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.
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