Interannual variations of the occurrence of epipelagic fish in the diets of the seabirds breeding on Teuri Island, northern Hokkaido, Japan
Introduction
The diets of breeding seabirds are known to reflect the abundance of prey fish in the surrounding waters of the seabirds colonies (Barrett, 2002; Hislop & Harris, 1985; Hatch & Sanger, 1992; Montevecchi, Birt, & Cairns, 1987; Montevecchi & Myers, 1995). Prey abundance for seabirds is typically dependent on marine environmental change (Aebisher, Coulson, & Colebrook, 1990; Durant, Anker-Nilssen, & Stenseth, 2003; Sydeman et al., 2001). Seabirds are active foragers, searching prey patches and selecting the most profitable prey within their foraging ranges (Hunt & Schneider, 1987; Suryan et al., 2002). Consequently, they may give us a strong, yet sometimes biased, signal of the changes in the total prey abundance (Cairns, 1987; Montevecchi, 1993). Species of seabirds with different foraging techniques, body sizes, and breeding modes, respond differently to prey stock changes (Cairns, 1987; Ainley, Sydeman, & Norton, 1995; Monaghan et al., 1996; Furness & Tasker, 2000). The monitoring of multiple seabird species, therefore, should be useful to reduce these biases. As data regarding the diets of breeding seabirds is easy to collect, we can potentially use it as a monitor of long-term fish stock changes (Montevecchi, 1993).
In the Sea of Japan/East Sea, along the western coastal side of Hokkaido, the strength of the warm Tsushima Current is known to affect marine ecological processes (Natsume, 1998; Takahashi et al., 2001). A total of 350,000 pairs of seabirds nests on Teuri Island (Osa & Watanuki, 2002), which is located on the northern boundary of this current, thereby giving us a good opportunity to study the variability of the ecological processes up to the topmost predators in this current system.
In order to use seabirds as an indicator of marine environmental change in the area of Teuri Island, we have been monitoring the summertime diets of three species of seabirds with varying foraging modes. These include black-tailed gulls (Larus crassirostris), rhinoceros auklets (Cerorhinca monocerata), and Japanese cormorants (Phalacrocorax filamentotus). This paper, describes the interannual changes of epipelagic fishes in their diets.
Section snippets
Study area and methods
The study was conducted on Teuri Island (44°25′N, 141°52′E), about 30 km off the coast of northern Hokkaido, in the Sea of Japan/East Sea (Fig. 1). Approximately 200,000–300,000 pairs of rhinoceros auklets, 10,000–20,000 pairs of black-tailed gulls and 900–1000 pairs of Japanese cormorants, breed on the island (Osa & Watanuki, 2002). Black-tailed gulls are surface foragers feeding primarily by surface-plunging and/or surface-seizing (Ashmole, 1971; Watanuki, 1987). Rhinoceros auklets are
Results
Rhinoceros auklets foraged on epipelagic fish (anchovy, 0-yr.-old sandlance, 0-yr.-old greenling, Table 1). Black-tailed gulls also foraged on epipelagic fish (0- and >1-yr.-old sandlance, anchovy). Japanese cormorants foraged on epipelagic (0- and >1-yr.-old sandlance, anchovy), epibenthic (>1-yr.-old greenling), benthic (rockfish, flatfish, blennies), and near-shore-living fish (naked sandlance) (Table 1). The most dominant diets were the anchovy for rhinoceros auklets; 0-yr.-old sandlance
Discussion
Prey switching from sardine to anchovy in the late 1980s was found in both rhinoceros auklets and black-tailed gulls, and might be attributed to the collapse of the sardine stock. The abundance of sardine around Japan decreased dramatically in late 1980s because of reproductive failure (Watanabe, Zenitani, & Kimura, 1995). Unfortunately, seabird diet data were not collected in 1986, 1988–1991, and abundances of sardine and anchovy during this period are unknown in the sea surrounding Teuri
Acknowledgements
We thank A. Kato, A. Takahashi, E. Hayashi, M. Kuroki, T. Takagi, S. Hikosaka, Y. Chin, M. Chyochi, M. Takenaka, K. Iseki, T. Kagami, M. Aotsuka, Y. Fukuda, for field assistance. This work was partly supported by a Grant-in-Aid (# 07780452, # 09680497) from the Japanese Ministry of Education, Science and Culture, PICES, and COE Program (Neo-Science of Natural History). We thank W. Sydeman for invaluable comments on the manuscript and R. Kitaysky for help with the English grammar.
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Present address: Division of Marine Environment and Resources, Graduate School of Fisheries Sciences, Hokkaido University, Hakodate 041-8611, Japan.