1
TROPHIC DYNAMICS OF ALBATROSSES ASSOCIATED WITH
SQUID AND LARGE-MESH DRIFTNET FISHERIES IN
THE NORTH PACIFIC OCEAN
1 2 3 Patrick Gould , Peggy ostrom , William Walker
1 Alaska Science Center, National Biological Service, 1011 E Tudor Road, Anchorage, AK, 99503, 2 Michigan State University, Department of Geological Sciences, 206 East Lansing MI, 48824, 3 National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 7600 Sand Point Way, NE, Seattle, WA, 98115
Author to whom mss correspondence should be sent:
Patrick J. Gould phone: ( 907) 786-3382 fax: (907) 786-3636 E-mail: [email protected]
Running head: Albatross trophic dynamics 2
TROPHIC DYNAMICS OF ALBATROSSES ASSOCIATED WITH
SQUID AND LARGE-MESH DRIFTNET FISHERIES IN
THE NORTH PACIFIC OCEAN
1 2 3 Patrick Gould , Peggy ostrom , William Walker
1 Alaska Science Center, National Biological Service,
1011 E Tudor Road, Anchorage, AK, 99503, 2 Michigan
State University, Department of Geological Sciences,
206 East Lansing MI, 48824, 3 National Marine Mammal
Laboratory, Alaska Fisheries Science Center, National
Marine Fisheries Service, NOAA, 7600 Sand Point Way,
NE, Seattle, WA, 98115
ABSTRACT: Diets of Laysan (LAA} and Black-footed albatrosses (BFA} killed in squid and large-mesh driftnets in the transitional zone of the North Pacific
Ocean were investigated by examining the contents of digestive tracts and o13c and o15N in breast muscle tissues. Stable isotope values were also obtained for nestlings of both species on Midway Island in the
Northwestern Hawaiian Islands. Combining digestive tract and stable isotope studies proved effective in interpreting intra- and inter-specific food habits of these North Pacific albatrosses. The results show that
(1} LAA occupy a lower trophic level than BFA. Based 3 on &15N values, there is a difference between the two species of one trophic level for non-breeding adults from the transitional North Pacific and 1/3 trophic level for nestlings from Midway Island; (2) both LAA and BFA prey on over 47 species (30 families} of marine organisms including coelenterates, arthropods, mollusks, fish, and carrion; (3) non-breeding individuals of both species supplement their traditional diets by taking advantage of prey made available by commercial fishing operations (e.g., net caught squid and offal); (4} while feeding from driftnets, LAA and BFA have almost identical diets including a percent index of relative importance of 41% and 39% respectively for neon flying squid; (5) in the absence of fisheries related foods, non-breeding LAA feed most heavily on fish, and BFA feed most heavily on squid; and (6) LAA carry higher loads of inorganic material, especially degraded plastics, than BFA. These results indicate that high seas driftnet fisheries provide high trophic level food supplements to the diets of albatrosses in the transitional North Pacific. These food supplements diminish the requirement for resource partitioning between non breeding LAA and BFA and thereby may alter historic distribution and abundance patterns. 4 KEY WORDS: Marine birds • Black-footed albatross • Laysan albatross . Trophic dynamics • Transitional North Pacific Ocean • Prey selection. o13c • o15N . Driftnet fisheries • Stable isotope
INTRODUCTION
Albatrosses are among the most frequent marine bird species to interact with commercial fisheries. Impacts of fisheries on these and other marine birds include direct mortality through entanglement in fishing gear, competition for resources, removal of natural competitors, concentration of food through the discard of bycatch and trapping of otherwise unavailable prey within nets, and supplementation of resources through disposal of offal. A direct affect of such impacts is a change in ecosystem energetics through the alteration of feeding relationships. Among albatrosses specifically, fisheries related activities could affect intra- and inter-species resource partitioning. Variability in food utilization within and between albatross species has been documented in the southern hemisphere (Prince 1980, Weimerskirch et al. 1986, Weimerskirch et al. 1988). This partitioning is manifested by differences in foraging ranges, feeding zones, and diets. Resource partitioning may occur in both the breeding and non-breeding seasons, and competition may be intensified in times of food scarcity. Commercial fishing operations may allow the overlap of otherwise disjunct albatross 5 feeding zones by providing a new, abundant, and easily obtained
food supply (Weimerskirch et al. 1988). Diet may be assessed directly through examination of digestive tract contents and indirectly through examination of feeding behavior and stable carbon and nitrogen isotopic analysis. Whereas analysis of digestive tract contents provides a short-term record of recently ingested material, isotope values are averages of the components of the diet that have been assimilated over a longer period of time (i.e., an estimated half life for turnover of muscle tissue is 27 days (Tieszen et al.
1983). The isotope technique is based on the observation that the Nitrogen or carbon isotopic composition (o 15 N and o13c, respectively) of an animal's tissue deviates by a consistent amount from its food source (Wada et al. 1987, Harrigan et al.
1989). Although the o13c of a consumer differs by approximately
1o/oo from its diet, the o15 N value of an organism is approximately
3o/oo greater than its food source (DeNiro and Epstein 1978,
DeNiro and Epstein 1981, Macko et al. 1982, Wada et al. 1987, Fry
1988, Minigawa and Wada 1984, Harrigan et al. 1989, Ostrom and
Fry 1994). Similar values for trophic level fractionation (shifts in isotope values between two successive trophic levels) have been observed for a variety of equatorial to polar marine communities and provide the basis for analysis of trophic structure (Parker 1964, McConnaughey and McRoy 1979, Dunton et al. 1989, Harrigan et al. 1989, Hobson 1990, 1991, 1993, Rau et al. 1992, Hobson et al. 1994). We use stable carbon and nitrogen 6
isotopic and digestive tract content analyses to examine trophic
relationships, resource partitioning, scavenging and fisheries
interactions in two congeneric species of albatrosses with overlapping ranges: Laysan (Diomedia immutabilis; LAA) and black footed (D. nigripes; BFA).
METHODS
study area and sample procurement
This study was conducted primarily in the transition zone
(35°N to 46°N and 170°E to 148°W) of the North Pacific Ocean
(Figures 1 and 2). This area marks a transition from subarctic to subtropical water and includes the subarctic and subtopic frontal zones (Roden 1980, Roden and Robinson 1988, Roden 1991).
The subarctic and subtropic frontal zones are areas of rapid change in water temperature and salinity. Major currents in the area move from west to east originating from the confluence of the cold oyashio and warm Kuroshio currents east of Japan. Along
170°E the Emperor Seamounts connect the Aleutian Islands with the leeward Hawaiian Islands. These seamounts are high enough to produce eddies, upwelling and other disturbances in overlying epipelagic waters (Royer 1978, Roden and Taft 1985, Roden 1991).
