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The Chukchi Sea environment changes considerably in physical and biological conditions, driven by the expanding volume of warm Pacific summer water. These environmental changes can affect the migration timing of baleen whales in the southern Chukchi Sea. However, few studies have been conducted in this area to determine the migration timing of bowhead whales (Balaena mysticetus), the only baleen whale species endemic to the Arctic region. In this study, we conducted a fixed passive acoustic monitoring of bowhead whales in the southern Chukchi Sea from July 2012 to October 2015 and compared the occurrence patterns of vocalizations to physical and biological environmental factors. Bowhead whale calls were detected in fall/winter and spring during the ice-freezing and retreating periods, respectively. The fall migration timing of bowhead whales through the southern Chukchi Sea was delayed in the years when the timing of sea ice formation was late, and it formed increasingly later in the years 2013, 2014, and 2012, in that sequence. Moreover, the sea surface temperature decreased before freeze-up, which affected the timing of fall migration of bowhead whales. There was no clear relationship between the occurrence of bowhead whale calls and the abundance of prey, especially in spring, suggesting that most bowhead whales use the southern Chukchi Sea as a corridor during their spring northward migrations. However, the occurrence of bowhead whales and high abundance of zooplankton in October–November present the possibility that bowhead whales expand their feeding area in the southern Chukchi Sea.

Information about the reproductive status of free-ranging pinnipeds provides useful insight into their population dynamics, which is essential to their management and conservation. To determine the reproductive status of individual animals, blood sampling is often required despite being impractical to collect in open water. Hair as an endocrine marker has been used to less invasively assess the reproductive status of terrestrial animals. However, it is unknown whether pinniped reproductive status can be assessed from hair samples. Here, we examine testosterone levels in hair obtained from 57 male northern fur seals and used it to compare their age class and spermatogenesis during the non-breeding season off Hokkaido. We isolated testosterone from the samples using gas chromatography and measured testosterone levels using time-resolved fluoroimmunoassay. Testosterone levels in hair increased towards the breeding season. In May, testosterone levels were the highest in seals aged between 4 and 7 years, followed by those over the age of 8 years and under the age of 4 years. Spermatids, the final phase of spermatogenesis, were present in the seals sampled between April and June, even though testosterone levels were low in April. The seals with spermatids in May showed the highest testosterone levels. Our results demonstrate that seals with higher testosterone levels in May are likely to be mature males (≥4 years). Since hair can be collected using biopsy darts in the field, it will be possible to less invasively determine testosterone levels of male seals in the future.

The Antarctic minke whale (Balaenoptera bonaerensis) is a seasonal migrator that feeds mainly on Antarctic krill at higher latitudes during the summer and travels to lower latitudes to breed in winter. A previous study using delta C-13 and delta N-15 values of baleen plates showed that pregnant Antarctic minke whales feed on Antarctic krill in the foraging season, with less feeding during the breeding season. However, long-term changes in the habitat use and diet of males remain unknown. In this study, we estimated the foraging habits of nine mature male Antarctic minke whales using the stable isotope signatures of baleen plates. From Bayesian stable isotope mixing models, it was found that Antarctic krill was the predominant prey of male whales, and prey species in lower latitudes were rarely incorporated into the baleen plates. This result is consistent with that of female Antarctic minke whales. Eight whales showed at least one oscillation in delta C-13 and delta N-15 values, whereas one whale (AM246) showed no variation in delta N-15 values. This possibly indicates that AM246 remained in the Antarctic water year-round. Our results suggest multiple migration patterns of male Antarctic minke whales.

Ice-breeding seals use vocal communication mainly during breeding season. The vocal function of aquatic mating, ice-associated ribbon seals (Histriophoca fasciata) has been unknown since they produce sounds in both breeding and non-breeding seasons. We examined the timing of their vocal presence in relation to environmental factors to infer the function of their calls in the Nemuro Strait, northern Japan, since this area is the possible southern limit of their breeding range. Vocalizations of ribbon seals were irregularly sampled from November 2012 to March 2014 in the Nemuro Strait and were compared with sea ice presence, time of day, and tidal currents. Ribbon seal downsweeps were detected in February 2013 and March 2014 only when sea ice was present along the Shiretoko Peninsula in the strait, with more detections in March leading up to the spring breeding season. Downsweep detections decreased in the middle of the day, indicating that ribbon seals were likely to be hauled out during this time. Vocalizing at night and early morning would probably reflect the increased opportunity for attracting females underwater. Our study suggests that the seal vocalizations in concurrence with sea ice presence in the Nemuro Strait could function as underwater communication for breeding.

