Description:

In the Arctic, warming air and ocean temperatures have resulted in substantial changes to sea ice, which is primary habitat for polar bears. Reductions in extent, duration, and thickness have altered sea ice dynamics, which influences the ability of polar bears to reliably access marine mammal prey. Because nutritional condition is closely linked to population vital rates, a progressive decline in access to prey or an increase in the energetic cost of accessing prey has the potential to adversely affect polar bear population dynamics. We examined long-term (1983–2015) patterns of spring body condition (indexed using residual body mass) and maternal investment (i.e., litter mass of cubs-of-the-year and yearlings; COY and YRL) of polar bears from Alaska’s southern Beaufort Sea to evaluate potential relationships with regional- and circumpolar-scale sea ice conditions and atmospheric patterns. The length of the summer Open-Water (OW) season (i.e., the period of time the sea ice is mostly absent from the continental shelf) increased at a rate of 18 days decade-1 over the study period. However, the OW season duration was not a strong determinant of spring residual body mass or litter mass. Residual body mass of independent (i.e., subadults and adults) female bears varied relative to age class, reproductive status, and the strength of the prior winter’s Arctic Oscillation (i.e., a circumpolar-scale mode of climate variability driven by long-term atmospheric patterns). Spring residual mass of independent males varied with age class and variation in wind speed (i.e., regional-scale short-term atmospheric patterns) during the winter of the year preceding capture. Over the study period, mean annual body mass of adult females unaccompanied by COY declined by 4 kg/ decade-1, while no temporal trends were evident in the mean annual body mass of adult females with COY, adult males, and subadults. Litter mass of YRL varied with capture date, maternal age class and mass, litter size, variation in winter wind speed (the year of and year preceding capture), and the strength of the prior winter’s Arctic Oscillation. Mean annual litter mass of COY decreased at a rate of 2.6 kg decade-1 and declined 0.68 kg for every 10 kg reduction in maternal mass. No trend was evident in the mean annual litter mass of yearlings. These findings suggest a nuanced response of the southern Beaufort Sea polar bears to environmental change, where some demographic groups (e.g., adult males and subadults) are presently more resilient than others to changes in the Arctic marine ecosystem.

The floating goby Gymnogobius petschiliensis Rendahl is an amphidromous fish, which migrates upstream from the sea in the juvenile period and then inhabits not only freshwater areas but also brackish water. Most of the species that have such migratory polymorphism switch their osmoregulatory mechanism in synchrony with the timing of migration, but in G. petschiliensis it is known that even individuals living in freshwater areas behaviourally prefer brackish water to freshwater conditions in a laboratory. To clarify whether this species lives in freshwater areas with high osmoregulatory cost, the relationship between physiological cost and salinity was investigated using adult fish of the species captured in a freshwater area. Adults were kept individually in 13 L tanks with various salinity levels under fasted conditions. Their loss of body weight in freshwater conditions was more rapid than in 2/3 seawater, whereas the frequencies of horizontal and vertical movement in 13 L tanks did not differ significantly between freshwater and 2/3 seawater conditions. This suggests that the weight loss reflects not behavioural performance but physiological cost. In addition, no adults died even when transferred from freshwater to seawater conditions, indicating that they have high tolerance to salinity change in estuarine areas even after migrating to freshwater areas. These results support the hypothesis that G. petschiliensis migrates to freshwater areas while maintaining high hypo-osmoregulatory ability and paying a high physiological cost. Because G. petschiliensis phylogenetically originates from the coastal species, this example of migration without completely switching the osmoregulatory mechanism may help us to understand the diverse processes of upstream migration in marine fish.

With Regards
Williston
Journal Coordinator
Global Journal of Research and Review