The eastern edge of the study area borders on the dilute domain identified by Favorite et al. {1976) as an area of considerable surface water dilution. 7 Specimens of breeding age, pre-breeding and recently fledged LAA and BFA and associated food items were obtained from driftnets set from May through November in 1990 and 1991 (Ito et al. 1993). Driftnets trapping these individuals had been soaking in the water for five to 20 hours prior to being taken shipboard. After removal from the nets, specimens remained on deck for several hours until they were identified, cataloged and frozen. Frozen specimens were shipped to the Burke Museum, Seattle, Washington, where they were autopsied and digestive tracts and breast muscle tissues were removed for dietary and geochemical analyses. Examination of reproductive tracts, bursa's, and molt patterns showed all specimens to be reproductively inactive. Nestling LAA and BFA were obtained from Midway Island in 1992. Midway is located in subtropical water near the southern border of the transitional zone {Figures 1 and 2). Breast muscle tissue was removed from recently deceased individuals and frozen prior to geochemical analyses. Tissues were selected only from individuals that contained some body fat. The presence of body fat indicated that factors aside from starvation, such as dehydration, may have contributed to the death of these individuals.
Digestive Tract Content Analysis
Digestive tracts (stomach and gizzard) from 247 nonbreeding albatrosses from the transitional North Pacific were examined 8 (Figure 1) . Each tract was carefully rinsed in order to recover tiny remains adhering to the lining. The contents were then sorted into four categories: cephalopods, fish, non-cephalopod invertebrates, and inorganic material. Tissue remains from each prey category were weighed and recorded independently. Inorganic items were also enumerated and weighed. Of the 247 digestive tracts examined, 87.4% contained at least one organic item, and 76.9% contained at least one piece of plastic or other inorganic item. We assumed that only the organic material represented food and therefore, conducted our analyses of organic and inorganic materials separately. Identification of remains of prey relied primarily on identification of fish otoliths, cephalopod beaks, and arthropod exoskeletons. Minimum number of these prey was determined by the greater number of left or right otoliths, upper or lower cephalopod beaks, or the number of paired but identical body parts (e.g., eyes) divided by two. Enumeration of individual prey through the use of otoliths, beaks and eyes works well only if the prey is consumed intact. In our sample, however, it is evident that in addition to ingestion of whole, smaller prey, the birds also scavenged portions of species which were too large to be consumed intact such as neon flying squid (Ommastrephes bartrami), Pacific pomfret (Brama japonica), and blue shark (Prionace glauca). In these cases, estimation of numbers of individual prey items relied heavily on identification and enumeration of fleshy 9 remains such as portions of tentacles and wings for squid and bones, scales and teeth for fish. As a result, the number of specimens of the larger species (e.g., neon flying squid) is an estimate of the number of individual prey which were partially scavenged upon and not the number consumed. In the case of non cephalopod invertebrates, the number of individuals of those that were crushed or compacted (e.g., a cluster of barnacles) was impossible to determine accurately. Squid stomachs appeared in both LAA and BFA samples. The contents of these squid stomachs represent a potential source of secondary introduction of beaks and otoliths into albatross digestive tracts and, thus, compromises our data to an unknown degree. To assess the relative importance of major prey items in albatross diets, we use the index of relative importance (IRI) modified from Pinkas et al. (1971). The IRI incorporates percentage by number (N), weight (W), and frequency (F): IRI = %F(%N + %W).
Age class analysis
We follow Broughton (1994) in dividing our samples into three age classes on the basis of the size of the bursa of Fabricius: <50 mm2 = breeding age; >75 and <500 mm2 = pre breeders; and >600 mm2 = newly fledged. Broughton used banded albatrosses that were obtained in our study. His data thus represent a subset of ours. 10
Isotope analysis
Breast muscle tissue from 129 LAA and 70 BFA from the transitional North Pacific, May-November, 1991, and from 31 LAA and 15 BFA from Midway Island, May, 1992, were retained frozen for isotopic analyses (Figure 2). In addition, an assortment of digestive tract items from the North Pacific specimens was frozen and retained. Muscle tissues from albatrosses and their prey were freeze dried, ground, lipid extracted, and ground again to a homogenous fine powder. Lipids were removed by soxhlet extraction using an azeotropic mixture of chloroform and methanol for seven hours. Lipid extracted samples were prepared for isotopic analysis by a modified Dumas combustion (Macko et al. 1987). Purified gases were obtained by cryogenic gas separation and subsequent isotopic determinations were performed using a
PRISM stable isotope ratio mass spectrometer (VG Isotech Ltd.). Samples of barnacle (Lepus), were emersed in 10% HCL to remove inorganic carbon and the resulting muscle tissue was processed as described above. Stable carbon and nitrogen isotope ratios are expressed as:
13 15 3 o C or o N = [ (Rsa""~/Rstandard) - 1] X 10 where R is 13Cf 12c or 15N/ 14N for o13c or o15 N, respectively. The standard for carbon is the Chicago Peedee Belemnite (PDB) and for nitrogen the standard is atmospheric N2 • Reproducibility of these measurements is 0.1%o (Table 1). 11 The average nitrogen isotopic composition of the diet of LAA and BFA can be estimated from mass balance equations:
where odiet = oconsuner - estimate of nitrogen isotopic discrimination between the consumer and its diet, fj = the fractional
contribution of the food item, ~ f . = 1, and 6 . = the isotopic 1=1 1 1
composition of the food item. For the purposes of this study,
J.Oo/oo was chosen for the estimate of isotopic discrimination between the consumer and its diet.
RESULTS
Classification of Digestive Tract Contents
Prey eaten by albatrosses include arthropods (6 species, 6 families), coelenterates (1 species, 1 family), gastropods (1 species, 1 family), squid (18 species, 10 families), octopus (2 species, 2 families), fish (17 species, 10 families), and mammals (1 species, 1 family). Cephalopods (especially neon flying squid) are the preeminent prey category for both LAA and BFA, although LAA select a wider variety of prey than BFA (Table 2). In the combined diets of LAA and BFA, 50.0% of the prey species 12
and 58.1% of the prey families are shared. Small fish and non
cephalopod invertebrates are relatively more important in LAA
diets than they are in BFA diets, while medium sized squid are
relatively more important in BFA diets. The predominance of
scavenged neon flying squid and Pacific pomfret in the digestive
tracts of both species, however, makes direct comparisons
difficult and possibly misleading.
