Bungo Nishizawa

Diets and body condition of polar cod (Boreogadus saida) in the northern Bering Sea and Chukchi Sea

To understand trophic responses of polar cod Boreogadus saida (a key species in Arctic food webs) to changes in zooplankton and benthic invertebrate communities (prey), we compared its stomach contents and body condition between three regions with different environments: the northern Bering Sea (NB), southern Chukchi Sea (SC), and central Chukchi Sea (CC). Polar cod were sampled using a bottom trawl, and their potential prey species in the environment were sampled using a plankton net and a surface sediment sampler. Polar cod fed mainly on appendicularians in the NB and SC where copepods were the most abundant in the environment, while they fed on copepods, euphausiids, and gammarids in the CC where barnacle larvae were the most abundant species in plankton samples on average. The stomach fullness index of polar cod was higher in the NB and SC than CC, while their body condition index did not differ between these regions. The lower lipid content of appendicularians compared to other prey species is the most plausible explanation for this inconsistency.

Seabird densities and species and hydrographic features across Amchitka Pass, Aleutian Islands

To examine mesoscale physical and biological factors affecting seabird communities, boat-based seabird censuses were conducted during a single cruise from the North Pacific to the Bering Sea across Amchitka Pass, central Aleutian Islands, during summer. Acoustic surveys of potential prey biomass and currents, and conductivity-temperature-depth profiler measurements for vertical profiles of the temperature, salinity, and fluorescence, were conducted simultaneously. The seabird density and number of species were highest in Amchitka Pass (283 birds km−2, 17 species), where the acoustically determined biomass was the highest, followed by the Bering Sea basin (100 birds km−2, 13 species) and North Pacific basin (4 birds km−2, seven species). Crested auklets Aethia cristatella were dominant (95% in number) in Amchitka Pass, while short-tailed shearwaters Ardenna tenuirostris were dominant (92% in number) in the Bering Sea basin. Surface fluorescence was higher in the Bering Sea basin (median: 1.3 volts) than in Amchitka Pass (0.9) and the North Pacific basin (0.6). Within the pass, the number of crested auklets was highest over the northern part of the sill, where the water column was well mixed, and the northward-flowing tidal currents increased the concentration of acoustically determined biomass in the subsurface layer. Thus, we hypothesize that small alcids were attracted to an aggregation of prey that resulted from tidal currents impinging on the sill.

Status of the Spectacled Guillemot (Cepphus carbo) in Japan

Abstract. —The Spectacled Guillemot (Cepphus carbo) is a seabird endemic to northeastern Asia. It is listed as a “Vulnerable” species in Japan, though little is known of its population status. Population surveys were carried out during 2011–2014 throughout their range in northern Japan. Twenty-eight known and six suspected breeding colonies were reported in this region during 1875–2010. Fourteen known and four suspected breeding colonies were found in the 2011–2014 surveys, with 1,169 individuals and 294 nests. Based on our analysis of the two survey periods, 19 colonies have apparently disappeared, and populations have decreased in 12 of the current colonies. Our models indicated that colonies with small numbers of individuals were most likely to disappear, whereas other environmental factors (i.e., colony type, latitude, and SST trends) had relatively minimal effects on local colony extirpation. Our study shows that the population status of the Spectacled Guillemot has deteriorated in Japan.