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Trophic Dynamics of Albatrosses Associated with Squid and Large-Mesh Driftnet Fisheries in the North Pacific Ocean

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Trophic Dynamics of Albatrosses Associated with Squid and Large-Mesh Driftnet Fisheries in the North Pacific Ocean 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
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