Size of Prey In Digestive Tracts
Direct measurement of prey lengths were obtained for the few
prey items that were recovered in tact. Lengths of partially
ingested or digested squid and fish were extrapolated from beak
and otolith sizes, respectively. The size of prey items from
both albatross species ranged from 1 mm (total length) for a
formicidae to 446 mm DML (dorsal mantle length) for two neon
flying squids. The one partially consumed giant squid
(Architeuthis sp.) in our data was estimated to be 1.5 m DML by using beak length - mantle length regression equations from a similar species from the north Atlantic (Clarke 1986) . Six intact lanternfish (Lampanyctus jordani) in LAA digestive tracts ranged from 46.3-106.5 mm (Mean= 77.8 mm), and 16 intact Pacific saury ranged from 120 to 210 mm (Mean= 163.4 mm). The dorsal mantle length of five club-hook squid (Onychoteuthis borealijaponicus) from BFA ranged from 66.3-100.4 (mean= 87.9) while two from LAA measured 77.7 and 287.9 mm. The estimated 13 lengths of neon flying squid partially consumed by our albatrosses (170 to 446 mm DBL) are representative of the size of squid taken by the commercial squid driftnet fleets (Murata and Hayase 1993). There was no significant difference between LAA and BFA in the lengths of neon flying squid scavenged (t=1.33, P=0.19, df=48).
Relative Percent Contribution of Prey in Digestive Tracts
The bulk of the material in our digestive tract samples from both LAA and BFA consisted of shredded freshly consumed tissue from neon flying squid and Pacific pomfret (see the weight percentages in Table 2). Since flesh is normally digested within hours of consumption, the last meal of both species in our sample was likely scavenged directly from driftnets just prior to death or entanglement. The percent frequencies, numbers, weights, and total Index of Relative Importance of this scavenged material in the digestive tracts of LAA were nearly identical to that of BFA (Tables 2 and 3). Even though there are no differences between LAA and BFA in the amounts of neon flying squid and Pacific pomfret scavenged from driftnets, there are some intra-specific differences between age classes. Breeding age birds of both LAA and BFA have higher %IRI values for neon flying squid and lower %IRI values for Pacific pomfret than fledglings (Table 3). Neon flying squid are larger and probably more easily obtained from nets than Pacific 14 pomfret and thus may be a more preferred food source. This suggests that older, more experienced and probably stronger, individuals out-compete younger individuals for neon flying squid. Younger birds consume larger amounts of the less preferred food such as Pacific pomfret. Using this same analyses, we detected no intra-specific differences between the sexes. Prey consumed prior to the last meal can be inferred by examining hard body parts (fish otoliths, squid beaks, arthropod exoskeletons) that resist digestion. such material is light in weight and has its greatest influence on dietary estimations if frequency of occurrence, percent numbers, and %IRI are used. Although neon flying squid still rank high in these terms, other prey items can be seen to be important in the overall diet.
Lanternfishes (myctophidae) and Pacific saury (Cololabis saira) are especially important in the diet of LAA, while squids (Gonatidae and Cranchiidae) are especially important for BFA (Table 2}. To remove the effects associated with LAA and BFA scavenging at driftnets, we excluded all neon flying squid and Pacific pomfret and re-evaluated the digestive tract data (Table 4}. The results show a greater contribution of fish and invertebrates to LAA diets and of medium to large squid to BFA diets (Figure 3}. In the absence of food made available by commercial fisheries, the relative importance of fish is greater than that of squid in LAA diets. Male and fledgling LAA rely on fish to a greater 15 extent than females and other age classes. Breeding-age LAA and BFA rely more on non-cephalopod invertebrates than other age classes (Table 4). Differences in prey selection between sexes and age classes of BFA are small relative to LAA. The only exception to this is that, in the absence of neon flying squid and Pacific pomfret from driftnets, male BFA rely heavily on non cephalopod invertebrates while female BFA do not (Table 4}.
Ingestion of Inorganic Material
Many species of marine birds ingest small pieces of plastic and other inorganic material (Day 1980, Azzarello and Van Vleet 1987, Ryan 1987, Moser and Lee 1992). Sileo et al. (1990) found that the digestive tracts of albatross nestlings contained the largest individual items and greatest diversity of plastic items of all 18 species of Hawaiian seabirds they studied. They found plastic pellets more often in LAA (52%) than BFA (12%). We found inorganic material in 94.2% of LAA digestive tracts and 60.2% of BFA tracts (Table 5). Most of the inorganic items were from artifacts (i.e., manufactured items). Identifiable artifacts included gloves, bottle caps, plastic wrap, pieces of styrofoam and cardboard, small screws, cigarette filters, and a cigarette lighter. Pieces of stone and pumice were more frequent in LAA digestive tracts than in BFA tracts although pieces of stone and pumice made up a greater percentage of the total number of inorganic items in BFA digestive tracts. Our specimens were 16
captured while feeding from driftnets so we examined our data
specifically for the occurrence of pieces of monofilament line.
In contrast to other inorganic material, monofilament was found
more frequently and in greater numbers in BFA than in LAA
digestive tracts.
Clearly, LAA have higher loads of inorganic and plastic
materials, but take, on average, smaller items than BFA (Table
5). Differences in quantity and quality of inorganic items
ingested by LAA and BFA suggest differences in foraging behavior
or feeding techniques. A greater amount of monofilament line in
BFA than in LAA digestive tracts suggests that BFA feed more
frequently from driftnets than LAA. Other albatross behaviors
that could account for inter-species differences include 1)
greater reliance on nocturnal feeding by LAA when birds are less
able to discriminate between small objects and thus take more
non-food items, and 2) heavier reliance by BFA on large animals,
thus reducing the intake of small objects such as pieces of
plastic. Interpretation of differences in the sizes and numbers
of ingested plastics between LAA and BFA might be confounded by
inter-species differences in abrasion and breakage due to muscle
contractions and to differential residence times of items in digestive tracts between LAA and BFA. our data are insufficient
to choose between these various options. It is, however, likely
that the results are a consequence of some balance between all of these processes. 17
Isotopic Analyses
The 615 N and 613c values of the muscle tissue of a consumer
are related to variations in both trophic level of food sources
and isotopic composition of organic material at the base of the
food chain. Both of these sources of variation influence the
nitrogen and carbon isotopic values of albatrosses. Individual
birds salvaged in the transitional North Pacific that have
recently arrived from their breeding grounds (e.g., Midway
Island) may exhibit isotopic values significantly affected by
their diets in the breeding area. These individuals are most
likely to arrive in early summer (June and July; Rice and Kenyon
1962, Fisher and Fisher 1969, Gales 1993). Consequently, isotope
data for albatrosses from the North Pacific Transition Zone were
stratified by species, age (fledgling, pre-breeder, and breeding
age) and time (June-July for newly arrived migrants, and August
November for longer term residents) . Within each age group of
LAA or BFA, no significant differences in o15 N or o13 c values were observed between time periods (ANOVA, p=0.01, df=2,103 and df=2,62 for LAA and BFA, respectively). Therefore, we did not stratify these data by season in further inter- and intra specific comparisons.
If influences from both changes in the trophic level of food sources and isotopic values at the base of the food chain still affect the June-July results, isotopic shifts associated with diet would have to be off set by variation in 613 C and 615 N at the 18
base of the food web that are of similar magnitude but opposite
sign. This possibility could occur but seems unlikely.