Fin whales (Balaenoptera physalus) undergo seasonal migration in the Arctic Sea. Because their migration and distribution is likely affected by changes in global climate, we aimed to examine the migration timing of fin whales, and the relationship with prey availability within the oceanographic environment of the Arctic Sea, using passive and active acoustic monitoring methods. Automatic Underwater Sound Monitoring Systems were deployed in the southern Chukchi Sea from July 2012 to 2014 to determine the acoustic presence of fin whales. Furthermore, water temperature and salinity were recorded by a fixed data logger. An Acoustic Zooplankton Fish Profiler was additionally deployed to estimate prey abundance through backscattering strength. Sea ice concentrations were obtained by remote sensing data. Fin whale calls were automatically detected using a custom-made software, and the per cent of half-hours containing calls were counted. Fin whale calls were detected from 4 August to 20 October 2012 (78 d) and 25 July to 1 November 2013 (100 d). The extended period of acoustic presence of fin whales during 2013 when compared with 2012 is likely related to a longer ice-free period during 2013. Furthermore, generalized linear model analyses showed that half-hour periods containing calls increased with a rise in water temperature and zooplankton abundance during the initial call presence period, while they decreased with a decrease in water temperature and salinity during the end of the call presence period. Our results suggest that the rise in water temperature and zooplankton abundance affect the timing of migration of fin whales in a way that is consistent with the expansion of their suitable habitats and the extension of their presence in the Arctic Sea.

KEIKO SEKIGUCHI, Graduate School of Arts and Science, International Christian University, 3–10–2 Osawa, Mitaka City, Tokyo 181-8585, Japan; HIROJI ONISHI, Faculty of Fisheries Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 041-8611, Japan; HIROKO SASAKI, Graduate School of Fisheries Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 041-8611, Japan; SHOTA HABA, Graduate School of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu City, Mie 514-8507, Japan; YUKA IWAHARA, Graduate School of Environmental Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 041-8611, Japan; DAISUKE MIZUGUCHI, Wildlife Research Center, Kyoto University, 2–24 Tanaka-Sekiden-cho, Kyoto City, Kyoto 606-8203, Japan; MAYUKO OTSUKI, Graduate School of Environmental Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 041-8611, Japan; DAISUKE SAIJO, School of Fisheries Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 0418611, Japan; BUNGO NISHIZAWA, Graduate School of Fisheries Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 041-8611, Japan; HIRONA MIZUNO and NAOKI HOSHI, School of Fisheries Science, Hokkaido University, 3–1–1 Minato-cho, Hakodate City, Hokkaido 041-8611, Japan; TAKEHIKO KAMITO, Department of Life Science, International Christian University, 3–10–2 Osawa, Mitaka City, Tokyo 181-8585, Japan.

A number of marine mammal species travel off Rausu, Shiretoko as feeding and breeding habitats, providing resources for tourism. However, seasonal sea ice and severe weather conditions have restricted visual observations of marine mammals off Rausu except for daytime observations between spring and fall. To compensate for this lack of information, fixed passive acoustic recording devices were deployed to monitor the presence of marine mammals during November 2012–March 2014. Pacific white-sided dolphin (Lagenorhynchus obliquidens) burst pulsed calls in July, November, and December; ribbon seal (Histriophoca fasciata) downsweeps in February and March; killer whale (Orcinus orca) pulsed calls from April to August; and sperm whale (Physeter macrocephalus) clicks in July and August were detected. Cetacean species were possibly involved in foraging since their pulsed calls or clicks were often detected during foraging. In contrast, ribbon seals were possibly involved in breeding behaviours as seal vocalizations are likely related to breeding activities. This passive acoustic monitoring will provide new insights into the ecology of diverse marine mammals off Rausu.

Environmental changes are considered to affect the migration and distribution of baleen whales in the Arctic region. We examined the seasonal acoustic presence of fin whales (Balaenoptera physalus) and the relationships between acoustics and oceanographic environments in the southern Chukchi Sea from July 2012 to October 2015. Fin whale calls were detected from 4 August to 20 October 2012, from 25 July to 1 November 2013, from 26 July to 14 November 2014, and 7 June 2015, only in the ice-free (sea ice concentration = 0%) periods. In addition, from the recordings in 2012 and 2013, fin whale calls were present in the periods of high prey abundance, and there were significant positive correlations among their call presence, prey abundance, water temperature and salinity. Furthermore, the result that the calls were detected in 7 June 2015 suggests that fin whales arrive much earlier at the southern Chukchi Sea than previously reported. Our study provide valuable information to assess the impact on marine ecosystem in the Arctic region by environmental changes.

Vocalizations of ice-breeding ribbon seals Histriophoca fasciata were recorded using underwater passive acoustic methods from November 2012 to March 2014 off the Nemuro Strait, Japan. Seal presence in the strait was examined in relation to the sea ice extent in the Okhotsk Sea. Ribbon seal downsweeps were only detected when sea ice was present in the strait (February and March), with more detections in March leading up to the spring breeding season. Since ribbon seals require ice for breeding, underwater communication for breeding could be needed during the sea ice presence in this strait. Northeasterly winds were another indirect driver of ribbon seal occurrence, since winds from this direction likely transport sea ice from the central Okhotsk Sea into the Nemuro Strait. Downsweep detections decreased in the middle of the day, which is consistent with observations that seals hauled out on the ice during this time, and thus were producing fewer underwater vocalizations. Our results suggest that a decrease...