96:203-207.
• Duffy, D.C. 1993. Stalking the Southern Oscillation: environmental uncertainty, climate change, and North Pacific seabirds. In Vermeer, K., K.T. Briggs, K. H. Morgan, and D. Siegel-Causy. (eds.) The status, ecology, and conservation of marine birds of the North Pacific. Can. Wild. Serv. Spec. Publ., Ottowa.
• Furness, R.W. 2002. Impacts of fisheries on seabird communities. Scientia Marina 67:33-45.
• Gjerdrum, C.A., M.J. Vallee, C.C. St. Clair, D. F. Bertram, J. L. Ryder, and G. S. Blackburn. 2003. Tufted Puffin reproduction reveals ocean climate variability. PNAS 100:9377-9382.
• Gould, P., P. Ostrom, and W. Walker. 1997. Trophic relationships of albatrosses associated with squid and large-mesh drift-net fisheries in the North Pacific Ocean. Can. J. Zool. 75:549-562.

Principal Investigators:

Dr. David C. Duffy
University of Hawaii at Manoa
Dept. of Botany
3190 Maile Way, St. John 409
Honolulu, Hawaii 96822
USA
Phone (808) 956-8218
FAX (808) 973-2936
email: dduffy@hawaii.edu

Mr. Jeremy Bisson
University of Hawaii at Manoa
Dept. of Zoology
2538 McCarthy Mall
Honolulu, Hawaii 96822 USA
email: jeremy_Reagh7@hotmail.com

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This page updated August 22, 2006