Furthermore, any isotopic differences at the base of the food web
between Midway and the transition zone or variations in food
sources that do exist, do not appear to be of such magnitude that
they result in differences in o13c and o15N values between birds
that are early migrants to the transition zone and those that
have been in residence there for at least one month. Given that
we know that dietary differences exist between birds from Midway
and those of the Transition Zone, we interpret the isotopic
comparison of nestlings to different age groups from the
Transition Zone in terms of dietary effects as opposed to
variations at the base of the food web. We recognize that
interpretations presented herein could be modified if further
understanding of spatial variations in isotopic differences at
the base of the food web become available.
The most salient feature of the isotope data is that for
each age class, average o15N and o13c values of LAA are lower than those of BFA (Figure 4). Differences in average o15N and o13c
values between LAA and BFA for breeding age (3.1 and O.So/oo, respectively) and pre-breeding birds (2.7 and 0.6%o, respectively) are indicative of approximately one trophic level.
Differences in average nitrogen isotope values between LAA and
BFA nestlings (1.3%o) and fledglings (1.0%o) suggest that there
is one third of a trophic level difference between the two species for these groups. Higher average o13c values of BFA 19
relative to LAA for nestling and fledgling age classes (0.9 and
0.6o/oo, respectively) are also consistent with an inter-species
difference in trophic level.
The magnitude of the range in o15N of LAA and BFA nestlings
on Midway is 1.6 and 1.7o/oo respectively. Among fledgling
albatrosses from the transitional North Pacific, the o15N values
fall within a 2.2 and 3.7%o range for LAA and BFA respectively.
The magnitude of the range in o15 N values among pre-breeders and
breeding age birds is even higher (>4.1o/oo for LAA and 2.7%o for
BFA). The increase in the magnitude of the range in o15 N values associated with fledgling, pre-breeding, and breeding age birds relative to that of nestlings suggests increased variance in diet
or foraging techniques once the nestlings have fledged and as they mature.
There are no significant differences in nitrogen and carbon
isotope values between males and females among either LAA or BFA
(Table 7). Intra-specific comparisons of age groups (Table 8) indicate that there are no significant differences in o15N values between nestling and fledglings for LAA and BFA. For LAA, the average o15N value of breeding age birds is significantly lower than that of nestlings and fledglings. For BFA, the average o15N value of breeding age birds is significantly higher than that of nestlings and fledglings. There are no significant differences in o13c values between age groups of BFA. The average o13c value of LAA nestlings is significantly different from both fledglings and breeding age birds. 20
DISCUSSION
Laysan and black-footed albatrosses co-exist in the
temperate North Pacific Ocean. Counts at colonies indicate that
LAA outnumber BFA by more than 12:1 (McDermond and Morgan 1993).
However, LAA are most abundant in the western Pacific while BFA
tend to be more evenly distributed. Thus, LAA outnumber BFA in
the west and BFA outnumber LAA in the east (Rice and Kenyon 1962,
Gould et al. 1983, Kuroda 1988, 1991, McDermond and Morgan 1993).
LAA weigh less, have longer and more slender bills, and better
night vision than BFA. Both species are surface feeders and take
food in the upper meter of the water column either by surface
seizing or contact dipping (Ainley and Sanger 1979, Harrison et al. 1983, McDermond and Morgan 1993). Although both albatrosses have access to the same food resources, these differences in morphology and spatial distribution could impose interspecific differences in diet. LAA, for example, may be more capable than
BFA of rapid retrieval of certain types of prey that are only active on the surface at night (Harrison et al. 1983, McDermond and Morgan 1993). In contrast, BFA may be better adapted to scavenging naturally occurring large carrion or refuse from ships than LAA (Harrison et al. 1983, Fefer et al. 1984).
The present study focuses on birds whose behavior is influenced by commercial fisheries. The entrapment of animals, especially large squid, in nets and the discarding of large amounts of offal from shipboard processing of commercial 21
fisheries catches, provide an easily found, easily taken, and
abundant source of food for many species of marine birds. The
interaction of albatrosses with commercial fisheries is
emphasized by the observation that albatrosses, especially BFA,
are well known ship followers and are among the 10 most
frequently killed marine birds in high seas fisheries. An
estimated 17,500 LAA and 4,400 BFA were killed in high seas squid
and large-mesh driftnet fisheries in 1990 (Johnson et al. 1993,
Gould and Hobbs 1993). This ratio of 4 LAA to 1 BFA caught in high seas driftnets is much lower than the ratio of 12 LAA to 1
BFA in the total population. This suggests 1) a greater attraction of BFA to fisheries activities, 2) differences in morphology and behavior make BFA more susceptible to entanglement
in driftnets, 3) the ratio of LAA:BFA is lower in the central
Pacific than it is in the western Pacific, or 4) some combination of these factors.
While scavenging from driftnets, LAA and BFA food habits are nearly identical. Net-caught neon flying squid form the major part of their diets (Tables 2 and 3). Digestive tracts of LAA and BFA also contained the remains of a wide variety of small prey items not normally entangled in driftnets (e.g., barnacles; small fish and squid). If neon flying squid and Pacific pomfret, the two species most frequently caught in high seas driftnets, are eliminated from dietary considerations (Table 4; Fig. 3), it becomes clear that in the absence of a fisheries related food supply, non-breeding LAA in the transitional north Pacific eat 22 more fish than squid while the reverse is true for BFA. Nitrogen isotope values indicate that pre-breeding and breeding age LAA are approximately one trophic level below BFA of corresponding maturity and that LAA nestlings and fledglings are about one-third of a trophic level below corresponding age groups of BFA (Figure 4). As evidenced by atypically high or low o15N values among fledglings, pre-breeders and breeders, a few individuals of both species appear to specialize at the extreme ends of their respective food chains. This suggests that certain LAA and BFA may rely exclusively on high trophic level food captured from driftnets or offal as sources of food while others may ignore the nets and commercial fisheries completely. The observation that LAA feed at a lower trophic level than BFA may be related to the suggestion of Harrison et al. (1982) that the differences in diets between the two species are related to LAA being better adapted to feeding at night than BFA. This could explain the greater use of lanternfish by LAA than BFA in our sample and the associated lower o15N of LAA relative to BFA. Given that lanternfish have a lower o15 N value than squid (Table 6), the proportionally higher percentage of lanternfish in the diet of a LAA would result in a decrease in their o15N values relative to those of BFA. Unlike our study, Harrison et al. (1982) did not show that LAA consumed more lanternfish than BFA. They found squid to be a major component of both LAA and BFA diets but that LAA food samples contained significantly less flying fish ova, by volume, 23
than BFA samples. These data were primarily based on food delivered to nestlings (95% of LAA and 85% of BFA samples). The comparison of our data to those of Harrison et al., clearly suggests that the diet of nestlings differs from that of birds within the Transition Zone. Furthermore, the emphasis of fish ova in BFA nestling diets, may explain differences in the o15N values between BFA nestlings and adults. If the fish and fish eggs consumed by BFA nestlings have a lower o15 N value than squid, the increased percentage of this food source could account for the 1.3%o decrease in the average o15N value of nestlings relative to the o15N of pre-breeding and breeding age BFA from the Transition Zone. Similarly, the higher percentage of squid in the diets of LAA from Midway might also explain our observation that for LAA the average o15 N of the nestlings is higher than that of breeding age birds from the Transition Zone.
