At-sea habitat use of breeding seabirds is strongly influenced by marine environmental features that vary over space and time. The use of bio-loggers allows researchers to track fine-scale movements ...of seabirds and provides opportunities to identify the primary factors affecting their area use for foraging. Using GPS loggers, we tracked chick-rearing rhinoceros auklets (
), which are wing-propelled divers, at Daikoku Island, eastern Hokkaido, Japan. The central phase for foraging activity on birds' trips was determined using a multiple change points model. To examine environmental factors explaining the distribution of the foraging phase, a generalized additive model was used where sea surface temperature, chlorophyll
concentration, bathymetry, and distance from the colony were explanatory variables. To obtain information supporting the behavioral tracking, prey items in the bill-loads of adult auklets were collected. We found that auklets foraged over the continental shelf shallower than the 200-m isobath and that distance from the colony was related to the area use. Adult auklets predominately brought back age-0 chum salmon (
), which was abundant in coastal waters along southeast Hokkaido during the study period. Our findings indicate that rhinoceros auklets rearing chicks, hence visiting nests frequently, on Daikoku Island can find suitable feeding grounds nearby.
To understand the environmental factors affecting the density of foraging seabirds across the cold-water belt in the southwestern Okhotsk Sea, we conducted a 1-day (180-km transect length) shipboard ...seabird survey off the northeastern coast of Hokkaido during summer in 2019, along with acoustic observations of potential prey (zooplankton and fish) biomass, thermosalinograph measurements, and CTD observations. Planktivorous short-tailed shearwaters
Ardenna tenuirostris
(66% of total seabirds) and piscivorous rhinoceros auklets
Cerorhinca monocerata
(28%) were predominant, but foraged in contrasting habitats. A large foraging flock of shearwaters was observed in the cold-water belt zone, including its front with coastal Soya Warm Current Water and the offshore Fresh Surface Okhotsk Sea Water, where surface chlorophyll
a
concentrations were the highest but not related to their prey (zooplankton) biomass at any spatial scale between 4.6 and 9.2 km. In contrast, the density of auklets was high in the coastal Soya Warm Current Water, where the acoustically determined fish biomass was large, and showed a positive relationship with the fish biomass especially in the lower layer (29–104 m depth) at any spatial scale. This species-specific difference in response to prey biomass might be related to prey-searching behaviors; i.e., rhinoceros auklets search prey underwater visually, but short-tailed shearwater can use both visual and olfactory cues to locate zooplankton patches from the air.
Seabird parents during chick rearing is hypothesized to regulate body mass to reduce flight costs and invest energy in current reproduction. Alcids have 2‒4 times higher wing-loading and higher ...flight costs than other seabirds. In particular, rhinoceros auklets
Cerorhinca
monocerata
(RHAU) carry the heaviest meals among alcids despite its medium-size, therefore, we expected that they might be more likely to keep their body mass small and within a narrow range during chick rearing. We examined between-breeding stage and interannual variations in RHAU body mass using 27-year monitoring data, then tested whether the interannual variation shown by the coefficient of variation (CV) in body mass during chick rearing was smaller than in other seabirds, and if their body lipid stores were smaller. RHAU during chick rearing have 15‒20 g lower body mass, corresponding to 5‒7% decrease of flight costs, than those during incubation. We found that CV of body mass in RHAU (1.4) was smaller than those of 10 other seabird species (1.7‒7.5), while CVs in provisioning metrics, such as meal mass, chick growth, fledgling mass, and fledging success, were the largest or second largest. RHAU body lipid stores during chick rearing (3.8‒4.0%) was also smaller than six other species (5.7‒9.5%). Results suggest that chick-rearing RHAU maintained a narrow range of body mass with minimum body lipid stores, possibly because of their greater wing-loading and heavier meals. Such constraints on body mass regulation might affect their variable investment in their chicks under environmental variability, as shown large variation in provisioning metrics.
Short-tailed shearwaters
Ardenna tenuirostris
are seasonally abundant consumers in the Southern Ocean, but their diet in this region is debated. Among birds found on vessels at high latitudes (
n
= ...11 live birds and 4 carcasses), at least six had recently ingested Antarctic krill
Euphausia superba
. Accumulated fresh remains with no flesh attached were also found in the gizzard of two birds. Liver isotopic values suggested that STSHs fed primarily on krill in southerly habitats. Clarifying whether this abundant predator commonly feeds on Antarctic krill is important to better understand and manage the Southern Ocean food webs.
Marine top predators are expected to adjust their foraging behaviour at multiple time scales concomitantly with changes in forage fish availability. Rhinoceros auklets
Cerorhinca monocerata
rearing ...chicks at Teuri Island, Japan Sea, fed on anchovy
Engraulis japonicus
in 2012 and 2013 (anchovy regime) but switched to sand lance
Ammodytes
spp in 2019 and 2020 (sand lance regime). Here, we studied their at-sea behaviour using the GPS locations of 33 birds and the depth-acceleration records of 26 birds, and compared their foraging behaviour between these prey regimes. At the trip scale, auklets used offshore waters (> 50 m sea depth) and coastal waters in the anchovy regime but used mainland coastal waters (< 50 m sea depth) in the sand lance regime. In the sand lance regime, the birds also conducted more overnight 2- to 4-day trips in 2020 and spent more time flying during 1-day trips as they fed in further areas compared to the anchovy regime. At the dive scale, auklets frequently dove to both < 5 m and 20–30 m depths in the anchovy regime but mainly to < 5 m depth in the sand lance regime. Within each dive, auklets showed a greater number of fast/strong wing stroke events in the anchovy regime than in the sand lance regime. These changes in auklet behaviour reflected the different habitats, depth distribution, and swim speed of the targeted prey species. Our study shows the behavioural flexibility of a wing-propelled flying-diving seabird in response to the inter-annual shifts in the dominant forage fish community. It also indicates the ecological constraints on the mechanisms determining nest productivity in this day-foraging/night-provisioning seabird.
Juvenile mortality, a key factor determining the stock sizes of salmonids, may be increased by predation both in rivers and along shores. Information on predation at sea, however, is scarce. We ...analysed the diet of a pelagic seabird, the rhinoceros auklet Cerorhinca monocerata, breeding on an isolated island off eastern Hokkaido, Japan, and estimated their consumption of juvenile chum salmon Oncorhynchus keta. The auklets brought juvenile chum salmon and Japanese anchovy Engraulis japonicus for their chicks and, in addition, fed on squid themselves. The fork length of most of the juvenile chum salmon was over 80 mm, indicating that they were migrating to offshore nursery area, the Sea of Okhotsk. Using a simple bioenergetics model, the consumption of juvenile chum salmon by auklets during the first half of chick-rearing period (late June to middle July) was 316.6 t in 2014 (296.4 t by adults, 20.2 t by chicks) and 11.7 t in 2015 (0 t by adults, 11.7 t by chicks). The otoliths of six out of 66 juvenile salmon found in the diet for chicks had thermal markings and indicated that these fish were released from the rivers along Pacific coast of Hokkaido. Our study first demonstrates the extent of the auklets' consumption contributing to the reduction of the survival in the open coastal area of juvenile chum salmon with the known stock origins.