The combined effect of an increase in the o15 N of LAA nestlings and decrease in the o15 N of BFA nestlings relative to birds in the Transition Zone, results in a much smaller inter-specific difference in o15 N for nestlings (1.3%o difference) than for non breeding birds from the transition zone (3.1%o difference).
Whether differences in diets of nestlings result from special energy requirements or from differences in foraging areas and food availability is unclear at the present time.
To explore the relationship between isotope and digestive tract data, average o15N values of LAA and BFA can be compared to predictions of isotopic composition of the diet based on mass 24
balance equations (Table 9; Harrigan et al. 1989, Ostrom and Fry
1993). The average o15N value of BFA is 3.1o/oo greater than that
of the predicted nitrogen isotopic composition of the diet. This
shift is consistent with previous estimates of the changes in
o15N between a consumer and its diet (Wada et al. 1987, Harrigan
et al. 1989, Ostrom and Fry 1994). The implication is that
material ingested from the driftnet environment by BFA (digestive tract data) is typical of their long term diet. The average o15N
of LAA on the other hand, shows a shift of only 0.4%o relative to the predicted nitrogen isotopic composition of the diet. This
indicates that the average prey consumed by LAA from driftnets is at a higher trophic level than that which was assimilated over many previous meals. Consequently, LAA must rely more heavily on
lower trophic level items than was indicated by the digestive tract content data. Their diet includes small prey (e.g., lanternfish) that are not available from driftnets and, therefore, only appear as a minor component of the digestive tract contents subsequent to feeding events that are influenced by the fishery.
Further understanding of resource partitioning between sexes or between age classes is difficult to assess from our sample.
In the presence of the fishing operations, food is abundant and readily available allowing potentially competing groups of albatrosses to overlap (Weimerskirch et al. 1988). In the north
Pacific, driftnet fisheries minimize competitive encounters not only between LAA and BFA, but between intra-species sex and age 25 cohorts. There is no significant difference in o13c or o15N values between the sexes in either LAA or BFA, and very little difference in IRI values of these birds when they are associated with fisheries operations (Tables 3,7 and 8). Significant differences in o15N values between nestlings and fledglings in comparison to breeding age birds may reflect differences in stable isotope abundance between the subtropical and transition zone feeding locations (Table 8). It may, however, be significant that in both LAA and BFA, neon flying squid are relatively less important, and Pacific pomfret relatively more important, for newly fledged individuals than for adults. The older, stronger, and more experienced individuals may out-compete younger birds for preferred food items (e.g., neon flying squid) caught in the driftnets. Intra-species food resource partitioning between sex and age cohorts becomes apparent when fisheries related food items are eliminated from the data set (Table 4). For LAA, IRI values indicate that females are generalists while males rely heavily on fish. Non-cephalopod invertebrates are relatively more important in the diets of breeding age LAA than in newly fledged young. For BFA, non-cephalopod invertebrates are relatively more important in male diets than in female diets. No differences in IRI values are evident between BFA age cohorts.
CONCLUSIONS 26
In the presence of high seas driftnet fisheries, LAA and BFA have similar diets. Both species feed heavily on animals trapped
in the nets and discarded during processing operations. When the dominant prey scavenged from the fisheries are removed from consideration, resource partitioning becomes evident between the species as well as between intra-specific sex and age cohorts.
Over the long-term, LAA feed one trophic level below BFA.
Commercial fisheries providing a bountiful food supply that diminishes resource partitioning between these two species. The advent of fisheries related food resources has the potential for seriously affecting albatross populations. Increased survival rates, especially in years of natural food shortage, and altered distribution patterns are only two of the possible population parameters that could be affected. Quantitative studies of both the birds and the fisheries are needed to quantify and model the impacts of this association, and similar studies are needed for the short-tailed albatross (Diomedea albatrus) . Now that large scale squid and large-mesh driftnet fisheries have ceased to operate in the north Pacific, a study similar to the one reported here would significantly increase our understanding of fisheries albatross relationships by providing an excellent before and after study design.
Acknowledgements. The National Marine Fisheries Service and U.S.
Fish and Wildlife Service provided financial and logistical support for this project. We especially thank Linda Jones of the 27
former, and Kent Wohl of the latter organization. We thank all
of the people who worked with us in the 1990-1991 High Seas
Driftnet Observer Programs, especially Skip McKinnel, Pacific
Biological Station, Nanaimo, Canada; Hiroshi Hatanaka, National
Research Institute of Far Seas Fisheries, Japan; Shean-Ya Yeh,
National Taiwan University and James Sha, Council of Agriculture,
Taiwan; and Yeong Gong, National Fisheries Research and
Development Agency, Republic of Korea, and especially the
scientific observers from all of these countries who obtained the
specimens for us. Sievert Rowher, and staff at the Burke Museum,
University of Washington, prepared the albatross specimens and
removed the digestive tracts and breast muscle tissues. Larry
Spear of Point Reyes Bird Observatory, sorted and weighed the
digestive tract contents. Ken McDermond, Theiry Work, and Jim
Ludwig obtained the albatross breast muscle tissue Midway Island.
Alan Kohn and Katie O'Reilly identified the non-cephalopod
invertebrates in the digestive tracts, and Martin Robards analyzed the inorganic material. We thank Patrick O-Connor,
Kevin Pilichowski, Lisa Koch, and Amanda straky of Michigan State
University for sample preparation for isotopic analysis and data base management.
We thank John Piatt, Alan Springer, Steven Ignell, and
Tanaka for reviewing the initial draft of this paper and xxxxx, xxxxx, and xxxxx for the peer review of the final draft.
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Table 1. Procedural reproducibility of isotopic measurements for lipid extracted muscle tissuefor four seabird specimens
c515N cS•Jc Species ( %o) ( %o)
Black-footed albatross 15.5 -18.6 15.4 -18.5 15.6 -18.6
Average ± so 15.6±0.1 -18.6±0.1
Laysan albatross 11.5 11.7 11.7
Average ± so 11.6±0.09
Laysan albatross 14.5 14.6 14.6
Average ± so 14.6±0.03 sooty shearwater 11.8 -18.3 12.1 -18.3 11.9 -18.3
Average ± so 11.9±0.1 -18.3±0.0 Table 2. Percent frequency of occurrence, number, weight, and index of relative importance (IRI) of prey items identified to lowest taxon in Laysan and black-footed albatrosses from the transitional North Pacific, 1990-1991. + = < 0.05. Values in parentheses below column headers refer to the total number or weight of stomachs and gizzards with food
Food Items Laysan Albatross Black-Footed Albatross Albatross Prey Prey Prey Albatross Prey Prey Prey digestive tracts number weight IRI digestive tracts number weight IRI (133) (411) (8280.6gms) (12797.6) (83) (312) (9307 .8 gms) (14298.6) Non-Cephalopod Invertebrates 23.3% 20.4% 4.7% 4.6% 16.9% 24.0% 1.3% 3.0% Unid. Non-cephalopod Invertebrates 0.8% 0.2% 0.3% + 4.8% 1.3% 0.1% + Arthropods 21.8% 19.0% 4.3% 4.0% 12.0% 22.8% 1.2% 2.0% Insecta 0 0 0 0 1.2% 0.3% + + Hymenoptera Formicidae 0 0 0 0 1.2% 0.3% + + Malacostraca 12.8% 4.6% 0.8% 0.5% 10.8% 2.9% 0.3% 0.2% lsopoda Anuropus bathypelagicus 2.3% 0.7% 0.1% + 1.2% 0.3% + + Leptostraca Nebaliidae 0.8% 0.2% + + 0 0 0 0 Decapod a 10.5% 3.6% 0.7% 0.4% 10.8% 2.9% 0.3% 0.2% Grapsidae Planes minutus 0.8% 0.2% + + 0 0 0 0 Oplophoridae Unid. Oplophoridae 0.8% 0.5% 0.1% + 2.4% 0.6% 0.1% + Unid. Shrimp 9.0% 2.9% 0.6% 0.2% 8.4% 2.2% 0.2% 0.1% Cirripedia Thoracic a 11.3% 14.4% 3.4% 1.6% 1.2% 19.2% 0.9% 0.2% Lepus fascicularis 3.0% 1.0% 2.3% 0.1% 0 0 0 0 Lepus sp. 6.8% 2.4% 1.1% 0.2% 1.2% 19.2% 0.9% 0.2% Lepus sp. (cyprids) 1.5% 10.9% + 0.1% 0 0 0 0 Coelenterates 1.5% 1.0% + + 0 0 0 0 Hydrozoa Velellidae
'"-~ Food Items laysan Albatross Black-Footed Albatross Albatross Prey Prey Prey Albatross Prey Prey Prey digestive tracts number weight IRI digestive tracts number weight IRI (133) (411) (8280.6 gms) (12797.6) (83) (312) (9307.8 gms) (14298.6) Velella sp. 1.5% 1.0% + + 0 0 0 0 Molluscs Gastropods 0.8% 0.2% + + 0 0 0 0 Mesogastropoda Janthinidae Janthina sp. 0.8% 0.2% + + 0 0 0 0 Cephalopods 81.2% 45.5% 74.3% 76.0% 91.6% 63.5% 75.9% 89.3% Unid. Cephalopoda 11.3% 4.4% 2.4% 0.6% 18.1% 5.1% 6.7% 1.5% Teuthoidea 74.4% 37.5% 68.2% 61.4% 80.7% 54.8% 69.2% 70.0% Unid. Teuthoidea 5.3% 1.7% 1.3% 0.1% 3.6% 1.0% 0.2% + Architeuthidae Architeuthis sp. 0.8% 0.2% + + 0 0 0 0 Ommastrephidae Ommastrephes bartrami 54.9% 23.8% 67.9% 39.3% 55.4% 26.6% 67.2% 36.3% Unid. Ommastrephidae 0.8% 0.2% 0.3% + 0 0 0 0 Onychoteuthidae Onychoteuthis borealijaponicus 4.5% 1.5% 1.7% 0.1% 12.0% 4.2% + 0.4% Unid. Onychoteuthidae 0.8% 0.2% + + 1.2% 0.3% 0.1% + Gonatidae 5.3% 1.7% + 0.1% 9.6% 2.6% + 0.2% Gonatopsis borealis 0 0 0 0 1.2% 0.3% + + Berryteuthis anonychus 0.8% 0.2% + + 1.2% 0.3% + + Gonatus sp. c.f. G. pyros 0.8% 0.2% + + 0 0 0 0 Gonatus sp c.f. G. berryi 3.0% 1.0% + + 4.8% 1.3% + + Gonatus sp. A 0.8% 0.2% + + 1.2% 0.3% + + Gonatus sp. 0 0 0 0 1.2% 0.3% + + Enoploteuthidae Abraliopsis affinis 0 0 0 0 1.2% 0.3% + + Octopotheuthidae Octopoteuthis deletron 3.0% 1.0% + + 7.2% 2.2% + 0.1% Food Items Laysan Albatross Black-Footed Albatross Albatross Prey Prey Prey Albatross Prey Prey Prey digestive tracts number weight lRl digestive tracts number weight IRI ( 1331 (4111 (8280.6 gmsl (12797.61 (831 (3121 (9307.8 gmsl (14298.6) Octopotheuthis sp. 2.3% 1.0% 0.5% + 2.4% 0.6% + + Unid. Octopotheuthidae 0.8% 0.2% + + 1.2% 0.6% + + Histioteuthidae Histioteuthis dofleini 1.5% 0.5% + + 6.0% 1.9% 0.3% 0.1% Histioteuthis sp. 0.8% 0.2% + + 6.0% 1.9% + 0.1% Mastigoteuthidae Mastigoteuthidae sp. 0 0 0 0 6.0% 2.2% + 0.1% Chiroteuthidae Chiroteuthis calyx 1.5% 0.7% + + 3.6% 1.0% + + Chiroteuthis sp. 0 0 0 1.2% 0.3% + + Cranchiidae 12.0% 4.1% 0.2% 0.4% 22.9% 9.9% + 1.6% Leachia dislocata 1.5% 0.5% + + 4.8% 3.8% + 0.1% Megalocranchia sp. 2.3% 0.7% + + 0 0 0 0 Taonius pavo 1.5% 0.5% + + 0 0 0 0 Taonius sp. A 1.5% 0.5% 0.2% + 8.4% 2.6% + 0.2% Taonius sp. 3.0% 1.0% + + 3.6% 1.0% + + Galiteuthis phyllura 1.5% 0.7% + + 3.6% 1.3% + + Galiteuthis sp. 0.8% 0.2% + + 2.4% 1.0% + + Octopod a 9.0% 3.6% + 0.3 8.4% 3.8% + 0.2% Ocythoidae Ocythoe tuberculata 9.0% 3.6% + 0.3 8.4% 3.5% + 0.2% Alloposidae Alloposus mollis 0 0 0 0 1.2% 0.3% + + Fish 45.1% 34.1% 21.0% 19.4% 33.7% 11.9% 20.8% 7.7% Unid. Fish 6.0% 2.9% 2.6% 0.3% 4.8% 1.3% 0.3% 0.1% Carcharhinidae Prionace glauca 0.8% 0.2% 0.3% + 1.2% 0.3% 2.3% + Searsiidae Sagamichthys abei 0.8% 0.2% + + 1.2% 0.3% + + Food Items Laysan Albatross Black-Footed Albatross Albatross Prey Prey Prey Albatross Prey Prey Prey digestive tracts number weight IRI digestive tracts number weight IRI (133) (411) (8280.6 gms) (12797.6) (83) (312) (9307 .8 gms) (14298.6) Photichtyidae lchthyococcus sp. 2.3% 0.7% + + 0 0 0 0 Myctophidae 24.8% 20.7% 0.1% 4.0% 6.0% 1.9% + 0.1% Electrona risso 8.3% 6.1% + 0.4% 2.4% 0.6% + + Symbolophorus californsis 6.8% 3.9% + 0.2% 0 0 0 0 Ceratoscopelas sp. 4.5% 1.7% + 0.1% 0 0 0 0 Lampanyctus ritteri 0.8% 0.2% + + 0 0 0 0 Lampanyctus jordani 7.5% 4.6% 0.1% 0.3% 1.2% 0.3% + + Lampanyctus sp. 0.8% 0.2% + + 0 0 0 0 Diaphus gigas 0.8% 0.2% + + 0 0 0 0 Diaphus theta 0.8% 0.2% + + 0 0 0 0 Notoscopelas japonicus 2.3% 0.7% + + 0 0 0 0 Unid. Myctophid 6.0% 2.4% + 0.1% 2.4% 1.0% + + Scomberosocidae Cololabis saira 6.8% 4.6% 3.7% 0.4% 1.2% 0.3% + + Moridae Unid. Moridae 0.8% 0.2% + + 1.2% 0.3% + + Macrouridae Coryphaenoides sp. 0.8% 0.5% + + 3.6% 1.3% + + Diretmidae Diretmus sp. C.F. D. argenteus 0.8% 0.2% + + 0 0 0 0 Bramidae Brama japonica 11.3% 3.9% 14.3% 1.6% 20.5% 5.8% 18.1% 3.4% Gemplyidae Gempylus serpens 0 0 0 0 1.2% 0.3% + + Mammals 0 0 0 0 2.4% 0.6% 2.1% + Cetacea Unid. Cetacean 0 0 0 0 2.4% 0.6% 2.1% +
,. 44
Table 3. Percent of IRI for major food groups in diets of albatrosses from the Transitional North Pacific including neon flying squid and Pacific pomfret
Sample Cephalopods Fish Non-cephalopod Neon Flying Pacific Size Invertebrates Squid Pomfret
Laysan Albatross:
Total 128 73.7 20.8 5.5 40.8 1.6 Breeding Age 30 83.8 5.2 11.0 57.3 0.4 Young pre-breeders 79 69.8 24.0 6.2 37.2 1.2 Newly Fledged 19 66.3 33.6 0.1 30.4 5.8
Male 58 72.7 24.1 3.2 38.7 0.9 Female 70 74.3 18.5 7.2 42.6 2.2
Black-footed Albatross:
Total 76 90.8 6.6 2.6 38.8 3.0
Breeding Age 21 94.5 1.7 3.7 50.8 1.0 Young pre-breeders 33 87.4 11.1 1.6 34.3 3.8 Newly Fledged 22 92.4 7.5 o. 1 33.0 4.2 Male 28 80.7 12.0 7.3 42.1 5.9 Female 48 95.5 4.1 0.4 36.4 1.6 45
Table 4. Percent of IRI for major food groups in diets of albatrosses from the Transitional North Pacific excluding neon flying squid and Pacific pomfret
Sample Cephalopods Fish Non-cephalopod Size Invertebrates
Laysan Albatross:
Total 82 27.4 49.5 23.2
Breeding Age 18 28.5 19.1 52.4 Young pre-breeders 53 23.2 54.2 22.7 Newly Fledged 11 43.6 56.2 0.2
Male 35 22.7 62.2 15.1 Female 47 37.6 32.6 29.7
Black-footed Albatross:
Total 60 87.8 4.8 7.4
Breeding Age 15 81.5 0.3 18.2 Young Pre-breeders 26 85.5 9.9 4.7 Newly Fledged 19 98.2 1.6 0.2
Male 23 59.1 4.9 36.0 Female 37 95.6 3.2 1.2 46
Table 5. Ingestion of inorganic material by Laysan (LAA) and black-footed (BFA) albatrosses in the transitional North Pacific, 1990-1991. The category "artifacts" contains all manufactured items including monofilament. Pieces of monofilament, however, were not weighed so that the weight of artifacts (*) excludes pieces of monofilament
Black-footed Lay san
No. of digestive tracts sampled 93 154
% of tracts with inorganic items 60.2 94.2 % of tracts with artifacts 53.8 93.5 % of tracts with monofiliment 11.8 5.2 % of tracts with pumice 10.8 16.2 % of tracts with stone 6.5 19.5
No. of ionorganic items 171 2265
% artifacts 80.1 95.7 % monofiliment 11.1 0.5 % pumice 11.7 1.6 % stone 8.2 2.7
Weight of artifacts* 27.1 gms 298.0 gms
artifact weight/digestive tract 0.3 gms 1.9 gms artifact weight/tract with artifact 0.6 gms 2.1 gms artifact weight/item 0.2 gms 0.1 gms
Length of largest artifact 60 mm 190 mm 47
Table 6. Average carbon and nitrogen isotope values of black-footed albatrosses, Laysan albatrosses, and prey items. All prey items were taken from albatross digestive tracts. 2SE = two standard errors about the mean
Species Sample c5 15N .snc Size Range Mean Range Mean Min Max ±2SE Min Max ±2SE Lays an albatross: Nestling 31 11.5 13.1 12.1±0.1 -20.1 -18.9 -19.4±0.1 Fledgling 14 11.6 13.8 12.5±0.3 -19.3 -18.4 -18.8±0.2 Pre-breeder 67 9.4 15.5 12.0±0.2 -19.8 -17.2 -18.8±0.1 Breeding Age 28 9.5 13.6 11.6±0.4 -19.6 -18.0 -18.9±0.1 Female* 76 9.5 15.5 12.0 0.2 -19.8 -18.0 -18.9 0.1 Male* 53 9.4 13.8 12.0 0.3 -19.7 -17.2 -18.8 0.1 Black-footed albatross: Nestling 15 12.6 14.3 13.4±0.3 -18.7 -18.0 -18.5±0.1 Fledgling 16 12.3 16.0 13.5±0.6 -18.9 -16.8 -18.2±0.3 Pre-breeder 28 13.0 16.8 14.7±0.4 -19.4 -17.1 -18.2±0.2 Breeding Age 20 13.4 16.1 14.7±0.3 -20.5 -17.0 -18.1±0.3 Female* 43 12.3 16.8 14.5±0.3 -20.5 -16.8 -18.3±0.2 Male* 26 12.5 15.8 14.1±0.4 -18.8 -17.5 -18.1±0.1
Neon flying squid 44 10.0 15.1 11.7±0.4 -20.9 -17.7 -18.4±0.2 Miscellaneous squid** 5 8.7 14.4 11. 6±1. 9 -19.1 -18.2 -18.5±0.4 Pacific pomfret 10 7.4 14.3 10.9±1.3 -20.4 -18.3 -19.2±0.5 Lanternfish 7 10.1 11.2 10.6±0.3 -20.4 -19.4 -19.9±0.3 Pacific saury 10 6.3 12.3 9.9±1.2 -20.3 -17.4 -18.7±0.7 Shrimp 5 5.8 10.3 8.0±1.8 -20.9 -18.4 -19.7±0.8 Barnacles 20 4.8 10.6 7.6±0.8 -20.0 -15.7 -19.1±0. 4 Pelagic crabs 5 4.4 8.3 6.5±1.3 -17.7 -15.2 -16.9±0.9
* Includes specimens intermediate between established age classesJr~,I~J,, ~~~~~~J ** Miscellaneous squid = Gonatidae, Octopoteuthidae, Histioteuthidae, and Cranchiidae 48
Table 7. Independent t tests of isotope values between sexes of Laysan and black-footed albatrosses from the transitional north Pacific, 1991. Sample sizes in parenthesis
LAYSAN ALBATROSS c515N c513c
Breeding Age t -0.907 -1.868 Females (21) vrs males (7) p 0.373 0.073
Newly Fledged t -1.319 -0.093 Females (8) vrs males (6) p 0.212 0.927
Total t -0.268 -1.541 Females (76) vrs males (53) p 0.789 0.126
BLACK-FOOTED ALBATROSS
Breeding Age t +0.272 +0.058 Females (14) vrs males (6) p 0.789 0.954
Newly Fledged t +1. 550 +0.576 Females (8) vrs males (8) p 0.143 0.574
Total t +1. 563 -0.856 Females (43) vrs males (26) p 0.123 0.395 49
Table 8. Independent t tests of isotope values between age classes of Laysan and black-footed albatrosses from Midway Island (nestlings, 1992) and the transitional North Pacific (fledglings and breeding aged birds, 1991). Sample sizes in parenthesis
LAYSAN ALBATROSS 15 15N 15 13C
Nestling (31) vrs t -2.417 +6.788 Fledgling ( 14) p 0.020 <0.001
Nestling (31) vrs t +2.602 +5.600 Breeding Age (28) p 0.012 <0.001
Fledgling (14) vrs t +2.980 +1.083 Breeding Age (28) p <0.001 0.295
BLACK-FOOTED ALBATROSS
Nestling (15) vrs t -0.449 -1.925 Fledgling ( 15) p 0.657 0.063
Nestling (15) vrs t -5.526 -1.986 Breeding Age (20) p <0.001 0.055
Fledgling (15) vrs t -3.487 +0.950 Breeding Age (20) p 0.001 0.349 50
Table 9. Mass balance equations for breeding age Laysan and black-footed albatrosses comparing fisheries related diets (FRO, from Table 5) to non-fisheries related diets (NFRO, from Table 6). Mean cSuN of birds is from Table 8
Diet Item Mean Percent Composition of Diet cS'~ Lays an Black-footed FRO NFRO FRO NFRO
Neon Flying Squid 11.7 27 0 43 0 Other Squid 11.6 57 29 51 82 Pacific Pomfret 10.9 5 0 1 0 Other Fish 10.2 0 19 1 0 Invertebrates 7.6 11 52 4 18
Mean c5 1 ~ of food 11.2 9.3 11.5 10.9 Mean c5 1 ~ of birds 11.6 11.6 14.6 14.6
Trophic Shift 0.4 2.3 3.1 3.7 Fig. 1. Study area showing collection locality of albatross specimens used in analyses of digestive tract contents ...... -;-"' !.-.... ~:' Aleutian Islands
47
0 oo.s;;;.0 • 45 0 cr ooo _..c§o o 0 !2l o o!i 4 ) 0 •o • Oo ~ g 0 t~• ~ • ~ 0 0 43 0 CD - ~ofJ' ~ 0 c ~o• • ~ ~ -diO~OOQ ~ 0 ~0 0 0~~· • .a "= 41 • • • 0. - 0 ....= If, 0 ·-«< 0 0 o •• ,..;! co •• (jl •$·0· oCll• • •• 0: • o. -= 39 0 t: 0 0 - • zc:> 37 • • 35 145 E 155 165 175 175 165 155 145W Longitude ';
Fig. 2. Study area showing collection locality of albatross specimens used in analyses of nitrogen and carbon stable isotope values in breast muscle tissue •W-.t,., ~ '·· '· '';"'~...... ~~ Aleutian Islands
47 0 • 45 oe 0 ... 0 • -~0 0 tr 0 eo fB 0~~ 0 0~ oo e. ~ • 0 • 43 0 (D - e ,.~ ~e .. 0'~:> 0 0 ~ 0 ooi~.@~ 1 - • -so 0 • 0 ~ OOo 0 0 • "CC 0 oe = 41 -0 ~- t"' ·-(IS - 0 0 o •• 0 ~- •• • ~ .., • e.ao• o .c 39 • • • n t: ~ ·- • z~ 37
35 165E 175 175 165 155 145W Longitude Fig. 3. Relative importance (% of IRI) of major food groups to the diets of Laysan (LAA) and black-footed (BFA) albatrosses comparing (A) diets including neon flying squid and Pacific pomfret scavenged from driftents to (B) diets excluding neon flying squid and Pacific pomfret ... fb
100 100 LAA BFA 80 80 -= -= "-4 -"-4 60 - 60 0 0
~ ~ R R u u 40 ...0 40 ...0 u u Q.. Q.. 20 20
0 0 A B A B
Cephalopod• :rhh lfon-aephalopod [j] [OJ Inw-erte'brate• Fig. 4. Stable isotope values of carbon ( c5 13C) and nitrogen ( .S 1'N) in breast muscle tissue of Laysan (LAA) and black-footed (BFA) albatrosses stratified by age class. Symbols represent means and standard errors 15 r- -
14 1- -
c715N ~+ 13 '"'" f BFA. _ ...... ~ ...... ~ ...... ~ *LA.A. 12 ... -
1 1 I I -20 -19 -18 -17 c713C G Ne•tlln; G Fledgling G Pre-breeder D Breeding Age